Sweet Potato(Ipomoea batatas)

Breeding

CROPSIRETA Publications No__/89

AUTHORS: Jill E. Wilson, Senior Fellow in Tropical CropProduction, Department of Agronomy and SoilScience, University of Hawaii at Manoa.

Finau S. Pole, Research Officer, ResearchDivision, Ministry of Agriculture, Fisheries, andForests, Kingdom of Tonga.

Nicole E. J. M. Smit, Root Crop Specialist,Tongan/ German Plant Protection Project,Kingdom of Tonga.

Pita Taufatofua, Senior Research Officer, ResearchDivision, Ministry of Agriculture, Fisheries, andForests, Kingdom of Tonga.

Assisted by Kim Des Rochers, Carl I. Evensen,and Dale O. Evans.

Illustrations by Shirley Tjendana.

Mention of trade names does not constitute IRETA approval tothe exclusion of suitable alternative products.

All or part of this publication may be reproduced for educationalpurposes. When doing so, please credit the University of theSouth Pacific Institute for Research, Extension and Training inAgriculture (IRETA).

Published April, 1989, by the Institute for Research, Extensionand Training in Agriculture with financial assistance from theFAO Root Crops Development Systems Project RAS/86/034.

IRETA PublicationsUSP Alafua Campus

P.O. Private BagApia, WESTERN SAMOA

Printing: Communications Support Centre

TABLE OF CONTENTS

Sweet Potato Breeding..................................................................................................... 1Steps in a Sweet Potato Breeding Programme ................................................................ 4Step 1 – Plant Crossing Block to Provide Improved Population of Seeds ...................... 5

The Crossing Block ............................................................................................. 7Floral Biology ...................................................................................................... 8Controlled hand-pollination ................................................................................. 9Harvesting Seeds ............................................................................................... 12Seed Storage ...................................................................................................... 13Record Keeping ................................................................................................. 14

Step 2 – Plant Seeds in Seedling Nursery...................................................................... 15Steps 3 through 9 – Evaluating and Selecting Clones ................................................... 16

Conducting Trials .............................................................................................. 17Step 10 – Name and Release the Best Clones ............................................................... 27Step 11 – Distribute New Cultivars to Farmers ............................................................. 27

Multiplication .................................................................................................... 28Recurrent Selection............................................................................................ 29Genetics ............................................................................................................. 31

Evaluating Specific Characters ...................................................................................... 32Plant Type .......................................................................................................... 32Stem Thickness .................................................................................................. 32Time from Planting to Harvest .......................................................................... 32Disease, Insect, and Nematode Resistance ........................................................ 33Yield .................................................................................................................. 34Tuber Characters ................................................................................................ 35Eating Quality .................................................................................................... 36Nutritional Value ............................................................................................... 38

Supplies ......................................................................................................................... 39

SWEET POTATO BREEDING

This manual is for researchers in thePacific region who are starting sweetpotato breeding programmes but lacktraining and experience in genetics andplant breeding. It is also for researchersand extension workers who are evaluatinglocal and newly introduced sweet potatogermplasm. It may also be useful toteachers and students of crop improvementcourses at the diploma, degree, andpostgraduate levels.

The objective of this manual is toprovide you with the methods andtechniques needed to carry out a sweetpotato breeding programme. Practicalaspects of breeding are emphasised, ratherthan theory. Use of technical terminologyis minimized. To keep the manual concise,detailed explanations are avoided whenpossible, and many concepts aresimplified. After learning the materialpresented here, you may want to read othertexts to gain a more completeunderstanding of plant breeding.

Not all sweet potato breeders use thesame breeding systems and techniques.The ones included here are appropriate tothe developing-country conditions thatmost of us face, including limitedlaboratory and field facilities and limitedfunds for labour, equipment, and supplies.

These methods and techniques haveproved successful in our breedingprogramme in the Kingdom of Tonga. Thisprogramme is a collaborative effortbetween the Tongan Ministry ofAgriculture, Fisheries and Forests, theUniversity of the South Pacific’s Institutefor Research, Extension and Training inAgriculture, and the Tongan/German PlantProtection Project.

The methods and techniquespresented here are important to thesuccess of your breeding programme.Equally important is your knowledge ofsweet potato production in your country,particularly your knowledge ofyield-reducing factors such asenvironmental stresses and disease,insect, and nematode pests. You mustalso be familiar with the other problemsfarmers face and with the sweet potatoquality characters favoured by farmersand consumers. Thorough knowledge ofthese factors is necessary to determinethe goals of your breeding programme.

There are 3 methods to obtainimproved cultivars (agriculturalvarieties) of sweet potato (Ipomoeabatatas) for distribution to farmers inyour country:

• Collecting, evaluating, and selectingfrom the local germplasm.

• Importing cultivars that have beenbred in other parts of the world andevaluating them under yourconditions.

• Breeding cultivars in your ownprogramme.

This leaflet will concentrate on thelast of these methods, that of breeding.However, much of the discussion underSteps 3 through 11, concerningconducting breeding trials and reducingexperimental error, will be useful if youare evaluating local germplasm orimported cultivars.

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Step 6evaluate clones in second

INTERMEDIATE TRIAL (IT-2)in season 2 and select best clones

Step 7evaluate clones in first

ADVANCED TRIAL (AT-1)in season 1 and select best clones

Step 8evaluate clones in second

ADVANCED TRIAL (AT-2)in season 2 and select best clones

Step 9evaluate clones in

ON-FARM TRIALS in several seasonsand several locations and select best clones

Step 10NAME best clone(s)

and RELEASE

Step 11DISTRIBUTE to farmers

MULTIPLYplanting material

3

STEPS IN A SWEET POTATO BREEDING PROGRAMME

A proposed breeding procedure isoutlined in Figure 1. In Steps 1 and 2,sexual propagation is used to produceseeds and seedlings. In Step 2, eachseedling is cloned by propagating itvegetatively, and in all the followingsteps, vegetative propagation is used.

The purpose of using sexualpropagation in Steps 1 and 2 is to creategenetic variability. Each seedlingproduced during sexual propagation isgenetically different from all others andis potentially a new, improved cultivar.

During Steps 3 through 5, youevaluate clones derived from theseedlings produced in Steps 1 and 2 todetermine which ones you will advanceto additional trials for further testing inSteps 6 through 9, which ones you willuse as parents in the next Crossing Block(Step 1), and which ones you willdiscard.

During the On-Farm Trials (Step 9),you will identify one or more clones thatboth you and the farmers like. You arenow ready to officially name and releasethese clones in Step 10 and distributethem to farmers in Step 11.

Figure 1 shows clearly thatmultiplication is a continuous activitythat begins after you harvest the HillTrial (Step 3), but the largestmultiplication takes place between Steps10 and 11 to produce enough plantingmaterial for distribution to farmers.

Why do you go through the process ofcompleting Steps 1 to 5 and then return someclones to Step 1? This is done to increaseyour chances of finding seedlings having allthe characters you need in an improvedcultivar. This process is called “populationimprovement”. During each breeding cycle,parent clones are selected andcross-pollinated in such a way that theresulting seedling population is improved.That is, it contains more good seedlings thanprevious seedling populations. The method ofpopulation improvement that many sweetpotato breeders use, and that we use inTonga, is called RECURRENTSELECTION. Recurrent selection, especiallythe step of choosing the parent clones thatyou plant in Step 1, determines the directionand success of your breeding programme; itis so important that a separate section isdevoted to it later in this manual.

After a clone has been named andreleased, we generally call it acultivar.

4

A clone consits of all thedescendants of a vegetativelypropagated individual.

Although Figure 1 may lookcomplex, it does in fact oversimplify arecurrent selection breeding programme,because it presents only one cycle.Recurrent selection is a long-term,continuous process with a new cycle ofbreeding (a new Crossing Block) startedevery year and with the chance for newand better clones ready for release everyyear.

Figure 2 tries to capture the morecomplex picture. It is important that abreeding programme be continued forlong enough to benefit from theinvestments made in the early years. Donot expect to find the “perfect” clone inthe first few years of the recurrentselection programme.

Your programme can include all 11steps outlined above, or it can begin at Step2 or 3 if you are collaborating with otherbreeding programmes. For example, if youreceive seeds from another breedingprogramme, then you would start at Step 2.If you receive tissue cultures of improvedclones from another breeding programme oryou are evaluating your local germplasm,then you would start at Step 3.

Step 1 – Plant Crossing Block to Provide Improved Population of Seeds

You can plant parent clones in a CrossingBlock isolated from other floweringsweet potatoes and allow them to beopen-pollinated by naturally occurringinsects. This is called a “polycrossnursery”. Or, you can make controlledhand-pollinations to ensure that specificcross-combinations are represented in theseeds you produce. We use acombination of open-pollination andhand-pollination.

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Pollinating in a Crossing Block

With open-pollination, you know thefemale parents of seeds but not themale parents. With controlled hand-pollination, you know both the femaleand male parents of the seeds.

Figure 2. Breeding is a long-term, continuous process with a new Crossing Block started every year. In this flow chart, 2crops of sweet potato are grown each year (Seasons 1 and 2). The breeding program could be speeded up if 3 crops ofsweet potato can be grown each year in your location.

Year 1 Year 2 Year 3 Year 4

Season 2 Season 1 Season 2 Season 1 Season2 Season 1 Season 2CB & SN HT PT IT - 1 IT - 2 AT - 1 AT - 2

CB & SN HT PT IT - 1 IT - 2CB & SN HT PT

CB & SN

Year 7 Year 6 Year 5Season 2 Season 1 Season 2 Season 1 Season 2 Season 1

Name & Release & Distribute On - Farm On-FarmName & Release & Distribute On - Farm On - Farm AT - 2 AT - 1

On-Farm On-Farm AT - 2 AT - 1 IT -2 IT -1AT - 2 AT - 1 IT - 2 IT - 1 PT HTIT - 2 IT - 1 PT HT SN & CB

PT HT SN & CBSN & CB

CB = Crossing BlockSN = Seedling NurseryHT = Hill TrialPT = Preliminary TrialIT = Intermediate TrialAT = Advance Trial

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The Crossing BlockSweet potato flowers best during shortdays. Winter is therefore the best time forproducing seed in the SouthernHemisphere.

In Tonga, we plant the crossing blockduring the first 2 weeks of April.Flowering begins about 1.5 months laterand continues for 3 months. This meansthat we pollinate from May throughAugust and harvest seeds from Junethrough September.

In the Crossing Block, plant vinecuttings of the parent clones around theplants. We use 1 x 1 m, with 2 cuttings ofthe same clone at each planting position.Usually, 10 plants (5 planting positions)of each clone are enough, although youmay need more plants of sparse-floweringclones and clones you plan to use in alarge number of crosses.

At each planting position, erect a2-m-tall stake. Train the main vines upthe stake and tie them to the stake withinexpensive material. We use “bushtwine”.

Staking facilitates hand-pollinationand insect-pollination, but staked plantsare easily damaged by wind. If yourCrossing Block is in a windy location,you should plant or construct awindbreak. We erect a “wall” of netshadecloth to block the prevailing wind.

Using a scissors, prune off all lateralbranches and the tops of long vines thathave reached the top of the stake. Becareful! Young flower clusters, whichare also in the axils of the leaves, areeasy to confuse with lateral branches. Donot prune off these flower clusters!

Do not fertilise the Crossing Blockheavily with nitrogen, because lush,leafy vines produce few flowers.

Insects and diseases can reduceflowering and seed set. The CrossingBlock should therefore be sprayedregularly, especially against sweetpotato, weevils, leaf scab, andcaterpillars that feed inside the flowers.To avoid killing bees and otherpollinating insects, apply pesticides inthe evenings, and use contactinsecticides or baits rather than thosewith residual activity.

Birds sometimes eat sweet potatoflowers. You can discourage them withbirdchasers, such as metal tin lids orplastic bags attached by short lengths ofstring to a lattice of string between thestakes.

Pollinating is easier if youlabel each stake with the clonenumber.

Treated stakes last longer andcan be reused another year.

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Floral BiologyMost sweet potato clones flowernaturally in the Pacific Islands andother tropical countries. It is notusually necessary to use grafting,girdling, or day-length control, whichbreeders in temperate regions use toinduce flowering during long days.

Flowers occur in inflorescences,clusters of up to 22 buds growing outof the leaf axils. Each flower opensonce, soon after daybreak, and usuallyfades by noon. It contains one stigmaon top of the pistil (the female part)and 5 anthers on top of the 5 stamens(the male parts). The height of thestamens varies in different clones. Ifthe stamens are shorter than the pistil,it is easy to find and pollinate thestigma, but if the stamens are the sameheight as or taller than the pistil, it is

difficult to find and pollinate thestigma.

The enlarged base of the pistilcontains 2 ovaries, and each ovaryhas the potential of producing 2seeds. Therefore each fruit, which iscalled a capsule, can contain amaximum of 4 seeds.Hand-pollinated capsules usuallycontain only 1 or 2 seeds, and mostopenpollinated capsules contain 2 to3 seeds.

Self-fertilisation in sweet potatois rare, because all clones have ahigh degree of self-incompatibility.Similarly, it may be difficult toobtain seed from crosses betweencertain parents, because cross-incompatibility also occurs.

calyx

corolla

pedicel

corolla

stigma

anthers

ovaries

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Before pollinating, you must ensurethat the flowers you plan tohand-pollinate are protected frompollination by insects. Protect theflowers on both female and male parentplants. To do this, select buds that willopen the following morning andprevent each flower from opening byclipping the tip of the corolla with anO.K. Fastener (see Supplies), largepaper clip, or small piece of drinkingstraw. Paper clips are the easiest topurchase, but we prefer O.K. Fastenersbecause they are lightweight. If youopen these O.K. Fasteners carefully,you can reuse each one about 10 timesbefore it breaks.

The best time to clip flowers forpollinating is late afternoon or eveningon the day before you want to pollinate.At pollinating time it is easier to findthe flowers you have clipped if youmark the plant with brightly-coloured,flagging ribbon, which you removeafter pollinating.

Before pollinating, small pieces of drinking strawor paper clips can be used to prevent flowersfrom opening. Note the size of these buds; theywill open the following morning.

Before pollinating, O.K. Fastenerscan be used to close corolla toprevent flowers from opening.

After pollinating, thecorolla is tied closed withstring.

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Controlled hand-pollinationTo carry out a controlledhand-pollination in sweet potato, thereare 4 steps:1. preventing insect-pollination before

hand-pollinating,2. hand-pollinating,3. preventing insect-pollination after

hand-pollinating, and4. labelling.

Since self-fertilisation rarely occursin sweet potato, it is not necessary toemasculate (remove anthers from) thefemale parent unless you are carryingout genetic studies.

The following morning,pollinate between sunrise and noon.Take a clipped flower from the maleparent plant and carry it to thefemale parent plant. Gently removethe clip from the female flowerwithout tearing the corolla, andspread the petals open. Remove theclip from the male parent, peel backthe corolla to make a handle, andrub the anthers gently over thestigma of the female parent.

To prevent contamination of thefemale parent after pollination,carefully tie the corolla closed sothat insects cannot reach the stigma.We use lightweight knitting wool orstring.

Pollinating

Flower from male parentprepared for pollinating

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NOTE: If you are producing seedsfor a genetic study, you shouldemasculate the female parent toeliminate all possibility of self-pollination. To do this, select budsthat will open the following morning.Use a one-sided razor blade to cut thecorolla into 2 equal parts, from the tipdown to the base. Do not damage theovaries. Gently pull down each half ofthe corolla to remove the attachedstamens. Cover the exposed pistil witha short length of drinking straw thathas been closed at the top, pollinateas usual, and cover the pistil againwith the drinking straw. If thedrinking straw is paper, close it bybending over the top; if it is plastic,close it with an O.K. Fastener orstaple. Remove the drinking straw 2or 3 days after pollinating to permitthe ovaries to expand.

To reduce contamination byundesired pollen, wash your hands orclean them with a damp cloth eachtime you change to a different maleparent.

Label each pollination with thedate and the names or numbers ofthe parents. Tie the label onto thepedicel of the individual flower thatyou pollinated, not below theinflorescence, which may containopen-pollinated flowers. We usesmall, white merchandise tags forlabelling.

Your success rate for hand-crossing is affected by the weather,the health of the plant, the parentsused, and your skill in pollinating. Inthe Tongan programme, about 48%of the flowers we pollinate developinto capsules, with an average of 2seeds in each capsule.

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Throughout this manual, we have usedthe term sweet potato TUBER, ratherthan thetechnically correct term STORAGEROOT or FLESHY ROOT. We havedone this because tuber is commonlyused and understood by agricultureworkers and students in the PacificIslands.

Unless you want a particularmaternally-inherited character, youcan try using each clone as both amale parent and a female parent.

When you are first learning topollinate, it is useful to also writeon the pollinating label yourinitials, the time of day, remarksabout the weather (very hot, rainy,etc.), and other comments that willhelp to improve your pollinatingsuccess rate.

By convention, breeders write thefemale parent first when labelling across. Therefore, in the sketch on p.9, 85022-12 is the female parentand 83003-100 the male parent.

Harvesting SeedsSeeds mature 4 to 6 weeks afterpollination. Harvest each seedcapsule when it is fully brown andthe pedicel is dried and shrivelled.Check the Crossing Blockregularly, capsules that are left toowill dehisce (split open), scatteringthe seeds on the ground. Pick eachhand-pollinated capsule, makingsure that its label is attached. Labelopen-pollinated capsules with thename or number of the femaleparent.

In the lab, thresh out the seedsand discard any that arelightweight or show signs ofinsects or fungus. Put sound seedsinto labelled envelopes, record thenumber of seeds on the envelope,and store. You can put seeds fromdifferent hand-pollinated capsulesinto one envelope if they have thesame female and male parents, or,in the case of open-pollinatedseeds, if they have the same femaleparent.

To simplify record-keeping, weoften bulk together differentcrosses, for instance, all open-pollinated seeds harvested fromclones that performed well in theAdvanced Trial. When bulkingtogether more than one cross, youcan make sure that all parents areequally represented by bulkingequal numbers of seed from eachfemale parent or each crosscombination.

capsules

seeds

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NOTE: Seeds with low viabilityare usually lightweight and canbe separated from sound seeds byfloating. Put seeds into acontainer of water to which a fewdrops of a surfactant or detergenthas been added. Discard thelight seeds that float. Save theheavy, sound seeds that sink tothe bottom, but be sure tothoroughly dry them beforestoring.

Seed StorageSweet potato seeds will remain viable forup to 20 years in wellcontrolled storageconditions (18oC, 50% RH), and for atleast 5 years when stored in a desiccatorin the refrigerator. We make aninexpensive desiccator from a tin or glassjar with a tight-fitting lid. In the bottomof the tin or jar we place a layer of thedesiccant, silica gel. The indicating typeis best, since you can see by the changein colour from blue to pink that it needsto be dried.

If you do not have silica gel, youcan use uncooked rice that has beenbaked in a low-temperature oven untilvery dry and light brown. Try to find atleast a small quantity of indicatingsilica gel to mix with the baked rice soyou can see when the rice needs to bedried again. Place a piece of screenover the desiccant before adding theenvelopes of seeds.

You can make an inexpensivedesiccator for storing seeds.

packets of seed

screen

silica gel or baked rice

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Record KeepingRecord keeping is an important part ofa breeding programme. With somemethods of breeding, like pedigreebreeding, it is essential that you be ableto trace the pedigree of a clone backthrough many generations. Withrecurrent selection, however, knowingthe pedigree of each clone is notalways important, and that is why weoften bulk seeds together. There aresome instances when knowing thepedigree of a clone will help improveyour breeding programme, and that iswhy we keep seeds of some crossesseparate.

Different breeders prefer differentmethods of recording pedigrees. Hereis the one that we use:

When seeds are harvested from theCrossing Block, each bulked seedlot orindividual cross is assigned a familynumber, for example, NIS/86001. NISindicates seed (S) of sweet potato(I=Ipomoea) produced in Nualei (N),Tonga, and 86001 is the family number(86 = 1986, the year the seed washarvested).

When the seedling nursery isharvested, we assign a clone number toeach seedling that is selected to go intothe Hill Trial. This clone numberconsists of the family number followedby -1, -2, -3, etc., for example,86001-1. This clone number is aPERMANENT number, and does NOTchange from year to year.

Keep handwritten records in abound record book–do NOT risk typingerrors.

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Step - 2 Plant Seeds in Seedling Nursery

Scarifying SeedsSince sweet potato seeds have very hardseedcoats, they germinate slowly and irregularlyunless they are scarified. The easiest way toscarify large quantities of seeds is to soak themin concentrated sulfuric acid.

If you have only a few seeds, you canhand-scarify them by scratching a small notch ineach seedcoat with a sharp needle or a small,3-cornered file. Do not scratch the round side ofthe seed, since this will damage the embryo.

For very important lots of seeds, you canacid-scarify first, and then hand-scarify anyseeds that have not germinated or swollen 2days after moistening them in petri dishes.

We use the following method for scarifyingseed: Caution: THE FUMES OF SULFURICACID ARE DANGEROUS, SO WORK IN AFUME CABINET OR OUTSIDE AWAYFROM BUILDINGS AND PEOPLE. We pourconcentrated sulfuric acid (98% H2SO4) into aGLASS beaker large enough for the quantity ofseeds we want to treat. We wrap the seeds inacid-proof fly screen (test a small piece in theacid before wrapping the seed) and label with ametal label. Then, using the metal wire on thelabel as a handle, we soak the seeds in thesulfuric acid for 40 minutes. Still working in thehood or outside, we pull the bundle of seeds outof the sufuric acid and rinse in 2 beakers ofwater, one after the other. Then it is safe to takethe bundle to a tap and rinse the seeds underrunning water for 5 to 10 minutes. Using thismethod, we normally get 95% germination.

Seed Germination and Seedling RearingImmediately after scarifying and rinsing theseeds, place them in petri dishes lined with wetfilter paper, paper towel, or tissues. Do not addtoo much water; there should be no free water inthe bottoms of the petri dishes. Up to 150 seedswill fit into one petri dish. Cover the petri dishesand keep them in the lab at ambient temperatureand light. Germination usually starts 1 to 2 daysafter wetting. Transplant germinated seeds to thenursery bed as soon as you can see the radicles,because transplanting is very difficult if you

delay until the radicles are longer.

We also plant seeds that are swollen buthave not yet produced radicles; they will usuallygerminate in the nursery bed.

Plant germinated seeds at a spacing of 30 cmbetween rows and 15 cm between plants.

For our nursery beds, we mix togethertopsoil, sand, and rotten (composted) chickenmanure and fumigate with Basimid. (Check tosee if this is legal in your area.) If vine growthduring the season is not vigourous enough, wefertilise with NPK. And we use slug bait toprotect the young seedlings.

Seedlings are ready to harvest when most ofthem are large enough to provide 3 vine tipcuttings. In Tonga, this is about 2.5 months aftersowing.

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Steps 3 Through 9 - Evaluating and Selecting Clones

Which characters should you evaluatewhen selecting clones? This will dependon the goals of your breeding programme,but to give you some ideas, we haveincluded in Table 1 the characters that weevaluate in each trial in the Tonganbreeding programme.

Evaluating large numbers of breeding clones infield trials requires careful observation of manyplant and tuber characteristics. It requirespatience, persistence, scientific skills and a “goodeye”. Plant breeding is both an art and a science.

In these steps, you evaluate thebreeding clones in a series of 6 trialslocated on the research station,followed by trials in farmers’ fields.Note that the Intermediate andAdvanced Trials and the On-FarmTrials are each planted in 2 seasons,wet and dry. Our goal is to findbreeding clones that performexcellently in the dry season and atleast satisfactorily in the wet season.

At the end of each trial, youmust make decisions whether toselect a clone for further trial, or todiscard it, or to use it as a parent inthe Crossing Block. Usually, youselect 10 to 50% of the clones forfurther trial.

In Hill and Preliminary Trialsyou have large numbers of clones,but each clone is represented byonly a few plants. In Intermediateand Advanced Trials you have fewerclones, but more plants in eachclone. For instance, in a Hill Trialyou might have 1000 clones with 3plants/clone, compared to theAdvanced Trial with 5 clones and24 plants/clone/replication, 4replications. With the small numberof plants/clone in Hill andPreliminary Trials, it is not possibleto judge very accurately suchcharacters as yield. That is why weuse subjective ratings during thesetrials and do not begin to actuallycount and weigh yield until theIntermediate Trials.

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Conducting TrialsHere are some pointers for conductingthese trials:

1. Trial Conditions. Ideally, trialsshould be carried out in as many sites aspossible, but practically we are oftenlimited to one or two sites. Therefore,choose your trial sites carefully toinclude as many as possible of therelevant biological and physicalconstraints faced by the farmers. Inother words, you should carry out yourtrials under conditions similar to thosefaced by farmers (or home gardeners).

Do not make the common mistakeof conducting your trials under optimumconditions by adding fertiliser andwater, controlling all pests and diseases,etc. If you do, you may end up selectingbreeding clones that perform well underoptimum conditions but perform poorlyunder the farmers’ less than optimumconditions. For instance, if farmers growmost of their sweet potatoes on lowfertility soil, then you should grow yourtrials without adding fertiliser. Iffarmers do not irrigate, you should notirrigate your trials, and so on.

Obviously, there are times when youmust break this rule in order toaccomplish your breeding goals. Forinstance, if you are selecting for leafscab resistance, you might need to applysmall quantities of overhead irrigation towet the leaves and maintain highrelative humidity, in order to encouragedisease spread. Also in your trials,especially the Hill Trial, you may needto use a spacing wider than the farmer’sin order to make evaluation possible.

2. Disease and Pest Control. In yourtrials, you should normally NOT controlthe diseases, insects, or nematodes towhich you are selecting resistance, evenif farmers do use control measures. Forinstance, many farmers in Tonga controlleaf scab with fungicides, but we do notapply fungicides to our breeding trials.

However, it may be necessary to usesome control measures to reduce damageby diseases, insects, or nematodes if thelevel of resistance in your breedingpopulations is still relatively low. Forinstance, the level of weevil resistance inour breeding clones is low. If we applyno control measures, the weevils destroymost tubers, and it is not possible for usto select for other characters such asyield and eating quality. We do,however, want to be able to identify anddiscard clones that are highly susceptibleto weevils. Therefore, we apply controlmeasures (hilling-up during the growingseason, weevil traps) that reduce weevildamage but do not eliminate it. Anotherexample is little leaf disease. The level ofresistance to this disease in our breedingclones is low, and if it is spreadingrapidly through our trials, we control itby rogueing out diseased plants.

If you are NOT evaluating yourbreeding clones for certain pests, youshould control them in your trials if thedamage they cause will obscure thecharacters you are evaluating. Forinstance, we control rats (with bait), leafminers, and horn worms whenpopulations are high.

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Table 1. Character evaluated in each trial in the Tongan breeding programme.

Trial Planting Pattern Characters

Seedling Nursery 30 x 15 cm; leaf scab score (at harvest), vine length,(SN) 1 plant/clone. vine thickness, twining (vine climbs or

does not climb), tuber skin colour, tuberflesh colour.

Hill Trial 100-200 x 90 cm between leaf scab score (average over season),(HT) planting points; 3 little leaf score, virus score, rose

plant/ planting point; beetle score, vine length, vine thickness1 planting point. twining, tuber skin colour, tuber flesh

colour, yield (high, medium, low),specific gravity of clones selected infield

Preliminary Trail 100 x 90 cm between leaf scab score (average over season),(PT) planting points; little leaf score, virus score, rose

3 plants/planting beetle score, vine length, vine thicknesspoint; 6 planting twining, tuber skin colour, tuber fleshpoints (use inside colour, tuber shape, weevil damage, tube4 planting points for smoothness, tuber cracking, precociousdata). tuber sprouting, tuber appropriate for

market and/or ceremony, tuber yield (high,medium, low), tube number (high, medium,low), tuber size (large, medium, small),specific gravity.

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Table 1 continued

Intermediate 100 x 90 cm between leaf scab score (average over season),Trials (IT - 1 and planting point; little leaf score, virus score, roseIT - 2) 3 plants/planting beetle score, vine length, vine thickness,

point; 6 planting twining, tuber skin clour, tuber fleshpoints/rep (use inside colour, tuber shape, weevil damage, tuber4 planting points for smoothness, tuber cracking, precociousdata); 3 reps; tuber sprouting, tuber appropriate for2 seasons. market and/or ceremony, marketable weight

tubers (weigh), marketable number tubers(count), edible weight tubers (weigh),edible number tubers (count), total weighttubers (weigh), total number tuberscount, field selection, specific gravity.

Advanced Trials 100 x 90 cm between same as IT plus tuber dry weight, eatingplanting points; quality.3 plants/plantingpoint; 6 plantingpoints/row, 4 rows/rep (use inside 8planting points fordata); 4 reps;2 seasons.

On Farm Trials spacing and planting leaf scab score (average over season),pattern determined by little leaf score, virus score, rosefarmer; number of beetle score, marketable weight tubersplants and reps (weigh), marketable number tubers (count),determined by edible weight tubers (weigh), edibleavailability of number tubers (count), total weight tubersplanting material and (weigh), total number tubers (count) withland; best located in farmer deciding which tubers arethe middle of farmer’s marketable and edible, eating quality asown sweet potato judged by farmer and family, farmer’sproduction field. overall opinion of breeding clones and

farmer’s choice of which ones he/she willgrow again.

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3. Local Checks. You must alwaysinclude in your trials one or more localcheck cultivars. A local check is acultivar that is presently popular andgrown by many farmers in your location,or a cultivar that is presentlyrecommended by the Extension Division.Why is it necessary to include a localcheck? Because the purpose of your trialsis to identify breeding clones that are asgood as, or better than, the cultivars thatfarmers are already growing. Localchecks are treated like breeding clones inthat they are randomised and included inall replications.

4. Susceptible and Resistant Checks. Ifyou are evaluating your breeding clonesfor disease, insect, or nematoderesistance, you should also include inyour trials a susceptible check cultivarand a resistant check cultivar for each ofthese constraints.

A susceptible check is one that youknow is susceptible to the disease (orinsect or nematode). It helps you todetermine whether the disease (or insector nematode) is present in your trial andat what level. For leaf scab, for instance,if your susceptible check is severelyinfected with the disease, then breedingclones in the same trial that show mild orno infection are most likely resistant tothis disease, and you can select them forfurther trial. However, if your susceptiblecheck does not become infected with leafscab, then you cannot evaluate thebreeding clones for resistance, becausethe disease is not present in the trial, orthe environment is not favorable for itsspread. If all rows of the susceptiblecheck in the different replications showsymptoms of the same intensity, thenyou know that the disease is uniformlyspread in the trial.

A resistant check helps you todetermine the LEVEL of resistance ofeach breeding clone. Is it the same,higher than, or lower than the resistantcheck cultivar?

In Intermediate, Advanced, and OnFarm Trials, treat the susceptible andresistant checks the same as thebreeding clones by randomising them ineach replication. In Hill and PreliminaryTrials containing large numbers ofbreeding clones, repeat the susceptibleand resistant checks several times, aboutonce for every 10 breeding clones.

5. Spreader Rows. When evaluating forresistance, it is important to have highlevels of the disease, insect, ornematode present in your trials so thatyou can distinguish between resistantand susceptible breeding clones.Therefore, locate your trials at siteswhere the disease, insect, or nematode isnaturally epidemic.

You can also increase the amount ofdisease inoculum available to infect thetrial by planting “spreader rows”–rowsof infected plants of a susceptiblecultivar. But, be very careful to plantthese rows so that all clones in the trialare an equal distance from the spreaderrows. See Figures 4, 5, 6, and 7 forexamples. Similarly, you can increasethe number of insect pests in your trialwith spreader rows of infested plants orplant parts. For example, spreadingaround tubers infected with weevils willincrease the weevil population in yourtrial.

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Figure 4. Example of the layout of part of a Hill Trial.Note positions of guard rows and spreader rows.

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Figure 5. Example of the layout of a Preliminary Trial. Notepositions of guard rows and spreader rows. Most ofyour Preliminary Trials will be larger than this one.

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Figure 6. Example of the layout of an Intermediate Trial. Notepositions of guard rows and spreader rows. Most of yourIntermediate Trials will be larger than this one.

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Figure 7. Example of the layout of and Advance Trial. Notepositions of guard rows and spreader rows. Most of yourAdvanced Trials will be larger than this one.

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6. Guard Rows. In all fieldtrials, guard rows (often called borderrows) are essential. All data plants (plantsthat you will observe, measure, andcollect data from) must be surrounded by4 other plants in order to eliminate the“wedge effect”. Or, if you are plantingmore than one cutting at a planting point,then all data planting points must besurrounded by 4 other planting points inorder to eliminate the “wedge effect”. SeeFigures 4, 5, 6, and 7 for examples. If youdo not take the precaution of plantingguard rows, then data plants on the edgeof the plot will have more space and willbe more vigourous and higher-yieldingthan plants in a typical field situation.

Extra guard rows can also be plantedto “guard” the trial from damage bytractors, trucks, and foot traffic. For guardrows, choose cultivars that have vinetypes similar to most clones in the trial.

In Preliminary and IntermediateTrials, the first and last planting points ineach row act as guard rows, and it iseasiest to use the same clone. This meansthat you plant 6 planting points in eachrow of a clone and harvest the inside 4 fordata.

7. Labelling and Mapping. At plantingtime, one of the most importantprecautions you should take to avoiderrors is to properly label all clones and tomap nursery beds, trials, andmultiplication blocks IMMEDIATELYafter planting. Label stakes may be lostbecause a farm worker needs wood to starta cooking fire or a stake to scrape mud offhis shoes! Make a permanent map of thetrial layout and keep it in a safe place sothat clones can be accurately located evenif every label is moved or destroyed. Fororientation, include compass directionsand permanent landmarks like buildingsor large trees.

In this permanent record, also writeplanting dates, harvest dates, dates ofimportant operations like hilling-up,fertiliser and pesticide applications, dateswhen data were taken, and informationon spacing, number of plants per plantingplace and row.

8. Vine Types. When you grow clones ofdifferent vine types in adjacent rows inyour trials, clones with short, compactvines and small leaves are sometimescovered by neighbouring clones withlong, vigorous vines and large leaves,especially during the rainy season whenvines are very vigourous. Consequently,the compact types may yield poorly, notbecause they are geneticallylow-yielding, but because their leaves areburied under other vines. This is not aproblem in Advanced Trials, in which4-row plots are used, or in On-FarmTrials, in which larger plots of each cloneare used, but it is a problem in Hill,Preliminary, and Intermediate Trials, inwhich single-row plots are used.

We reduce interrow competition inHill Trials by planting breeding andcheck clones 2 m apart with ascab-susceptible spreader row planted inbetween. At mid-season, when the longervines have started to invade neighbouringrows, we pull out these spreader rows,which have already completed theirfunction of spreading leaf scab inoculum.We also give the “benefit of doubt” tosmall-vine clones that have been coveredby their neighbours and select them forfurther trials even if their yields are low.In Preliminary and Intermediate Trials,we plant 2 separate trials, one for normal,long vine types and a second for compacttypes.

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9. Selection. To save land and labour, it isessential that you discard undesirable clonesas early in the trial sequence as you canaccurately identify them. However, in the Hilland Preliminary Trials, where there are noreplications and only a few plants of eachclone, you must be careful when evaluatingcharacters like yield that are stronglyinfluenced by the environment. Keeping thiscaution in mind, you should try to identifyclones that have poor yields and discard them,especially when your decision is reinforced byother negative characters such as diseasesusceptibility or late maturity.

In these unreplicated trials, concentratemore on highly heritable characters such asskin colour, flesh colour, and broad categoriesof vine type, such as compact and normalvining types.

Concentrate also on discarding the poorestcultivars rather than on selecting the best. Forexample, you may not be able to accuratelyidentify clones that are resistant to leaf scab,but you can discard any that show severesymptoms of the disease. Clones withobviously poor yield, poor tuber shape,veining, cracking, or otherwise unacceptabletuber appearance can be discarded in theseearly trials.

10. Experimental Error. There are severalways you can reduce variability and thusexperimental error in your trials. At planting,use all vine TIP cuttings of the same length,from plants that are the same age. Sort thesecuttings into piles according to vigour andhealth and plant the “best” in replication 1, the“second best” in replication 2, the “third best”in replication 3, etc. If you do not have enoughtip cuttings to plant the entire trial, you canuse non-tip cuttings for non-data plants.

Throughout the growing season, apply alltreatments uniformly to all clones in areplication. This includes treatments such asplanting depth and style, fertiliser, pesticides,rat bait, irrigation, hilling-up, and weeding.

Carry out operations like weeding andhilling-up by replication, and if you cannotcomplete the entire trial at one time, breakafter completing a replication, not in themiddle of a replication.

Each field worker treats sweet potato alittle differently, in terms of planting,weeding, hilling-up, etc., and therefore, ifmore than one worker is applying a treatment,assign one worker to complete one replication.For example, when we plant our trials, all ofreplication 1 is planted by one worker, all ofreplication 2 by another worker, etc. You mustalso take the same precautions whencollecting data. If it begins to rain for instance,finish the replication before you stop. If morethan one worker is collecting data, eachworker must complete a replication; NEVERhave different workers evaluate differentclones within the same replication.

From each trial, you usually select10 to 50% of the clones for plant-ing in the next trial. Therefore,you must start with a large numberof seedlings.For example:

2000 seedlings1000 seedlings in HT100 clones in PT25 clones in IT - 113 clones in IT - 27 clones in AT - 15 clones in AT - 22 clones in On-Farm

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Step 10 - Name and Release the Best Clones

Step 11 - Distribute New Cultivarsto Farmers

The breeder’s job is NOT completed until thenew cultivar has been distributed to farmers.This step requires large-scale multiplication ofhigh quality planting material andcollaboration with the Extension Division,farmer groups, shools, etc.

Both vine cuttings and tuber sprouts can be used to multiply clones.

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After you have made your final selections inthe On-Farm trials, you are ready to name andofficially release the selected breeding clonesas improved cultivars. To inform the publicabout this release, you can use radio,newspaper, extension leaflets, displays atagriculture shows, and demonstration plotsplanted by the Extension Division.

MultiplicationDuring each season it is necessary to multiplyenough planting material for the next season’strials. And after a cultivar has been released,large amounts of planting material need to bemultiplied for distribution to farmers.

Cuttings taken from 2.5-month-old plantsare best for planting trials. Therefore, it is notgood to take cuttings to plant the next trialfrom a trial you are harvesting. In Tonga, weplant one multiplication plot at the same timeas the trials. Then we take cuttings from thisfirst multiplication plot to plant a secondmultiplication plot 2.5 months before theexpected planting date of our next series oftrials. In this way, the second multiplicationcan provide cuttings that are just the right agefor planting the trials.

We keep multiplication plots sprayedagainst weevils, rose beetle, and leaf scabdisease, especially if we are multiplyingplanting material for distribution to farmers.

Use both vine cuttings and tuber sproutsfor multiplication. One vine cutting willproduce about 5 vine cuttings, and 1average-size tuber will produce about 20 vinecuttings.

When using vine cuttings formultiplications, you can increase themultiplication rate by using both tips andmiddle sections of the vines, but avoid usingvine bases, because they are likely to carryweevils.

Producing vine cuttings from tubers iseasy. Tuber dormancy varies, but tubers areusually ready to plant 2 to 3 weeks afterharvest, when they are just beginning tosprout. Cut average size tubers into 2 piecesand large-size tubers into 3 pieces. Plant thesepieces in furrows if rainfall is low to averageor on mounds or ridges if rainfall is high.Cover tuber pieces with 5 to 8 cm of soil.

Most tuber pieces produce a large numberof sprouts, but these are too crowded to growproperly. Therefore, when sprouts are youngpull off the weak ones, leaving only 4 or 5strong sprouts to grow. When these sproutsare about 30 cm long, pinch off their tips toencourage branching.

To keep each clone true-to-type, alwayscarefully select the planting material used formultiplication. It is also necessary to walkyour multiplication plots regularly and pullout all “off-type” plants. These off-type plantscan occur in your clones for several reasons.“Volunteer” plants may grow from tuber andvine pieces remaining from previous crops ofsweet potato grown on the same field.Naturally set seeds may fall to the soil,germinate, and the seedling vines may bemistakenly harvested together with the parentvines. Mutations occur frequently in sweetpotato. Usually these mutations are minor,such as small patches of different skin colouron tubers, but even these minor mutations cangradually lead to a “mixed” clone if they arenot detected and removed.

A few mutants may be better than theiroriginal clones, and you could evaluate themin the breeding trials.

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Recurrent SelectionThere are 3 steps in a recurrent-selection breeding programme:

1. Create a base population byselecting parent clones andintercrossing them in as manycombinations as possible.

2. Grow, evaluate, and select theseedling-derived clones grownfrom seeds produced in the abovestep, and intercross selectedclones in as many combinationsas possible.

3. Include new germplasm in thecrossing block when available(this is called “introgression”).

In a recurrent-selection breedingprogramme, the first time you selectand intercross parent clones you arecreating what is called the basepopulation or the original sourcepopulation. It is important to beginwith a wide genetic base. To do this,choose at least 20 parent clones thatare unrelated and have a wide rangeof characters. To widen the geneticbase, or to obtain specific charactersnot available in the local germplasm,it is often necessary to import seedsor vegetative material from othercountries. However, this importationmust be done in such a way that thereis no risk of introducing newdiseases and pests. Consult yourlocal agriculture department beforeimporting any plant material.

Seeds and PATHOGEN-TESTED(virus-indexed) plantlets growing astissue cultures are safe ways to importmaterials if you follow recommendedquarantine procedures. The first step inthe importation process is to obtain anImport Permit from your QuarantineDivision and then to cooperate fullywith the requirements for phytosanitarycertificates, post-entry quarantine, etc.

How do you select seedling-derivedclones for each cycle of intercrossing?You can base your selection on thephenotype of the seedling itself.However, when seedlings are planted ina Nursery Bed, you have only one plantof each growing in a situation that differsgreatly from a farmer’s field. Therefore,it is not possible to make accuratejudgements on most characters at theseedling stage. After cloning theseedlings and evaluating theseedling-derived clones in the trialsequence, your judgement of theirperformance becomes increasinglyaccurate as you advance from SeedlingNursery through Hill Trial, PreliminaryTrial, Intermediate Trials, AdvancedTrials, and finally to Farmer Trials.

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It takes about 2 years fromCrossing Block until the harvest ofthe first Intermediate Trial.Therefore, if you plant a CrossingBlock once each year, you will infact have 2 recurrent-selectionpopulations running at the sametime.

During the first 2 or 3 cycles ofyour recurrent-selection programme,apply only light selection pressure toallow maximum recombination tooccur. For example, in the first cycleyou could select and intercross allthe seedling-derived clones that youselect from the Hill Trial for plantingin the Preliminary Trial. In thesecond and third cycles, you couldselect and intercross all theseedling-derived clones that youselect from the Preliminary Trial forplanting in the first IntermediateTrial.

After completing these earlyrecombination cycles, you shouldincrease the selection pressure. Wedo this by selecting and intercrossingall seedling-derived clones that wehave selected from the firstIntermediate Trial for advancementto the second Intermediate Trial (seeFigure 1). We may also include inthe crossing block a few clones fromearlier trials that have goodcharacters but are not good enoughoverall to be selected for furthertrial–for example, a clone that has avery high level of scab resistance buthas poor tuber shape, or a clone withexceptionally high tuber number butwith small tubers. In these laterrecombination cycles, use 30 or moreparent clones.

Newly introduced germplasmcan be used in the base population orintrogressed (crossed) at any latertime.

With each cycle of recurrentselection your breeding populationwill improve. This means that thefrequency of desirable genes willincrease, and your chances of findingclones with all the characters youwant will increase.

If you have a clone with one or afew good characters but manyundesirable characters, you canintrogress it into your improvedpopulation without adding the genesfor the undesirable characters to yourimproved population. Plant the cloneto be introgressed in a border row ofyour crossing block. Use it as afemale, but prevent its pollen fromcontaminating your improvedpopulation. To do this, you caneither emasculate all flowers beforethey open and then permit them toopen-pollinate, or you can clip allflowers closed and use them asfemales in hand pollinations.Remove any buds you do not plan touse as females before they open.Evaluate the progenies from thesecrosses in the trial sequence andrepeat the procedure with theselections until they are improvedenough for most characters to beincluded as normal parents in therecurrent-selection crossing block.

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You can also base your selection on thebreeding value of a seedling-derivedclone, as judged by the performance ofits progeny (progeny testing). This isoften a better criterion than thephenotype of the seedling-derived clone.However, progeny testing adds anumber of lenghty steps to the breedingprogramme and for this reason isusually not used in sweet potatobreeding programs.

GeneticsThe sweet potato is a polyploid with 6 setsof each chromosome (that is, a hexaploid),giving a total of 90 chromosomes. It isalso heterozygous. Consequently thegenetics of this crop are complicated, andsegregation of characters always occurs inthe offspring (progeny). Sweet potato hasboth qualitative and quantitativecharacters. Qualitative characters aredistinct and discontinuous and are easilydistinguished from each other. Forexample, stem colour is a qualitativecharacter. Generally only 1 or 2 sets ofgenes control each qualitative character.

In contrast, quantitative characters areindistinct and continuous and graduallygrade into each other, and usually manysets of genes are involved. For example,the shape and yield of tubers arequantitative characters.

For some characters, improvementthrough selection is rapid, but for othercharacters it is slow. This is determined bythe heritability of the character. If acharacter has high heritability, the progenywill resemble the parents, andimprovement through selection will berapid.

Other characters have lowerheritabilities such as yield, fibre content,tuber shape, tuber cracking, and number oftubers.

If a character has low heritability,improvement through selection will oftenbe slow. It is sometimes possible toincrease low heritabilities by increasingthe number of replications in trials andusing better screening techniques, such asartificial inocultation. Also, whenheritabilities are low, it may be better tobase your selection on progeny testingrather than on the parents.

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It has been determined that fleshcolour, flesh oxidation, % dryweight, % crude starch, % crudeprotein, skin colour, resistanceto root-knot nematode, and vinelength all have relatively highheritabilities.

Evaluating Specific Characters

Stem ThicknessYou can distinguish between THICK,MEDIUM, and THIN stems. Vines withthick stems produce strong cuttings,which establish well at planting. Incomparison, thin-stem vines produceweak cuttings, which often die soon afterplanting. However, we have found thatweevils damage thick stems moreseverely than thin stems. Therefore, weselect clones with medium stems, sincethese establish well enough in the fieldbut are not as susceptible to weevildamage as thick stems.

Our data suggest that stem thicknessis also maternally inherited. Therefore,when making crosses, you should use theclone with the best stem thickness as thefemale parent.

Time from Planting to HarvestTo decide when to harvest your on-stationtrials, consider the needs of your farmers.Most commercial farmers prefer earlycultivars that give high yields 4 or 5months after planting. For manysubsistence farmers, however, an earlyharvest is not as important as being ableto harvest over a long period of time. Foron-farm trials, the farmer should makethe decision about when to harvest.

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Plant TypeVariability in plant type includes longvines compared to compact vines, fewbranches compared to many branches,vines that produce all tubers at theplanting point compared to those thatproduce tubers both at the plantingpoint and on the vines, and climbingvines compared to nonclimbing.

The ideal plant type or types foryour location will depend on the needsof your farmers. Compact vines areoften best for intercropping. Long,vigorous, many branched vines oftencompete best with weeds. Plants withmany branches produce more plantingmaterial.

Subsistence farmers in somecountries prefer vines that can beharvested over many months, becausethey produce tubers both at the plantingpoint and on the vines. In contrast,commercial farmers usually prefervines that produce tubers only at theplanting point so they can be harvestedall at one time. Most farmers do notlike climbing vines, because they arevery thin and make poor plantingmaterial.

Stem colour and leaf shape areusually not important to farmers.

OUR DATA SUGGEST THATPLANT TYPE IS MATERNALLYINHERITED. This means that many ofthe offspring of a cross will have aplant type similar to the female parent.Therefore, when making crosses, usethe clone with the best plant type as thefemale parent.

Disease, Insect, and NematodeResistanceIt is important that you develop areliable rating scheme that candistinguish resistant from susceptibleclones. Your rating scheme shouldhave at least 3 classes, but it must notbe so complex that it is difficult touse on a large number of clones inthe field.

For evaluating leaf scab diseasein the field, we use a scale of 0 to 4,with 0 meaning no plants in the cloneshowing symptoms of the disease,and 4 meaning all plants in the cloneshowing severe symptoms, and 1, 2and 3 are of intermediate severity.

For little leaf disease and thevarious virus diseases, we have notyet found good sources of resistance,but we always discard those clonesthat have obvious symptoms of thesediseases.

For evaluating weevils, we use ascale of HIGH, MEDIUM, andLOW, with High meaning that a highpercentage of the tubers (more than50%) are damaged by weevils andLow meaning that less than 10% ofthe tubers are weevil infested. Tuberplacement affects weevil resistance,and therefore we prefer clones thathave deep tubers and long “necks”attaching them to the stem base.

Most of our breeding clones, aswell as all Tongan local cultivars, aresusceptible to rose beetle, but we donote which breeding clones are lesssusceptible or more susceptible thanaverage.

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In Tonga we developed a method ofartifical inoculation to evaluateseedlings for leaf scab resistance.Pieces of disease stems and petioles areincubated in petri dishes to produceinoculum (above). Seedlings are sprayedwith inoculum and then placed in plasticbags to provide the high humiditynecessary for disease development(below).

Yield on the vine

Yieldunder theplantingpoint

YieldTOTAL YIELD of tubers is made up ofEDIBLE YIELD plus REJECTS(nonedible including weevil damaged,rotten, and very small tubers). Edibleyield is made up of tubers that are goodenough to be sold (MARKETABLEYIELD) and tubers that can not bemarketed but would be eaten by farmfamilies. Which yields should youmeasure? If you are breeding newcultivars for subsistence farmers, edibleyield is the most important. If you arebreeding for commercial farmers, thenmarketable yield is the most important tomeasure. In Tonga, many farmers growsweet potato to feed their families, butthey also sell the extra tubers on themarket. Therefore, we measure bothedible and marketable yields. Total yieldis usually not a very relevantmeasurement.

The yield of tubers harvested fromunder the planting point is easier tomeasure than the yield of tubers thatgrow on the vines. But if vine tubers areimportant to your farmers, then you mustmake the effort to measure this yield.

For all farmers, ability to producehigh yields in the wet season isparticularly important.

In Tonga our goal is to select clonesthat produce excellent yields in the dryseason and at least reasonable yields inthe wet season. This is why theIntermediate, Advanced and On-FarmTrials are each grown in a wet as well as adry season.

Tuber yield is determined by acombination of TUBER NUMBER andTUBER WEIGHT. Clones that producelarge numbers of medium-sized tubersand clones that produce medium numbersof large tubers may have the same yieldsbut may not be equally acceptable tofarmers. Farmers in the Pacific Islandsoften prefer large tubers, even if the yieldis somewhat lower.

In Tonga, sweet potato plays animportant role in feasting andpresentations and extra large tubers areneeded for these ceremonial occasions.This may also be true in your location.We therefore note during harvest thosebreeding clones that can be used only foreating and marketing and those thatproduce enough extra-large tubers forceremonial use. It is not necessary for allnew cultivars released from a breedingprogramme to be appropriate forceremonial use, but at least a few shouldbe.

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Tuber CharactersIs there a preference for a certain tubershape in your location? If there is, thenyou should select breeding clones withthat shape. We classify tuber shape asROUND, ELONGATEROUND (oval),and ELONGATE.

Smoothness indicates how easy ordifficult it is to wash and peel the tubers.We use the categories SMOOTH,MEDIUM, and ROUGH.

Cracked tubers are unattractive anddifficult to wash and peel. One type ofcracking is also a symptom of root-knotnematode attack. Therefore, you shoulddiscard breeding clones that have a highpercentage of cracked tubers at harvest.For cracking, we use the ratings MANY,FEW, NONE.

Another undesirable character isprecocious tuber sprouting, whichmeans that tubers sprout before harvesttime. We rate precocious sprouting asABSENT or PRESENT.

Farmers and consumers may have astrong preference for certain tuber skinand flesh colours, or they may acceptmany different colours. Be certain thatyou understand the situation in yourlocation before you begin yourbreeding programme.

Note that tubers with orange andbright yellow flesh are higher invitamin A than tubers with other fleshcolours, but they are also sweet tastingand moist, with low percent dry weight.Therefore, they are often not preferredby Pacific Islanders.

In breeding populatins, tuber characters are very variable. The tubers on the left are undesirable becausethey have poor shapes, many cracks, and/or are rough. In contrast, the tubers on the right have goodshapes, few or no cracks, and are smooth.

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Tubers that are damaged by rats orweevils will often sprout. There-fore, evaluate precocious sproutingonly on undamaged tubers.

Eating QualityGenerally, Pacific Islanders prefer hard,dry, low-sugar sweet potatoes.Therefore, percent dry weight is a goodmeasure of eating quality. In Tonga forexample, cultivars having dry weightsof 30% or higher are preferred to thosewith dry weights of 25 to 28%.

We measure percent dry weight ofall clones in Advanced Trials. To dothis, we select 5 marketable tubers ofeach cultivar from each replication. Wetreat each tuber as a separate sample.We scrub it in water with coconut huskor an other abrasive material, cut off theends as for cooking, chop the rest of thetuber into cubes 1 cm square or smaller,and place it into a paper bag or smallaluminum foil baking pan. We thenweigh each sample to determine itsfresh weight, dry it in a laboratory ovenat 100oC until it has reached constantweight (that is, when further dryingdoes not lower the weight), and finallyweigh it again to determine dry weight.

You calculate percent dryweight as:

dry weight x 100% dry weight = fresh weight

Often there is not enough ovenspace and labour to determine percentdry weight of the large number ofbreeding clones in Intermediate,Preliminary, and Hill Trials. When thisoccurs, specific gravity can be used toestimate dry weight and eating quality

We estimate specific gravity bydetermining whether a tuber sinks orfloats in a solution of common tablesalt dissolved in water. ForIntermediate Trials we select 5marketable tubers of each clone fromeach replication. For the unreplicatedPreliminary and Hill Trials we select 5marketable tubers of each clone that hasperformed well for all other characters.

To estimate specific gravity, makeup 1 bucket for each specific gravityrating by adding a specific amount ofsalt to 6 litres of water (see Table 2).Treat each tuber as a separate sample.Wash it in water. (Unlike dry weights,it is not necessary to scrub off the skin.)Then place it in Bucket 0. If it sinks,move it to Bucket 0.5, then Bucket 1.0,then Bucket 2.0, etc, until the tuberfloats to the surface of the salt solution.Give the sample the specific gravityrating of the bucket in which it firstfloats, and then average the ratings ofthe 5 tubers to obtain a mean rating forthe replication (Intermediate Trials) orclone (Preliminary and Hill Trials).

Clones that Tongans usuallyconsider “excellent eating quality” havespecific gravity ratings of 3 or higher,and clones they consider “poor eatingquality” have specific gravity ratings of1 or lower.

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Do not forget to “tare” (deduct) theweight of the paper bag or bakingpan when you weigh fresh and drysamples. And when the relativehumidity is high, you must weigh drysamples IMMEDIATELY afterremoving them from the oven becausethey quickly absorb moisture fromthe air and gain weight.

Table 2. How to prepare a series of salt solutions to estimate the specific gravityof sweet potato tubers.

Specific Amount of salt (grams) ActualGravity to dissolve in 6 liters Specificrating of water Gravity

0 0 1.0000

0.5 150 1.0173

1 300 1.0347

2 600 1.0680

3 900 1.1002

4 1200 1.1316

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To save time when measuringspecific gravity of large numbers ofclones in Hill Trials, we use onlyone bucket of salt solution, with thelowest acceptable specific gravityrating for that season, usuallyBucket 1. If 3 or more of the 5tubers from a clone float in this saltsolution, we discard the clone.

Of course, the best method todetermine the eating quality ofbreeding clones is to evaluate them intaste tests. However, taste tests aretime consuming and only a few clonescan be accurately “tasted” at any onetime. Therefore, taste tests mustusually be delayed until Advanced andOn-Farm Trials.

There are several methods ofconducting taste tests, some of themvery complicated. However, we havefound that a relatively simple test isaccurate enough. From eachreplication of Advanced Trials, weselect 2 average tubers of eachbreeding clone and each checkcultivar. We cook these in the “umu”,since this is the most common methodof cooking sweet potato in Tonga.Prepare your tubers using the mostcommon cooking method in yourlocation. We then peel the cookedtubers, cut them into small pieces, andput each clone on a separate plate thathas been labelled with a code number(not the actual number that sometasters might recognize). We invite atleast 20 Tongans to taste severalpieces from each plate and fill out anevaluation form. On this form we askthem to rate each clone as 4 (verygood, I like it very much), 3 (good), 2(OK), or 1 (poor, I do not like it). Wealso ask them to write remarks abouteach one, explaining why they do ordo not like it.

Of course, taste is not the onlyfactor that determines the ratingsthat our tasters give to a sample.They are also influenced by howthe sample looks (colour,crumbling), its texture, and thepresence of fibres. All these factorsare important in determining eatingquality.

Nutritional ValueProtein content, protein-inhibitor content, andvitamin content are very important nutritionalfactors that should be evaluated in your sweetpotato breeding programme. However, mostbreeding programmes cannot afford theexpensive equipment and highly trainedtechnicians required to measure thesenutritional factors. If possible, you shouldestablish a collaborative project with anoverseas institution where you can sendsamples of clones from Advanced Trials foranalysis.

Vitamin A is the only nutritional factorthat is easy to determine. It is related to fleshcolour, and clones with orange and brightyellow flesh have high Vitamin A content.

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Be sure that you use “heat-proof” labels to identify differentclones during cooking. Metallabels are best, and somemarking pens will last throughthe “umu”. For boiling, you cancut the tubers into differentshapes for identification.

Tongan taste-testers make negativeremarks like “too sweet”, “soft”,“stringy”, “sour” and positive remarkslike “good taste”, “firm”, “attractivecolour”.

SUPPLIES

Pollinating Clips778C O.K. Fasteners, 10 mm, 13/32in., No. 1-B, 100 in a box; fromLabelon Corporation, Canandaigua,New York 14424, USA.

Merchandise Tags for labelingpollinationsStandard quality, white, 1-15/16 x1-1/4 in., No. 6, No. 12-203, 1000 ina box; from Denney-ReyburnCompany, West Chester,Pennsylvania 19380, USA.

Whit e Plastic Labels formarking basketsDiamond-Lox, white, 5-1/2 x 1/2 in.,1000 in a box; from A.M. Leonard,Inc., P.O. Box 816, Piqua, Ohio45356-0816, USA.

Flagging Ribbon for markingFlagging, polyethelene or vinyl, 1-3/16 in. wide, many colours, 100 yds/roll; from Forestry Suppliers, Inc.,205 West Rankin St., P.O. Box 8397Jackson Mississippi 39204-0397,USA.

Aluminum Labels for markingfield stakesAl Tags, 3/4 x 3 in., 9 in. wire, canbe permanently embossed with penor pencil, 1000 in a box; fromForestry Suppliers, Inc., 205 WestRankin St., P.O. Box 8397 JacksonMississippi 39204-0397, USA.

Field Record BooksLietz Level Book, No. 8152-55, 4-1/2 x 7-1/4 in., water repellent; fromForestry Suppliers, Inc. 205 WestRankin St., P.O. Box 8397 JacksonMississippi 39204-0397, USA.

Plastic Net Bags for cuttings andsmall lots of tubersSold for packaging onions,potatoes, etc., available in rolls oflong tubes that can be cut to anylength, and in many bright colours.

Nursery Marking PensPermanent, sunproof, waterproof,fine point, Number UH0960; fromThe John Henry Company, 5800W. Grand River, P.O. Box 17099,Lansing, Michigan 48901-7099,USA.

a - O.K. Fastenersb - Merchandise Tagsc - Flagging Ribbond - Al Tagse - Diamond-Lox Labels

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