Managing Soil Borne Diseases

Transcription

1 Managing Soil Borne Diseases PMA-5205 Spring 2017 Evan Johnson Plant Pathologist UF-IFAS-CREC

2 Citrus Root Health Acquisition of the most important resources applied to the tree is via the root system Most of these inputs are now regulated by law: fertilizers, irrigation water, disease and pest control chemicals HLB (Greening) drastically reduces fibrous roots compelling careful management of the above resources to sustain root health

3 Promoting Root Health is to maximize water, nutrient use efficiency, and anchorage of the tree Structural Roots Fibrous roots Photo courtesy WS Castle

4 Promoting Root Health is to maximize water, nutrient use efficiency, and anchorage of the tree

5 In Hurricanes Jeanne and Francis young trees w/ large tops and/or heavy fruit load were pushed over

6 Mature trees were also pushed over due to shallow root systems and poor root health

7 Phytophthora-Diaprepes (PD) complex caused disintegration and breakage of structural roots

8 Key pathogens, pests and their interactions affecting root health Phytophthora diseases Phytophthora spp. Diaprepes root weevil (DRW) Diaprepes abbreviatus Huanglongbing (HLB) Candidatus Liberibacter asiaticus

9 Root attack directly and indirectly affects: Fruit size Yield Post harvest quality Tree survival

10 Phytophthora diseases damage all parts of the tree Foot rot of trunk and limbs Fibrous and structural root rot Brown rot of fruit Yield reduction estimate: 3-6% per yr or approx. $20 mil in crop loss

11 Phytophthora root rot damages fibrous roots Impairs water and nutrient uptake Healthy Reduces CHOs allocated for next crop season Damaged

12 Foot rot damages major roots and trunk Blocks delivery of water and nutrients up to the canopy Blocks carbohydrate movement to the roots

13 Root and trunk diseases: initial canopy symptom is yellow vein chlorosis Yellow vein of foliage in fall/winter (resembles HLB) Poor growth and dieback of shoots

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

15 Girdling of the trunk bark results in tree collapse

16 Bark cracks, remains firm Water soluble gum is exuded Lesions spread around the trunk, or crown below the soil line Lesions may heal Foot Rot symptoms

17 Fibrous Root Rot Root cortex soft and water soaked Cortex sloughs leaving white thread-like tips Water and nutrient uptake impaired Stored carbohydrates depleted

18 Root rot decline of mature trees difficult to diagnose Reduced fruit size and/or number Loss of leaves Twig dieback Decline in yield is slow but annual

19 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 with high water tables P. palmivora Sporangia elongated P. nicotianae Sporangia round

20 Life cycle of Phytophthora

21 Phytophthora infection cycle under favorable conditions Zoospores release from sporangia swim or splash to the root, bark or fruit Spores encyst, germinate, and infect tissues within 24 hr

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 in post harvest Hamlin Grapefruit

23 Brown Rot affects early season varieties Coincidence of rains w/color break Most fruit loss in older, large trees Fruit can not be harvested until infected fruit fall Hamlin most affected Navel and grapefruit (shows up in the packinghouse box)

24 Periodic rain & 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 managements Avoid premature drop of fruit to reduce inoculum (not possible post HLB era) Raise tree skirts to increase air movement and promote drying of foliage Apply preventative sprays of phosphites, copper Phosphite sprays in July and October if rainy season is prolonged

27 Wet conditions favor root infection cycles Susceptibility of roots highest during very wet to very dry cycles Wetting and drying increases root exudation attracts zoospores HLB infected roots also exude more exudates that promotes infection

28 Flooding damages roots predisposes roots to infection Occurs in soils with restricted drainage Over irrigation High rainfall in summer Tropical storms!!!

29 Prolonged O 2 deprivation kills roots Root regeneration depends on proper aeration Disfavors fungal attack of new roots Monitor water table to maintain drainage in the soil profile so root regeneration can occur

30 Root and bark age and activity: periods of susceptibility Young roots > old Root flushes more susceptible Foot rot risk highest in spring & fall as CHOs move up & down in trunk cambium Fall roots store CHOs for next spring fruit Fungal infection depends on concentration of root CHOs

31 Soil Temperature, Rains & Root Flushes Fungus is dormant in winter (soil temp <59 o F) Spring flush of roots >68 o F after shoot flush/fruit set 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

32 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 Oct Dec Aug SC CM Propagules (cfu/cm 3 soil) Disturbed Undisturbed Apr Jun Oct Dec Aug SC CM Month Month

33 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

34 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

35 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

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

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

38 Field Monitoring of Phytophthora spp. Soil populations used to predict the need for management Soil sampling is most important step Commercial labs are available to conduct assays P. palmivora can be distinguished from P. nicotianae on selective medium

39 Sample collection Samples sites at random in the problem area (not the worst or the best trees) Collect root-soil cores from 1/2 between trunk & dripline samples/10 acre Composite in a 1 gal. resealable bag, store cool not cold Ship to lab within hr

40 Interpretation of Phytophthora population counts Vary seasonally and annually, resample each year Thresholds to predict damage are imprecise Populations < 5 propagules/cm 3 soil are considered non-damaging Populations of >10-20 may be damaging depending on disease history, soil type, topography, irrigation method, rootstock cultivar, HLB disease status

41 Replanting after tree losses Use resistant or tolerant rootstocks in area with history of foot rot Inspect roots of nursery trees before planting Plant trees with the bud union well above the soil line

42 Adequate drainage and proper irrigation is essential Keep area around trunk clear of weeds and avoid wounding Remove wraps early in spring and treat area for fire ants Fire ants feed on and damage bark predisposing to Phytophthora infection Managing replants in groves

43 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

44 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