WSU Whatcom County Extension

Integrated Pest Management for Blueberries

Phytophthora Root Rot

(Phytophthora cinnamomi)

Diseases

 

Symptoms

Phytophthora root rot may occur in poorly drained sites or in low areas of fields.  Early symptoms are yellowing or reddening of leaves and lack of new growth, followed by leaf drop.  Below ground symptoms vary from slight necrosis of young rootlets to extensive necrosis that turns crowns and main roots reddish brown. Plants may eventually die.

 

Life History

The pathogen lives in the soil and produces swimming spores that infect the roots.  Hardy chlamydospores (the primary overwintering structures) are released into the soil as the affected roots break down.  Abundant soil moisture and temperatures between 68°F and 90°F (20°C -32°C) promote disease development.

 

Monitoring

Plants with root rot may take several years to show noticeable symptoms.  Pay attention to plant vigor, especially during periods of hot temperature.  Monitor more closely those areas with low drainage.

Pull plants suspected of Phytophthora root rot and inspect roots for symptoms.   

 

Thresholds and Management

No thresholds exist; decision making around this pest complex is complicated.  Growers should consider age of planting, size of effected area, and variety.

The best control strategy involves prevention.  Either avoid planting in poorly drained sites and select fields that have naturally good drainage or improve drainage by tiling.  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 and do not draw water from sources that are known to be infested with P. cinnamomi.   Use effective systemic fungicides (will not cure severely diseased plants) as a drench around the base of infected plants as a disease repressor. Stresses such as over-fertilization with nitrogen and herbicide injury seem to accelerate the death of infected plants.

 

Resources

Michigan State University, Michigan Blueberry Facts, Phytophthora
http://www.blueberries.msu.edu/phytophthora.htm

Oregon State University Extension, An Online Guide to Plant Disease Control, Blueberry-Root Rot (Phytophthora)
http://plant-disease.ippc.orst.edu/ShowDisease.aspx?RecordID=186

 

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