Monday, October 10, 2011

Crown Rot of Strawberry

Strawberry growers in KY have reported losses of up to 25% in new strawberry plantings this month due to crown rot issues.  Incidentally, last month a commercial strawberry supplier announced possible shipments of transplants and plugs infected with the pathogen that causes Phytophthora crown and root rot.   Several Kentucky strawberry growers submitted plant samples UK Plant Pathology Diagnostic Labs for diagnosis of this crown rot disease. Meanwhile, many growers applied the Phytophthora-specific fungicide Ridomil to their fields.  In spite of the assumptions, diagnosticians confirmed that all of the samples had Anthracnose crown rot. 

Various pathogens can cause rapid loss of new transplants, and as we saw in this example, the diseases may have very similar symptoms.  Correct diagnosis is critical for effective control.  Below are brief descriptions of some common diseases of strawberry transplants.
1.       Anthracnose Crown Rot – caused by the fungus Colletotrichum fragariae.  Dormant spores can remain on plant tissue until ideal conditions arise.  Warm, rainy/irrigated, humid conditions cause the fungus to begin growing again.  Upon reinfection of plants, the fungus produces new growth and sporulates.  These spores splash up and infect crowns, as well as petioles, leaves, and fruit.  As crowns are infected, plants wilt and die suddenly.  If crowns are cut lengthwise, crowns are firm, and red streaking or marbling is apparent.   Roots usually appear healthy.  Abound (azoxystrobin), Cabrio (pyraclostrobin), or Pristine (boscalid + pyraclostrobin) are effective fungicides.  Control of this pathogen is important to reduce disease incidence in spring.  Apply fungicides as foliar sprays, through drip irrigation, or as a transplant dip according to label instructions.

2.       Crown Rot, Phytophthora Crown Rot – caused by the water mold Phytophthora cactorum.  Warm wet conditions, as well as poorly drained soil encourage infection. This pathogen causes crown and collar rots, leaf blights, and fruit rots in strawberry and various other plants.  Symptoms begin in the upper part of the crown, with youngest leaves wilting first.  Above ground symptoms appear before roots are affected.  Upon splitting the crown, inner tissue is extensively soft, brown, and necrotic.   Ridomil (mefenoxam) may be applied as a drench application, while Aliette or ProPhyt (phosphite) can be applied as foliar spray or transplant dip according to label instructions. 

3.       Red Stele – caused by the water mold Phytophthora fagariae.  This water mold requires wet conditions to establish and infect.  Cool wet conditions are ideal for this pathogen, at which time it can extend throughout root systems quickly.  Water runoff and farm implements can spread the pathogen from plant to plant or from field to field.  Roots begin rotting from the ends, infecting upward toward the crown.  Cutting roots lengthwise reveals the red-colored stele.  Do not plant strawberry where Red Stele occurred in the past.  When conditions become hot and dry, this pathogen can produce survival structures that are heat and fungicide resistant.  Applying Ridomil (mefenoxam) to soil before planting can eliminate a significant amount of inoculum.  Alternatively, Ridomil can be applied through drip irrigation systems shortly after planting.  Phosphites such as Aliette or ProPhyt are effective as transplant dips or as preventative foliar applications only.  Follow label instructions for rates and other application regulations. 

Disease management begins with prevention.  Purchase disease-free transplants.  In the event of disease, consider field conditions and symptoms, then contact your county agent for advice.  Keep in mind that proper diagnosis is critical for disease control.  Your agent can assist with submission of plant samples to the UK Plant Disease Diagnostic Lab.  More information on strawberry disease control can be obtained from PPFS-FR-S-05 and ID-44.

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