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2008 Florida Citrus Pest Management Guide: Phytophthora Foot Rot and Root Rot1
J.H. Graham and L.W. Timmer2Foot rot results from infection of the scion near the ground level, producing bark lesions which extend down to the budunion on resistant rootstocks. Crown rot results from infection of the bark below the soil line when susceptible rootstocks are used. Root rot occurs when the cortex of fibrous roots is infected, turns soft and appears water-soaked. Fibrous roots slough their cortex leaving only white thread-like stele. Foot rot and root rot can be caused by P. nicotianae or P. palmivora. When managing Phytophthora-induced diseases, consider integration of cultural practices (e.g., disease exclusion through use of Phytophthora-free planting stock, resistant rootstocks, proper irrigation practices) and chemical control methods. Phytophthora management with chemical control should not be considered until other potential causes of decline in tree production are evaluated and corrected. See also ENY-606 Nematodes .
Cultural practices to manage P. nicotianae. Field locations not previously planted with citrus are probably free of citrus-specific P. nicotianae. Planting stock should be tested free of Phytophthora spp. in the nursery and inspected for fibrous root rot in the nursery or grove before planting. In groves with a previous history of foot rot, consider use of Swingle citrumelo for replanting. Swingle citrumelo is normally resistant to foot rot and roots do not support damaging populations once trees are established. Cleopatra mandarin should be avoided because it is prone to develop foot rot when roots are infected in the nursery or when trees are planted in flatwoods situations with high or fluctuating water tables and fine-textured soils. Trees should be planted with the budunion well above the soil line and provided with adequate soil drainage. Overwatering, especially of young trees, promotes buildup of populations in the soil and increases risk of foot rot infection. Prolonged wetting of the trunk, especially if tree wraps are used on young trees, should be avoided by using early to midday irrigation schedules. Control of fire ants prevents their nesting under wraps and causing damage to moist, tender bark.
Sampling for P. nicotianae. Population densities of the fungus in grove soils should be determined to assist in decisions to treat with fungicides. Soil samples containing fibrous roots should be collected during the spring through fall (March to November) from under the canopy within the tree dripline. Individual small amounts of soil from 20 to 40 locations within a 10-acre area are composited into one resealable plastic bag to retain soil moisture. Samples must be kept cool but not refrigerated for transport to the analytical laboratory. Currently, populations in excess of 15 to 20 propagules per cm3 soil of P. nicotianae are considered damaging. The same soil sample should be tested for populations of nematodes, to assess whether they occur at damaging levels.
Larval feeding injury to citrus roots and its relationship to invasion by Phytophthora spp. Association of Phytophthora root rot with root damage by larvae of Diaprepes abbreviatus has been called the Phytophthora-Diaprepes (PD) complex (see ENY-611 Citrus Root Weevils ). Surveys have identified a far more severe interaction of P. palmivora with Diaprepes root weevil than where P. nicotianae is the predominant interacting pathogen. The damage caused by P. palmivora is often associated with poorly-drained, fine-textured soils and with the rootstocks normally tolerant of P. nicotianae, Swingle citrumelo and Carrizo citrange. In the more severe form, structural roots collapse from what appears to be moderate larval damage followed by aggressive spread of P. palmivora through the roots. Because trees on Swingle citrumelo are more severely affected by the Diaprepes root weevils than surrounding groves on sour orange, there is a particular concern about young plantings on Swingle citrumelo that replace blocks on sour orange. Swingle citrumelo consistently supports higher soil populations of P. palmivora than trees on Cleopatra mandarin or sour orange.
Rootstock susceptibility to damage by the PD complex depends on which Phytophthora spp. is present and whether the soil and water conditions are conducive to the fungus or to rootstock stress. In most situations, P. nicotianae is the predominant pathogen and Swingle citrumelo appears to perform acceptably as a replant in weevil-infested groves, provided soil conditions are suited for this rootstock (e.g., sandy soil texture, well-drained, favorable pH and calcium carbonate status, etc.). When P. palmivora is present in poorly-drained soils high in clay, pH and calcium carbonate, Diaprepes root weevils render normally tolerant Swingle citrumelo and Carrizo citrange susceptible to Phytophthora root rot infection. Thus, tolerance of Swingle citrumelo is restricted to the ridge and certain flatwoods soils.
Chemical control. Use of fungicides in young groves should be based on rootstock susceptibility, likelihood of Phytophthora infestation in the nursery, and history of Phytophthora disease problems in the grove. For susceptible rootstocks, such as Cleopatra mandarin and sweet orange, fungicides may be applied to young trees on a preventive basis for foot rot. For young trees on other rootstocks, fungicide treatments should commence when foot rot lesions develop. The fungicide program for foot rot should be continued for at least one year for tolerant rootstocks, but may continue beyond the first year for susceptible stocks. In mature groves, the decision to apply fungicides for root rot control is based on yearly soil sampling to indicate whether damaging populations of P. nicotianae occur in successive growing seasons. Time spring and fall applications to coincide with periods of susceptible root flushes in the spring (after the spring leaf flush) and fall (October to November). Soil application methods with fungicides should be targeted to undercanopy areas of highest fibrous root density. To avoid leaching from the root zone, soil-applied fungicides should not be followed by excessive irrigation. Aliette and Ridomil are both effective, but alternation of the materials should be practiced to minimize the risk of the development of fungicide resistance.
Management of the Phytophthora-Diaprepes complex. Selection of tolerant rootstocks for replanting Diaprepes root weevil-affected groves aids in management of future losses. For existing trees, fungicides in conjunction with careful water and fertilizer management have been utilized to maintain tolerance to Diaprepes root weevil and Phytophthora damage. Fertigation maximizes efficiency of water and nutrient uptake by the roots in well-drained soils. However, use of fertigation to regenerate roots is limited in poorly-drained soils and high water tables typical in the flatwoods. In these situations, there may be increased reliance on fungicides to control root damage by Phytophthora spp.
Based on studies of the PD complex, aggressive control of Diaprepes root weevil larvae and adult stages should be implemented as soon as infestation is discovered to minimize the more severe Phytophthora damage that follows larval feeding on roots. The IPM program may include carefully scheduled fertigation in well-drained soils to promote regeneration of fibrous roots after damage. In the flatwoods, IPM may include use of fungicides under the following conditions: 1) the soils are fine textured, poorly-drained or high in pH and calcium carbonate status, 2) the trees are on rootstocks susceptible to either or both Phytophthora spp., and 3) populations are above the damaging levels (15 to 20 and 40 propagules per cm3 soil for P. nicotianae and P. palmivora, respectively).
Fungicides are not recommended for Phytophthora control in citrus nurseries.
Recommended Chemical Controls
READ THE LABEL.Rates for pesticides are given as the maximum amount required to treat mature citrus trees unless otherwise noted. To treat smaller trees with commercial application equipment including handguns, mix the per acre rate for mature trees in 250 gallons of water. Calibrate and arrange nozzles to deliver thorough distribution and treat as many acres as this volume of spray allows.
Tables
Table 1. Recommended Chemical Controls for Phytophthora Foot Rot and Root Rot - Fosetyl AL and Phosphite Salts Products
Pesticide
FRAC
MOA1
Mature Trees
Rate/Acre2
Method of Application
Comments
Aliette WDG3,4
13
Protectant and curative systemic. Buffering to pH 6 or higher is recommended to avoid phytotoxicity when copper has been used prior to, with, or following Aliette. Nonbearing
5 lb/100 gal. Do not exceed 5 lb/acre.
Foliar spray
0.5-1 lb/gal
Trunk paint or spray5
Up to 5 lb/acre
Microsprinkler
Adjust rate according to tree size
Bearing 5 lb/acre
Foliar spray in 100-250 gal/acre. Do not exceed 1500 gal/acre.
Apply up to 4 times/year (e.g., March, May, July and September) for fibrous root rot control.
5 lb/10 gal/acre
Aerial
Fly every middle. 5 lb/acre
Surface spray on weed-free area followed by 0.5 inch irrigation or by microsprinkler in 0.1-0.3 inch of water.
Apply up to 4 times/year (e.g., March, May, July and September) for fibrous root rot control. Phostrol 13
Protectant and curative systemic. Do not apply when trees are under water stress or high temperature conditions. Bearing or Nonbearing 4.5 pt/acre
Foliar spray
Apply up to 4 times/year (e.g. March, May, July and September).
Bearing or Nonbearing 2-5 pt/5 gal
Trunk paint or spray Trunk paint or spray ProPhyt 13
Protectant and curative systemic. Do not apply when trees are under water stress or high temperature conditions. Nonbearing 2 pt/100 gal
Foliar spray Adjust rate according to tree size. Bearing 4 pt/acre
Foliar spray Apply up to 4 times/year (e.g., March, May, July and September) for fibrous root rot control.
1Mode of action class for citrus pesticides from the Fungicide Resistance Action Committee (FRAC) 2003. Refer to ENY624, Pesticide Resistance and Resistance Management, in the 2008 Florida Citrus Pest Management Guide for more details.
2Lower rates may be used on smaller trees. Do not use less than the minimum label rate.
3For combinations of application methods, do not exceed 4 applications or 20 lb/acre/year.
4Fungicide treatments control fibrous root rot on highly susceptible sweet orange rootstock, but are not effective against structural root rot and will not reverse tree decline.
5Apply in May prior to summer rains and/or in the fall prior to wrapping trees for freeze protection.
Table 2. Recommended Chemical Controls for Phytophthora Foot Rot and Root Rot - Mefenoxam and Copper Products
Pesticide
FRAC
MOA1
Mature Trees
Rate/Acre2
Method of Application
Comments
Ridomil Gold SL3,4
4
Protectant and curative systemic. Do not apply tank mixes of Ridomil and residual herbicides to trees less than 3 years old. Apply herbicide first, then wait 3-4 weeks to apply Ridomil.
Nonbearing
1 qt/acre of treated soil surface
Surface spray on weed-free area, followed immediately by 0.5 inch irrigation or by microsprinkler in 0.1-0.3 inch of water.
Apply every 3 months for maximum control; in most cases a late spring and late summer application should be sufficient.
1 qt/grove acre
Through irrigation injection 1-1.5 oz/100 gal
Soil drench; apply 5 gal of mix in water ring.
Apply every 3 months for maximum control; in most cases a late spring and late summer application should be sufficient. Bearing 1 qt/acre of treated soil surface
Surface spray on weed-free area, Followed immediately by 0.5 inch irrigation or microsprinkler in 0.1-0.3 inch of water.
Apply 3 times/year (late spring, summer, early fall).
1 qt/grove acre
Through irrigation injection Bearing or Nonbearing
1 qt/10 gal
Trunk paint or spray May be applied up to 3 times/yr.
Ridomil Gold GR2
4
Nonbearing
40-80 lb/acre of treated soil surface
Broadcast application under the canopy. If rain not expected for 3 days, follow by 0.5-1.0 inch of irrigation.
Apply every 3 months for maximum control; in most cases a late spring and late summer application should be sufficient.
1-1.5 oz/tree
Within 3 days of application, apply 5 gal water/tree.
Apply every 3 months for maximum control; in most cases a late spring and late summer application should be sufficient. Bearing 40-80 lb/acre of treated soil surface
Broadcast application under the canopy. If rain not expected for 3 days, follow by 0.5-1.0 inch of irrigation.
Apply 3 times/year (late spring, summer, early fall). UltraFlourish3,4
4
Protectant and curative systemic. Do not apply tank mixes of UltraFlourish and residual herbicides to trees less than 3 years old. Apply herbicide first, then wait 3-4 weeks to apply UltraFlourish.
Nonbearing
2 qt/acre of treated soil surface
Surface spray on weed-free area, followed immediately by 0.5 inch irrigation or by microsprinkler in 0.1-0.3 inches of water.
Apply every 3 months for maximum control; in most cases a late spring and late summer application should be sufficient. 2 qt/grove acre
Through irrigation injection 2-3 oz/100 gal
Soil drench; apply 5 gal of mix in water ring.
Apply every 3 months for maximum control; in most cases a late spring and late summer application should be sufficient. Bearing
2 qt/acre of treated soil surface
Surface spray on weed-free area, followed immediately by 0.5 inch irrigation or microsprinkler in 0.1-0.3 inch of water.
Apply 3 times/year (late spring, summer, early fall). 2 qt/grove acre
Through irrigation injection Copper-Wettable Powder
M9
0.5 lb (metallic) Cu/1 gal water
Trunk paint5
Protectant Copper-Count-N
M9
1 qt in 3 qt water
Trunk paint5
Protectant. Do not apply to green bark; may cause gumming. 1Mode of action class for citrus pesticides from the Fungicide Resistance Action Committee (FRAC) 2003. Refer to ENY624, Pesticide Resistance and Resistance Management, in the 2008 Florida Citrus Pest Management Guide for more details.
2Lower rates may be used on smaller trees. Do not use less than the minimum label rate.
3Do not exceed the equivalent of 6 lb a.i./acre/year of mefenoxam-containing products.
4Do not apply to bare roots or higher than 1 qt/acre of treated soil surface to citrus resets or trees less than 5 years old to avoid potential phytotoxicity.
5Apply in May prior to summer rains and/or in the fall prior to wrapping trees for freeze protection.
Footnotes
1. This document is PP-156, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date printed: December 1995. Date revised: November 2007. This publication is included in SP-43, 2008 Florida Citrus Pest Management Guide. A copy of this publication may be found at http://edis.ifas.ufl.edu/CG009. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. For a copy of this handbook, request information on its purchase at your county extension office.2. J.H. Graham, professor, Soil and Water Science Department and L.W. Timmer, emeritus professor, Plant Pathology Department, Citrus REC, Lake Alfred, Florida; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.
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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
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