Citrus Root Diseases

Citrus Pathology - Fall 2019

Evan JohnsonPlant PathologistUF-IFAS-CREC

Role of Roots

• Anchor tree– Structural roots

• Uptake of water and nutrients– Fibrous or feeder roots

Structural

Fibrous

Anchoring tree

• Wind

Courtesy of Mongi Zekri

Courtesy of A. Singerman

Fibrous root system

• Gathers water and nutrients

• Short-lived and mines the soil

Pathogens can compromise root health

• Fibrous roots– Limits water uptake capacity

• Structural roots– Poorly anchored– Reduces fibrous roots

Root damage directly or indirectly affects

• Fruit size

• Yield

• Post harvest quality

• Tree survival

Key pathogens, pests, and their interactions affecting citrus roots

• Phytophthora diseases– Phytophthora spp.

• Diaprepes root weevil (DRW)– Diaprepes abbreviates– Diaprepes/Phytopthora complex

• Huanglongbing (HLB)– Candidatus Liberibacter spp.

Phytophthora causes disease in all parts of the tree

• Root Rot

• Foot Rot

• Brown rot of fruit

• Yield reduction estimate = 3-6% per yr

Oomycetes and Fungi are Not Closely Related

Stramenopila/Oomycota

(“Oomycetes”)

Fungi

Differences Between Oomycota & EumycotaFeature Oomycete True Fungi

Neighboring taxonomic groups Diatoms and golden brown algae Animals

Hyphal architecture Aseptate tubular hyphae Either single cell or septated hyphaeOne or more nuclei per compartment

Vegetative hyphae ploidy Diploid except in cells leading to gametes (haploid)

Typically haploid or dikaryotic; often with stable or semi-stable diploid stage post-mating

Typical genome size (Mb) 50-250 10-40

Major components of cell walls Cellulose and glucose polymers Chitin and/or chitosan and glucans

Asexual spore types Undesiccated, unicellular sporangia (multinucleated cells)

Desiccated single or multicellular conidia (one nucleous/cell)

Motile asexual spores Nearly universal biflagellated Rare, only chytrids, and monoflagellated

Sexual spores Oospores on the end of specialized hyphae with one zygotic nucleous

Varied types, often occur in large numbers within complex structures

Modified from Howard S. Judelson & Flavio A. BlancoNature Reviews Microbiology 3:47-58

Causal species of Phytophthora diseases

• Phytophthora nicotianae(parasitica) – common cause of foot rot and root

rot

• Phytophthora palmivora– causes brown rot of fruit – root rot in poorly drained soils

• Phytophthora citrophthora– Causes gummosis in Mediterranean

climates

P. palmivoraSporangia elongated

P. nicotianaeSporangia round

Life cycle of Phytophthora

Phytophthora infection cycle

• During favorable (warm, wet) conditions, zoospores release from sporangia– Swim or splash to root, bark, or

fruit

• Spores encyst, germinate, and infect tissues within 24 hr

Phytophthora root rot

• Damages fibrous (feeder roots)

Healthy Damaged

(Fibrous) Root Rot

• Preferentially infects young growing root tips

• Root cortex soft and water soaked

• Cortex and epidermis slough leaving thread-like tips

• Water and nutrient uptake impaired

• Stored carbohydrates depleted

Young tree canopy symptom of root rot is ‘Yellow vein chlorosis’

• Yellow veins in fall/winter foliage

• Poor growth and dieback of shoots

• Can resemble HLB

Root rot decline is difficult to diagnose in mature trees

• Reduced fruit size and/or number

• Loss of leaves• Twig dieback• Decline in yield is slow but

annual• Confounded with HLB

Foot rot

• Infects bark of roots and trunk

• Kills cambium layer

• Blocks carbohydrate movement to the roots

• Initial canopy symptoms similar to root rot ‘yellow vein chlorosis

Foot rot symptoms

• Bark cracks– Remains firm

• Water soluble gum exuded• Lesions spread around the trunk

or crown below the soil line• Lesions may heal

Terminal symptoms of foot rot

• Leaf drop

• Branch dieback

• Fruit drop

• Death of tree

Girdling of the trunk results in tree collapse

Brown rot of early season fruit

• Light brown leathery decay• White fungal growth on surface

under humid conditions• Infected fruit have sharp,

pungent odor• Infection spreads post-harvest

Hamlin

Grapefruit

Brown Rot affects early season varieties

• Rains coincident with color break• Most fruit loss in older, larger

trees• Fruit cannot be harvested until

infected fruit fall– Post harvest spread

• Hamlin most affected• Navel and grapefruit (shows up in

the packinghouse box)

Periodic rain and prolonged leaf wetness = Brown rot

Date

Rainf

all (i

nche

s)

0

1

2

3

4

June July Aug Sept Nov Dec JanOct

12Brown rot recorded in 1997

8/4/97

10/3/

97

11/12

/97

11/25

/97

12/12

/97

12/29

/976 13 20 27 4 11 18 25 1 8 15 22 29 5 12 19 26 3 10 17 24 7 14 21 28 5 12 19 26 2 9 16 23 30

Date

Leaf

wet

ness

(hou

rs)

0

4

8

12

16

20

24

June July Aug Sept Nov Dec JanOct6 13 20 27 4 11 18 25 1 8 15 22 29 5 12 19 26 3 10 17 24 7 14 21 28 5 12 19 26 2 9 16 23 30

Brown rot control with cultural and fungicide management

• Avoid premature drop of fruit to reduce inoculum– Not possible with HLB

• Raise tree skirts– Increase air movement and drying– Reduce contact with inoculum

• Apply preventative phosphite or copper sprays

• Phosphite sprays in July– October if rainy season is prolonged

Complete Disease Cycle

• Roots most susceptible during very wet to very dry cycles

• Wetting and drying increase root exudation that attracts zoospores

• HLB-affected roots also exude more sucrose that promotes infection

Root and bark age and activity: periods of susceptibility

• Young roots > old• Root flushes more susceptible• Foot rot risk highest in spring and

fall– Carbohydrates moving in trunk

• Fall roots store carbohydrates for next spring flowering

• Phytophthora infection depends on carbohydrate concentration in roots

Soil temperature, rain and root flushes

• Phytophthora is dormant in winter– Soil temp <59oF

• Spring root flush – After shoot flush/fruit set with soil

temp >68oF and rains start• Fall root flush after last summer

shoot flush• Soil populations increase on root

flushes

Root Flushes and Seasonal Phytophthora Activity

Month

Soil

prop

agul

es (p

rop/

cm3 )

0

10

20

30

40

50

60

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Spring Shoot Flush Shoot Flush Shoot Flush

Flowering/Fruit Set

Root Flush Fall Root FlushRoot Flush

Flooding damage to roots predisposes them to infection

• Occurs in soils with restricted drainage

• Over irrigation• High rainfall in summer• Tropical storms and hurricanes

Prolonged O2 deprivation kills roots

• Root regeneration depends on proper aeration

• Disfavors Phytophthora attack of new roots

• Monitor water table to maintain drainage– Allow root regeneration– Reduce infection

Field monitoring of Phytophthora spp.

• Soil populations used to predict damage and need for management

• Soil sampling is most important step

• Commercial labs available• P. palmivora can be distinguished

from P. nicotianae on selective medium

Sample collection• Sample sites at random in the

problem area – Worst trees may have too few roots to

support large population• Collect root/soil cores halfway

between trunk and dripline• 20-40 samples/10 acres• Composite and store in resealable

bag– cool not cold

• Ship to lab within 24-48 hr

Interpretation of population counts

• Vary seasonally and annually– Resample each year

• Thresholds to predict damage are imprecise• Populations of < 10 propagules/cm3 are considered non-

damaging• 10-20 propagules/cm3 depends on soil type, irrigation method,

rootstock, etc.

Phytophthora prevention

• Avoid highly susceptible rootstocks– Plant graft union 6-9 inches above soil

• Adequate drainage and proper irrigation are essential

• Keep area around trunk clear of weeds and avoid wounding

• Remove trunk wraps early in spring and treat for fire ants

• Fire ants feed on and damage bark

Tolerance/Resistance of Swingle citrumelo (SC) P. nicotianae compared to Cleopatra mandarin (CM)

(mg/

cm3 s

oil)

UndisturbedDisturbed

01234

Prop

agul

es (c

fu/c

m3 s

oil)

Roo

t mas

s de

nsity

0102030

Rainfall (cm)

CM

SC

Apr Jun Oct DecAug

Month

Apr Jun Oct DecAug

Month

Disturbed Undisturbed

0

20

40

600

20

40

60SC

CM

01234

Susceptibility of scions to Foot Rot

• Grapefruit - Very susceptible• Lemons - Very susceptible• Limes - Susceptible• Mandarins, tangerines, other hybrids - Very susceptible• Sweet oranges - Very susceptible Because all scions are susceptible they are grown on Phytopththora ‘tolerant’ or ‘resistant’ rootstocks

Rootstock susceptibility - Foot rot• Very Susceptible• Tolerant to susceptible• Tolerant to susceptible• Tolerant• Tolerant• Moderately resistant• Resistant• Resistant

• Sweet orange• Cleopatra mandarin• Rough lemon• Sour orange• Volkamer lemon• Carrizo citrange (hybrid)• Swingle citrumelo (hybrid)• Trifoliate orange

Rootstock susceptibility to root rot depends on Phytophthora spp.

P. nicotianae P. palmivora

Sweet orange Susceptible ?

Cleopatra Susceptible Tolerant

Rough lemon Susceptible ?

Sour orange Susceptible Tolerant

Volkamer lemon Tolerant Tolerant

Carrizo citrange Tolerant Susceptible

Swingle Moderately Resistant Susceptible

Trifoliate orange Resistant Susceptible

Cleopatra mandarin rootstock trees infected in the nursery showing damage that started belowground

Cleopatra mandarin rootstock trees showing damage that started on the sweet orange due to burial of the budunion at planting

Fungicide use in young trees

• Warranted only if cultural practices fail

• Treatment decision based on:- Rootstock susceptibility- Likelihood of nursery

infection- History of site, e.g. Diaprepes

Management of root health in mature groves

• Most commonly used rootstocks are susceptible

• Proper irrigation scheduling and soil drainage are essential

• In high water table areas -installation of tile drains and ditch maintenance

5 minute Break

Phytophthora/Diaprepes ComplexHLB of citrus roots

After the break