Downy Mildew of Grape: An Overwintering Pathogen

Downy mildew (Plasmopara viticola) is a major disease of grape here in Kentucky. Infection by this pathogen can weaken plants, infect fruit and buds, and reduce photosynthesis.

Downy mildew disease symptoms develop in spring during warm, wet weather when temperatures range from 65 to 75 degrees. Fungal masses then begin to grow across plant surfaces, infecting tissue along the way. Symptoms usually subside when summer temperatures become hot and dry, but the disease quickly returns when temperatures begin to drop again in the fall.  

The pathogen, however, does not disappear in winter. It can survive mild winters in infected dormant canes and begin to grow again in spring (sporangia).
During this time of year, the pathogen also generates specialized overwintering spores (oospores). Leaf litter on the vineyard floor creates the ideal environment for winter survival of these spores.

It is a misconception that downy mildew is a late season disease. Very early in the spring, when temperatures begin to creep into the 50’s, the overwintering spores germinate on the ground, new spores (zoospores) splash up onto vines, and infection occurs. This infection can remain inactive until temperatures rise above 65 degrees, and the disease cycle begins once more.

 Control of downy mildew begins with removal of leaf litter. This helps eliminate early sources of the fungus. Fungicides can be applied after harvest to destroy spore-producing fungi, and dormant sprays can be used before bud break to help eliminate any pathogen that may have overwintered in canes.

 Understanding a pathogen’s life cycle is the first step in maintaining a disease-free vineyard. Control of downy mildew before it becomes a problem is the best way to manage the disease.

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.

Soybean Cyst Nematode in Nurseries

Nursery growers who ship plant material across state lines should be aware of regulations enforced by national plant protection standards.  In Kentucky, one such regulated pest is the soybean cyst nematode (SCN). 

So, what if your nursery doesn’t grow soybean? 

 SCN can infest weeds in nurseries, which subsequently can be transported in ball-and-burlapped materials.  Exports to states such as Pennsylvania may require State Phytosanitary Certificates (www.kystateent.org).

Publication ID-110 provides more information on management recommendations for nurseries and common weed hosts of SCN.