Peach Phytophthora Root Rot – How To Treat A Peach With Phytophthora Rot


By: Mary H. Dyer, Credentialed Garden Writer

Phytophthora root rot of peach is a destructive disease that afflicts peach trees around the world. Unfortunately, the pathogens, which live under the soil, may go unrecognized until the infection is advanced and symptoms are obvious. Read on to learn more.

About Phytophthora Root Rot of Peach

Trees with peach phytophthora root rot are usually found in soggy, poorly drained areas, especially where the soil stays heavy and wet for 24 hours or more.

Phytophthora root rot of peach is somewhat unpredictable and may kill the tree gradually over a few years, or an apparently healthy tree may decline and die suddenly after new growth appears in spring.

Symptoms of peach with phytophthora rot include stunted growth, wilting, reduced vigor and yellowing leaves. Leaves of trees that die slowly often display a reddish-purple coloration in autumn, which should still be bright green.

Phytophthora Root Rot Control

Certain fungicides are effective for treating young trees before symptoms appear. This is critical if you’re planting trees where phytophthora root rot of peach has been present in the past. Fungicides may slow progression of phytophthora root rot if the disease is spotted in the early stages. Unfortunately, once phytophthora root rot takes hold, there isn’t much you can do.

That’s why preventing phytophthora root rot of peaches is important and your best line of defense. Start by selecting peach tree varieties that are less susceptible to disease. If you don’t have a good spot for peaches, you may want to consider plums or pears, which tend to be relatively resistant.

Avoid locations where the soil remains wet or is prone to seasonal flooding. Planting trees on a berm or ridge may promote better drainage. Avoid overwatering, especially in spring and autumn when the soil is most susceptible to soggy conditions and disease.

Treat soil around newly planted peach trees using a fungicide registered for treatment of phytophthora root rot of peaches.

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How to Manage Pests

Peach

Phytophthora Root and Crown Rot

Pathogen: Phytophthora spp.

(Reviewed 4/10 , updated 4/10, pesticides updated 9/15 )

SYMPTOMS AND SIGNS

Symptom expression depends upon how much of the root or crown tissues are affected and how quickly they are destroyed. Generally, crown rots advance rapidly and trees collapse and die soon after the first warm weather of spring. Leaves of such trees wilt, dry, and remain attached to the tree. Chronic infections, usually in the roots, cause reduction in growth and early senescence and leaf fall. These trees may be unthrifty for several years before succumbing to the disease. Phytophthora infections typically kill young trees because their root systems and crown areas are small compared to those of mature trees.

COMMENTS ON THE DISEASE

Periods of 24 hours or more of saturated soil favor Phytophthora infections. Conversely, good soil drainage and more frequent but shorter irrigations reduce the risk of root and crown rot. Rootstocks vary in susceptibility to the different Phytophthora species none are resistant to all pathogenic species of the fungus. Thus, the success of a rootstock may depend in part upon the species of Phytophthora present in the orchard.

MANAGEMENT

The most effective ways to manage Phytophthora root and crown rot are to select a good planting site, select an appropriate rootstock, and properly manage irrigation water. Avoid over-irrigating in spring and fall when soil temperatures are most conducive to disease development and water use by the tree is low. Planting on raised berms can also help with disease management.

Chemical Control
Fungicides are available to treat soil around newly planted trees. If there is a history of Phytophthora root rot in the orchards and problems are anticipated, treatments may be warranted.

PUBLICATION

UC IPM Pest Management Guidelines: Peach
UC ANR Publication 3454

Diseases

J. E. Adaskaveg, Plant Pathology, UC Riverside
R. A. Duncan, UC Cooperative Extension Stanislaus County
J. K. Hasey, UC Cooperative Extension Sutter/Yuba counties
K. R. Day, UC Cooperative Extension Tulare County

Acknowledgment for contributions to Diseases:

Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2017 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California


Contents

Phytophthora palmivora produces abundant sporangia on V-8 agar under continuous fluorescent light. However, light is not required for sporangia production on infected papaya fruit. Sporangia are usually produced in clusters sympodially. Sporangia are papillate and ovoid with the widest part close to the base. They are easily washed off and each detached sporangium contains a short pedicel. The average size of the sporangia is 50×33 µm with a length of about 1.6 times longer than it is wide. Sporangia germinate directly in a nutrient medium by producing germ tubes that develop into mycelial masses. In water, however, zoospores are released from germinating sporangia. Zoospores aggregate and form distinct patterns at 16 °C in water.

Chlamydospores produced in infected papaya fruit and pure papaya juice are thick-walled. However, chlamydospores produced in papaya juice at lower concentrations or in other kinds of fruit juice are mostly thin-walled. In the presence of nutrients, chlamydospores germinate by producing germ tubes that continue to grow and form mycelial masses. In water, chlamydospores germinate by producing short germ tubes, each with a sporangium at the tip.

Sexual reproduction in Phytophthora palmivora requires the presence of opposite mating types known as A1 and A2. Both A1 and A2 isolates can produce zoospores by selfing when stimulated by sex hormones produced by A2 and A1, respectively. Light is inhibitory to zoospore formation but stimulatory to zoospore germination. Mature zoospores can be induced to germinate by treatment with 0.25% KMnO4 for 20 min and incubation under light during germination.

Although sporangia and zoospores may survive in soil for short periods, chlamydospores are the main survival structure for P. palmivora in nature. Zoospores are capable of long-term survival but do not play a significant role in the disease cycle because sexual reproduction in P. palmivora requires the presence of opposite mating types, and the chance for this to occur in nature is very low.

During rainy periods, chlamydospores in soil may germinate in water to produce sporangia and release zoospores. The impact of falling rain drops may splash zoospores into air in droplets. The zoospore-containing droplets may be further dispersed by wind and become the inoculum for infecting fruit and occasionally stems of papaya in the fields. The pathogen produces abundant sporangia on the surface of infected fruit that are further dispersed by wind-blown rain and cause outbreaks of Phytophthora fruit rot in the same and nearby orchards. Chlamydospores formed in fallen fruit survive in soil and serve as the main source of inoculum for infection of roots of papaya seedling in subsequent plantings.

Phytophthora root rot of papaya seedlings is most serious during rainy periods. Under waterlogged conditions, P. palmivora may attack roots of papaya older than three-months of age, the time at which they become resistant to the pathogen under normal conditions. Therefore, Phytophthora root rot may occur on papaya at any age in poorly drained areas. Waterlogged conditions appear to weaken the defense mechanism of papaya roots against invasion by the pathogen. Mobility of zoospores of P. palmivora under such conditions also may contribute to the severity of the disease due to their attraction by papaya roots.

Favorable temperature is also a contributing factor to the severity of Phytophthora diseases because of its effect on growth and sporulation of the pathogen. Phytophthora palmivora has an optimum temperature for growth of 30 °C, a maximum temperature of 36 °C and a minimum temperature of 12 °C. The pathogen produces the most sporangia at 25 °C but no sporangia are produced at temperatures higher than 35 °C or lower than 15 °C.

Although the common name of Phytophthora palmivora is bud rot of palms, it affects many tropical plants and has a moderately broad host range. P. palmivora is well studied in coconuts and papaya trees, however there are multiple hosts that are less commonly studied. One common symptom of P. palmivora is fruit rots which are found in papaya, citrus, coconuts, durian, and cacao. Root rots are another symptom of P. palmivora and can be seen in red maples, citrus, papaya, mango, durian, and black pepper. Another symptom is the presence of cankers which are found in red maple, papaya, rubber, mangos, and cacao. Bud rots can also be seen in papaya and coconuts infected with P. palmivora. Bud rots are also found in Palmyra palms and coconut palms. Collar rots are found on citrus, mango, and black pepper infected with P. palmivora. The signs of P. palmivora are microscopic and can be differentiated from other oomycetes by the presence of oval shaped papillate sporangia with short pedicles and spherical oogonia with narrow stalks [3] (Widmer, 2014).

Rain and wind are the two major factors in the epidemiology of Phytophthora fruit rot of papaya. Rain splash is needed for liberation of sporangia of P. palmivora from the surface of infected fruit into the atmosphere and for projection of the soil inoculum into air. Wind is required for dispersal of the inoculum once it reaches the air. Therefore, wind-blown rain is essential for initiation of the primary infection and the development of epidemics in papaya orchards. Phtophthora palmivora also cause fruit rot, bud rot, etc.. Bud rot of coconut (cocos nucifera)is very common in India.

Atmospheric temperature of 18-20 °C along with high humidity activates the pathogen

General control Edit

Since P. palmivora is an oomycete the simplest management technique is to control the amount of water present in the soil. Techniques for controlling moisture include: monitored watering, pruning to increase airflow and decrease humidity in the soil, as well as making sure that areas where potential hosts are planted are not prone to flooding, oftentimes this includes planting on an incline. Other means of cultural control for P. palmivora include mulching to reduce the number of spores released via rain splash, complete removal of infected host plants and materials, and in some cases the use of companion crops. Companion crops are planted in the same fields as the host plant and are used to divert some of the pathogen away from the hosts, an example being planting bananas and avocados in the same field. Chemical control methods for P. palmivora include: protectant fungicides such as the Bordeaux mixture, phosphonates which control the mycelial growth of the pathogen, dithiocarbamates such as Mancozeb, and phenylamides which control the spread of the pathogen from the roots of the host. Host resistance is also a method of controlling the pathogen, resistant plants generally have thicker cuticles which inhibits the ability of the pathogen to enter the host.

Non-chemical control in papaya Edit

Root rot of papaya seedlings, caused by P. palmivora, in replant fields can be controlled with the virgin soil technique. Virgin soil (soil in which papaya has never been grown in before) is placed in planting holes about 30 cm in diameter and 10 cm deep with a mound about 4 cm high. Roots of papaya plants are protected by the virgin soil during the susceptible stage, and become resistant to the pathogen when they extend to the infested soil. Trees established with the virgin soil method in the replant fields produce fruit as abundantly as those growing in the first planting fields. The virgin soil method has the advantages of being relatively inexpensive, very effective and nonhazardous.

Cultural practice is also important in the management of Phytophthora diseases of papaya. Incidence of Phytophthora root rot of mature trees in waterlogged areas during the rainy periods can be greatly reduced by improving drainage in the orchards. Infected fruit on the trees and those that have fallen to the ground should be removed to reduce the inoculum for aerial infection of fruit and stems, and infection of seedling roots in subsequent plantings.

Because P. palmivora infects multiple hosts that hold an economic significance including cacao, coconut, papaya, mango, olive trees, and black pepper, this is a pathogen of great concern. The pathogen is found in various regions of the planet ranging from Africa, India, South America, and even the temperate regions of North America. It has been estimated that 10-20% of all cacao is lost due to Phytophthora Pod Rots (PPR) which includes P. palmivora. Due to P. palmivora’s dependence on moisture, the annual yield loss fluctuates and in some years losses have been as high as 75% in some regions. This impacts the cost of cacao, and thus the pathogen controls the cost and availability of products such as chocolate. In mangoes, the pathogen is known to kill young plants, specifically nursery plants. This impacts the long-term number of commercially available plants which could lead to potentially lower crop yields. In coconuts, the expected yield losses caused by P. palmivora have been up to 2.5% per month during the rainy season, this can impact coconut product manufacturing such as coconut oil. In the 1970s P. palmivora had such a severe impact on black pepper plants in Brazil that it was no longer commercially grown, and it is considered the most detrimental pathogen of black pepper. As previously stated impacts of P. palmivora commercially cause it to be a pathogen of significant importance.


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Management

Management of Phytophthora root rot involves the use of resistant rootstocks, irrigation management, fungicides, and fumigation.

Cultural Control

Provide adequate soil drainage and avoid over irrigation. If destruction of feeder roots is minimal, corrective action may include increasing irrigation intervals, switching to alternate middle row irrigation or a different irrigation system such as mini sprinklers, and installing subsoil tiles.

Resistant Rootstocks

When replanting or establishing new plantings, choose resistant rootstocks where possible, but also consider tolerance to other diseases, nematodes, and cold. The most tolerant rootstocks are trifoliate orange, swingle citrumelo, citrange, Alemow, and sour orange.

Organically Acceptable Methods

Use cultural controls and resistant rootstocks in an organically managed citrus grove.

Monitoring and Treatment Decisions

If a tree growing on susceptible rootstock looks stressed, dig up some soil and check the feeder roots. Sample for P. parasitica during July through September, and P. citrophthora throughout the year:

  • Randomly select 20 to 40 locations within a 10-acre orchard block with mild to moderate expected Phytophthora tree decline.
  • Sample from aroung the tree drip line or near irrigation emitter where roots are concentrated.
  • Put composite samples in a sealed plastic bag, but do not refrigerate or overheat.
  • Ship within 24 to 48 hours to a lab where propagule count per unit of soil and root infestation are determined.
  • Phytophthora populations of more than 15 to 20 propagules per gram of root zone soil may warrant a pesticide application. When planting or replanting in soil infested with Phytophthora, or when a susceptible rootstock has to be used, fumigation may be feasible if no other adverse conditions persist.

Common name Amount to use REI‡ PHI‡
(Example trade name) (hours) (days)
UPDATED: 9/15
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees, and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
A. FOSETYL-AL
(Aliette WDG) 5 lb/100 gal 12 NA
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phosphonate (33)
COMMENTS: NONBEARING TREES ONLY. Foliar spray, 60-day interval. Do not apply more than 5 lbs Aliette/acre per application.
B. PHOSPHORUS ACID
(Fosphite) 1–3 qt/acre 4 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phosphonate (33)
COMMENTS: Allow 10 days before applying a copper-based compound following an application of Fosphite. Allow 20 days before applying Fosphite following a treatment with a copper product. Do not apply with copper-based fungicides or fertilizers.
C. MEFENOXAM
(Ridomil Gold) Varies with method of application and size of tree 0 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phenylamide (4)
COMMENTS: Applications made in early spring and fall.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1,4,9,11, or 17 before rotating to a fungicide with a different mode of action Group number for fungicides with other Group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode of action Group number.
NA Not applicable.
Common name Amount to use REI‡ PHI‡
(Example trade name) (hours) (days)
Pesticide precautions Protect water Calculate VOCs Protect bees
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
PREPLANT
A. METAM SODIUM* §
(Vapam) 75 gal/acre See label NA
. . . or . . .
16 fl oz/tree (8 ft diameter canopy) See label NA
COMMENTS: Apply with 6 to 12 inches of water. Do not plant for at least 45 days. Fumigants such as metam sodium are a prime source of volatile organic compounds (VOCs), which are a major air quality issue.
B. CHLOROPICRIN* §
(Chloropicrin 100) 350 lb/acre See label NA
. . . or . . .
16 oz/tree (8 ft diameter canopy) See label NA
COMMENTS: Use lower rate on sandy loam and high rate on heavier soils or high clay. Inject 8 to 10 inches deep, 12 to 18 inches apart, and tarp immediately. Do not plant for at least 3 months.
NONBEARING TREES
A. MEFENOXAM
(Ridomil Gold SL) 1–1.5 fl oz/100 gal water for soil drench See label See label
. . . or . . .
1–2 qt/acre for soil surface spray 48 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phenylamide (4)
COMMENTS: For citrus in nurseries: Apply at planting and at 3-month intervals during growing season. As a drench, apply 100 to 250 gal mixture per 1000 ft of row on an area wide enough to cover the root system. As a soil surface spray, apply as a broadcast or banded surface spray to seedbeds, liners, or bedded stock in sufficient water to obtain uniform coverage of the root system. For use on resets or new plantings: Apply at planting and up to three applications at 3-month intervals to coincide with root growth flushes during the growing season. As a drench, apply 5 gal mix around tree base within the watering ring. As a soil surface spray, apply in sufficient water to obtain coverage of the soil surface wetted by irrigation. Apply spray to the soil surface beneath the tree canopy. Follow immediately with an irrigation sufficient to wet the soil to 1 ft.
B. FOSETYL-AL
(Aliette WDG) 5 lb/100 gal per acre 12 365 (1 year)
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phosphonate (33)
COMMENTS: For use on trees in nurseries only. Apply in 100 gal water/acre to susceptible varieties as a foliar spray when conditions favor the disease. Trees should be sprayed to wet at the time of planting. Do not exceed four applications per year or 20 lb/acre per year.
C. POTASSIUM PHOSPHITE
(K-Phite 7LP) See label 4 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phosphonate (33)
COMMENTS: For use on all susceptible citrus. Apply in 100 to 250 gal/acre spray to wetness when conditions favor disease development. Do not exceed four applications of this product per year.
BEARING TREES
A. MEFENOXAM
(Ridomil Gold SL) 1–2 qt/acre 48 0
. . . or . . .
0.75–1.5 fl oz/1000 sq ft 48 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phenylamide (4)
COMMENTS: Apply two to three times per year to coincide with flushes of root growth. Apply in a banded surface spray under tree canopy. Up to three applications may be made per year.
B. MEFENOXAM
(Ridomil Gold GR) Label rates 48 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phenylamide (4)
COMMENTS: Apply in March to April followed by one or two applications at 3-month intervals to coincide with root flushes rate depends on tree size and the number of applications per year. Apply 0.5 to 1 inch water after application.
C. FOSETYL-AL
(Aliette WDG) 5 lb/acre 12 30
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phosphonate (33)
COMMENTS: Apply to susceptible varieties as a foliar spray when conditions favor the disease. Spray to wet. Do not exceed four applications or 20 lb/acre per year. Do not allow livestock to graze in sprayed citrus groves.
D. POTASSIUM PHOSPHITE
(Prophyt) 4 pt 4 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Phosphonate (33)
COMMENTS: For use on all susceptible citrus. Apply in 100 to 250 gal/acre spray to wetness when conditions favor disease development. Do not exceed four applications of this product per year.
E. OXATHIAPIPROLIN
(Orondis) 2–9.6 fl oz/acre 4 0
MODE-OF-ACTION GROUP NAME (NUMBER 1 ): Oxy-sterol-binding protein inhibitor (49)
COMMENTS: For use on all susceptible citrus. Apply in 100 to 400 gal/acre spray to wetness when conditions favor disease development. Minimal re-application interval is 30 days. Do not make more than two applications of this product per year and do not use more than 19.2 fl oz/acre per year. Do not make more than two sequential applications before rotating to another mode of action. When three or more applications are needed for disease management, do not apply this product more than 33% of the total number of applications. May be applied as a soil or trunk spray or by chemigation.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
§ Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.
No information
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441


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