WSU Whatcom County Extension

Integrated Pest Management for Raspberries

Phytophthora

(Phytophthora rubi)

Insects and Invertebrates

 

Phytophthora

 

Symptoms

Phytophthora root rot is most commonly associated with heavy soils and portions of fields that are slow to drain. Symptoms include a general lack of vigor and a sparse plant stand. Primocane leaves will initially take on a yellow, red, or orange color and may begin scorching along the edges. Floricanes of affected plants will have weak lateral shoots. Below ground symptoms vary from slight necrosis of young rootlets to extensive necrosis that turns crowns and main roots reddish brown. Plants may eventually wilt and die, especially with the first onset of warmer weather. Frequently, new shoots develop from the healthier portions of the crown.  Infected plants frequently occur in patches, which may spread along the row if conditions remain favorable for disease development.

 

Life History

Phytophthora can survive in the soil for several years and persists primarily as mycelium in infected roots or as dormant resting spores in the soil. When the soil is moist, reproductive structures (sporangia) are formed upon the infected tissue or by germinating resting spores (oospores) in the soil. Within each of these structures, a number of individual spores called zoospores are formed. These zoospores are expelled into the soil during periods when the soil is saturated with water. The zoospores have "tails" (flagella) that allow them to swim through the water-filled soil pores to reach new plant parts. Upon reaching a plant root or crown, the zoospores become attached to fine roots and begin the infective process. They grow through the root tissue and may grow into the plant crown and damage it.  As water remains standing and oxygen is depleted from the root zone, the plant is progressively less capable of resisting the fungus's attempts at invasion, and infection becomes more likely and severe.  Each new infection site is a potential source of additional resting spores and zoospores, allowing for epidemic disease development in sites that are subjected to repeated periods of standing water. Although the optimum season for infection is not known for certain, it is likely that spring and fall are particularly favorable periods. However, it is assumed that infection can occur throughout the growing season if soil moisture conditions are favorable.

 

Phytophthora

 

Monitoring

Evaluate your field in early spring making note of low areas that either contain standing water or were slow to drain, especially if heavy rains were experienced that winter. Return to those areas in June with the onset of hot weather to evaluate Phytophthora symptoms.

At harvest, began scouting 3-5 sites per field (depending on field size) looking for reduced number of primocanes, wilted primocanes, and fruiting canes with yellow or scorched lateral shoots. These symptoms are more likely to be found in low areas where saturated soil conditions persist, which favor infection and spread of disease.  However, the pattern of disease plants is determined by the presence and movement of the disease organism and therefore can be found in both well-drained and poorly drained soils. Record a 0-5 visual rating at each site and continue to monitor you field until shortly after harvest ends. Also look for Phytophthora symptoms when performing other field activities.

Poor growth in low-lying areas may also be due to anaerobic (low oxygen) conditions which are unfavorable for normal root function. Phytophthora may not even be present, yet plants show symptoms typical of Phytophthora infection. Remove susceptible plants and examine the crown area by scraping away the epidermis, which will reveal a distinct margin between healthy (white) and disease (red) tissue, if infected. A lack of fine feeder roots is also associated with Phytophthora root rot.

 

Thresholds and Management

No threshold or tolerance level available. Decision making around this pest complex is complicated. Growers should consider age of planting, size of effected area, and variety.

Presence of both above ground and root/ crown symptoms indicates that Phytophthora is the causal agent and that fall, and possibly spring soil-applied fungicides may be appropriate. Use effective systemic fungicides as a drench around the base of infected plants as a disease repressor; these fungicides will not cure diseased plants. Confine treatment to those areas where scouting indicates a problem and lab results confirm presence of the pathogen. Stresses such as over-fertilization with nitrogen and herbicide injury can accelerate the death of infected plants.

The best control strategy involves prevention. Avoid planting in poorly drained sites and select fields that have naturally good drainage or improve drainage by tilling. Purchase disease free nursery stock from a reputable grower or grow rooted cuttings or nursery plants on raised beds. Avoid over irrigating when soil temperatures are high.

 

Resources

Ohio State University Extension: Phytophthora Root Rot of Raspberry
http://ohioline.osu.edu/hyg-fact/3000/pdf/HYG_3207_08.pdf

Oregon State University Extension, Plant Disease Control: Raspberry – Root Rot
http://plant-disease.ippc.orst.edu/ShowDisease.aspx?RecordID=954

University of California, UC IPM Online, Caneberries: Phytophthora Root Rot
http://www.ipm.ucdavis.edu/PMG/r71100811.html

Washington State University Whatcom County Extension: Survey for Phytophthora fragariae var. rubi in pre-plant stocks and planted root sample of red raspberry in Washington State
http://whatcom.wsu.edu/ag/IPM/phytophthora.htm

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WSU Whatcom County Extension 1000 N. Forest St., Suite 201, Bellingham, WA 98225 (360) 778-5800 whatcom@wsu.edu