Selecting for Resistance to Plant Disease: Part I

Martha Rosemeyer

Organic Seed

May 20 2004

Outline

Importance of disease resistanceIdentifying symptoms of disease (those

plants that are not resistant!)Types of pathogensKoch’s postulatesThe disease triangleImplications for selection

Most plants are resistant to most pathogens Most plants grow relatively unhindered by

diseases Why?

One of most powerful methods for organic farmer, and coming back in importance

What is a disease?

“A harmful deviation from normal functioning of physiological processes”

“Dis-Ease” Plant diseases cause loss of 12% of food

crop worldwide (Science 257: 482, 1992)

What is a plant pathogen?

Any organism which can cause plant disease

What are the mechanisms of resistance to disease?

Structural- pathogen cannot enter- sticky or tough

Chemical- pathogen enters but is hindered by “plant secondary chemicals” of plant used in plant defense can inhibit insect growth, e.g. tannins are protein-

binding Organic foods higher in antioxidants because

insects bite and induce formation

Mechanisms of resistance to plant disease cont.General systemic PR (pathogenesis-related)

proteins act like immune system Example- pathogen cell wall degrading

Specific antibody-like resistance genes (vertical) Genetic engineering moving them between species Gene cloned “osmotin” that inhibit fungal

reproduction by bursting spore cases of Phytopthora infestans

Other resistance mechanisms

Hypersensitive reaction- immediate death of tissue (necrosis) around the point of entry of the pathogen

Systemic induced resistance (SIR) heightened immunity to plant pathogens

Usually mechanism of resistance is not apparent

However we can still select plants that are resistant

Let’s now turn to susceptibility, so we know which plants to eliminate in our screening for disease resistance (whether vertical or horizontal)

In selection the observation of symptoms is key!

Many plant problems caused by non-pathogens in PNW, like unfavorable weather and poor growing conditions or herbicide

Only 1/3 caused by insect pest or pathogen (WSU extension bull. 1247)

Non-pathogens may be identified by regular pattern, more than one crop affected

General steps to diagnosisExamine leaf or affected plant part and find the time

course of symptoms Is it environmental condition, nutritional problem, pest or

disease?

Examine pattern of symptoms in plot- random or regularFind appropriate resources

Insect references or on-line Web databases Host Disease Index; Web: Cornell site

Take to plant disease diagnosticianPerform Koch’s postulates

General bacterial vs. fungal leaf symptoms

Bacterial, usually Fungal, usually

Mildew vs. Virus or nutrient deficiency

Non-pathogenic conditions

Chemical spray injury Salt or chemical injury

Nutrient symptoms can also be confused

Insect damage can cause disease like symptoms

Look for the insectCan cause chlorosis, leaf curl, distorted growth

Blue aphid symptoms on alfalfa

Red currant in OR

Aphid damage can manifest in various symptoms AND most importantly they can vector virus

Sooty mold

Fungus complex living on aphid“honeydew” (excrement)

Other: Walnut Wilt of Tomato (allelopathy caused by juglone)

MacNab et al. 1983

Lightning damage

MacNab et al. 1983

Types of plant pathogens and their symptoms

Viruses (NA plus protein shell)Bacteria (single cell, no nucleus)Fungi (most multicellular, have a nucleus)Nematodes (multicellular organisms)

Most organisms do not cause disease-- less than 0.5%

VirusesParry 1990

MacNab et al. 1983 MacNab et al. 1983

Cucumber mosaic virus

Wide host rangeSymptoms:

chlorosis distortion of leaf

Tulip Breaking Virus

•First plant disease recorded in 1576 byCarolus Clusius in Netherlands •Realized viral nature in 1900s•Potyvirus- forms filaments strands of protein coat with nucleic material inside

Bacteria

Bacterial Spot of Tomato Xanthomonas vesicatoria

MacNab et al. 1983

Angular leaf spot of strawberryXanthamonas frageriae

Symptoms: watersoaked lesions leading to necrosis and chlorosisAvoid contaminated plant material when planting (crowns)

Fungi

MacNab et al. 1983

Late blight of tomato and potato

Late Blight of Tomato and Potato

MacNab et al. 1983

Check disease cycle to know when crop may be susceptible so select at correct time

Schumann 1991

Disease started plant pathology as a discipline

Potato famine of 1846 in Ireland (8 million population) caused by Late Blight of Potato, pathogen is fungus Phytopthora infestans

1 million people die, 1.5 million emigrate to US or Canada

Observations of these organisms were previously seen as result not cause of the disease. Accepted Pasteur’s “germ theory of disease” originally proposed in 1863.

Result of the Irish Potato Famine

30% of ppn died or emigratedSchumann 1991

How do you know any pathogen causes your disease?

Koch’s postulates German 1843-1910

Grew microrganisms on potato slices and then gelatin

Student was Petri

Isolated cholera and anthrax

Koch’s postulates first demonstrated with anthrax

Anthrax disease of herbivores caused by the bacteria, Bacillus anthracis

Wade, N. New York Times. October 23, 2001

Koch’s postulatesThe disease must always be associated with the

pathogenThe pathogen is isolated from the diseased tissue

in vitroThe pathogen is inoculated onto susceptible host

with resulting symptoms of disease as was originally found

The pathogen is re-isolated from the host tissue and found to be the same

Control of Late Blight of PotatoMajor problem for organic farmers

yields are about 30% lower and tuber size small

Major control is resistant varieties problem is keeping up with the pathogen

Irrigate using drip, hill soil, kill foliage before harvest

Fungicides Org: Copper sulfate and lime (Bordeaux) Conv: copper, Maneb, Mancozeb, et al.

Sclerotinia Rot of Tomatofungus Sclerotinia sclerotiorum

Often disease of Sunflower and Pepper in PNW

MacNab et al. 1983Symptoms vs. signs (the visible organism)

Clubroot of Cabbage(Slime mold)Plasmodiophora brassicae

Serious disease of crucifersworld wide

First see wiltLongevity in soil

Liming may reduce or completely control the disease. The pH of the soil should be raised to 7.2 or higher with hydrated lime

Resistance in cabbage ‘Badger Shipper’ and turnips, radish and rutabaga

Tulip Fire or Tulip blight Botrytis tulipae (Fungus)

Attacks all parts of plantBy far most common and

serious diseaseCan cause complete lossControl- remove diseased

plant and bulbRotate- 2 years minimum

Blue mold of Tulip, Penicillium sp.

Grows especially on damaged bulbsAvoid damaging bulbs

Blue Mold of Onion and GarlicPenicillium sp. (Fungus)

Appears during harvesting and storage

Watersoaked lesions to green/blue powdery mold

Dry immediately store at 41°F with low relative humidity

Blue mold of onion

Early Blight of Potato and TomatoAlternaria solani (Fungus)

Early in seasonSame disease affects

tomato, potato, eggplant, pepper

Look for target like spots

Can infect tuber when lifted out of the ground through infested ground

Can affect crown of stemControl: Minimize leaf

wetnessRotate 3-4 yearsRemove infected materialResistance

Tomato Varieties Resistant to Early Blight

Mountain

Mountain Fresh

Plum Dandy

•Supreme

•Big Beef

•Floramerica •Juliet

Potato Varieties Less Susceptible to Early Blight

Kennebec

Root Knot Nematodes(there are other pathogenic ones as well)technically a parasite

Root knot nematode of bean

Dindal’s, Foodweb of the Compost PileCardona et al. 1982

Nodules vs. Gall?

Just because the pathogen is there will disease be present?

A susceptible host plantA pathogen capable of

causing diseaseEnvironmental

conditions Need all 3! So if your

plant does not appear susceptible, do you know if it is or not?

If disease is present you can select it out of your breeding population

If no disease present you don’t know if you don’t have the right environment for expression of disease or the pathogen is not present

Summary

Know your germplasm and know your potential plant disease, pest and abiotic problems, so you can select appropriately

ResourcesAmerican Phytopathological Society series for specific crop

plants.DeAngelis J., et al. 2000. 2000 Insect Management

Handbook for the PNW. OSUPscheidt, J. and C. M. Ocamb. 2000 Plant Disease

Management Handbook.OSUhttp://www.ippc.orst.edu/cicp/Vegetable/veg.htmhttp://plantclinic.cornell.edu/FactSheets/http://www.ipm.ucdavis.edu/PMG/

http://vegetablemdonline.ppath.cornell.edu/

ResourcesMacNab AA, AF Sherf and JK Springer. 1983. Identifying Diseases of Vegetables. Penn. State Press. and the newer Sherf and MacNab. Vegetable Diseases and their Control

Englehard, AW (Ed.) 1989. Management of Diseases with Macro- and Microelements. APS. Huber, Take-all and nutrition, pp 46-74

Agrios, G. Plant Pathology. Several EditionsHoward (Ed.). Diseases and Pests of Vegetable Crops in CanadaBio-Integral Resource Center (BIRC). Berkeley CAKF Baker and RJ Cook. 1982. Biological Control of Plant

Pathogens. APS

Brassica flower