Citrus Root Diseases. Citrus Pathology Fall Evan Johnson Plant Pathologist UF-IFAS-CREC

Transcription

1 Citrus Root Diseases Citrus Pathology Fall 2017 Evan Johnson Plant Pathologist UF-IFAS-CREC

2 Role of Roots Anchor tree Structural roots Structural Uptake of water and nutrients Fibrous or feeder roots Fibrous

3 Anchoring tree Wind Courtesy of A. Singerman Courtesy of Mongi Zekri

4 Gathers water and nutrients Fibrous root system Short lived and mines the soil

5 Pathogens can compromise root health Fibrous roots Limits water uptake capacity Structural roots Poorly anchored Reduces fibrous roots

6 Root damage directly or indirectly affects Fruit size Yield Post harvest quality Tree survival

7 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.

8 Phytophthora causes disease in all parts of the tree Root Rot Foot Rot Brown rot of fruit Yield reduction estimate = 3 6% per yr

9 Oomycetes and Fungi are Not Closely Related Fungi Stramenopila/ Oomycota ( Oomycetes )

10 Differences Between Oomycota & Eumycota Feature Oomycete True Fungi Neighboring taxonomic groups Diatoms and golden brown algae Animals Hyphal architecture Aseptate tubular hyphae Either single cell or septated hyphae One 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) 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. Blanco Nature Reviews Microbiology 3:47-58

11 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. palmivora Sporangia elongated P. nicotianae Sporangia round

12 Life cycle of Phytophthora

13 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

14 Damages fibrous (feeder roots) Phytophthora root rot Healthy Damaged

15 (Fibrous) Root Rot Preferentially infects young growing root tips Root cortex soft and water soaked Cortex sloughs leaving with thread like tips Water and nutrient uptake impaired Stored carbohydrates depleted

16 Young tree canopy symptom of root root is Yellow vein chlorosis Yellow veins in fall/winter foliage Poor growth and dieback of shoots Can resemble HLB

17 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

18 Infects bark of roots and trunk Foot rot Kills cambium layer Blocks carbohydrate movement to the roots Initial canopy symptoms similar to root rot yellow vein chlorosis

19 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

20 Terminal symptoms of foot rot Leaf drop Branch dieback Fruit drop Death of tree

21 Girdling of the trunk results in tree collapse

22 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

23 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)

24 Periodic rain and prolonged leaf wetness = Brown rot 12 4 Brown rot recorded in /4/97 10/3/97 11/12/97 11/25/97 12/12/97 12/29/ Rainfall (inches) 3 2 Leaf wetness (hours) June July Aug Sept Oct Nov Dec Jan June July Aug Sept Oct Nov Dec Jan Date Date

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26 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

27 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

28 Root and bark age and activity: periods of susceptibility Young roots > old Root flushes more susceptible Foot rot risk higest in spring and fall Carbohydrates moving in trunk Fall roots store carbohydrates for next spring flowering Phytophthora infection depends on carbohydrate concentration in roots

29 Soil temperature, rain and root flushes Phytophthora is dormant in winter Soil temp <59 o F Spring root flush After shoot flush/fruit set with soil temp >68 o F and rains start Fall root flush after last summer shoot flush Soil populations increase on root flushes Soil propagules (prop/cm 3 ) Root Flushes and Seasonal Phytophthora Activity Spring Shoot Flush Shoot Flush Shoot Flush Flowering/Fruit Set Root Flush Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Root Flush Fall Root Flush

30 Flooding damage to roots predisposes them to infection Occurs in soils with restricted drainage Over irrigation High rainfall in summer Tropical storms and hurricanes

31 Prolonged O 2 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

32 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

33 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 samples/10 acres Composite and store in resealable bag cool not cold Ship to lab within hr

34 Interpretation of population counts Vary seasonally and annually Resample each year Thresholds to predict damage are imprecise Populations of < 10 propagules/cm 3 are considered nondamaging propagules/cm 3 dependes on soil type, irrigation method, rootstock, etc.

35 Avoid highly susceptible rootstocks 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 Phytophthora prevention

36 Tolerance/Resistance of Swingle citrumelo (SC) P. nicotianae compared to Cleopatra mandarin (CM) Root mass density (mg/cm 3 soil) Disturbed Undisturbed Rainfall (cm) Apr Jun Aug Oct Dec SC CM Propagules (cfu/cm 3 soil) Disturbed Undisturbed Apr Jun Aug Oct Dec SC CM Month Month

37 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

38 Rootstock susceptibility Foot rot Sweet orange Cleopatra mandarin Rough lemon Sour orange Volkamer lemon Carrizo citrange (hybrid) Swingle citrumelo (hybrid) Trifoliate orange Very Susceptible Tolerant to susceptible Tolerant to susceptible Tolerant Tolerant Moderately resistant Resistant Resistant

39 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

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

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

42 Warranted only if cultural practices fail Treatment decision based on: Rootstock susceptibility Likelihood of nursery infection History of site, e.g. Diaprepes Fungicide use in young trees

43 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

44 15 minute Break Phytophthora/Diaprepes Complex HLB of citrus roots After the break