By the Chemtura AgroSolutions team of experts
Playing catch-up with powdery mildew is usually a losing proposition. Preventing this costly disease is a better course of action. That’s because it can lie in wait for weeks, showing no hint of danger to your crop, then flare up suddenly when conditions are right.
Here’s a quick refresher on how this disease acts in your vineyard.
The fungus survives the winter as dormant mycelium in plant buds or as chasmothecia (spore structures). Chasmothecia are the most important sources of overwintering infection. They mature in late summer and fall on infected green tissue and are washed onto the cordons and spurs with fall and winter rainfall.
In spring the overwintering chasmothecia produce spores called ascospores, which are the primary source of infection. Ascospores are released when 0.1 inch of rain or irrigation is followed by 13 hours of leaf wetness and temperatures are between 50° and 80° degrees F.
Symptoms of powdery mildew infections include red blotchy areas on dormant canes. On leaves initial symptoms appear as pale, yellow or yellow-white spots on the upper leaf surface. Signs of the pathogen appear a short time later as white, webby mycelium. As spores are produced, the infected areas take on a white, powdery or dusty appearance. On fruit and stems the pathogen appears as white, powdery masses that may colonize the entire berry surface.
The rule of thumb is, beginning seven to 10 days after this initial infection, monitor vineyards for the presence of powdery mildew by collecting 10 to 15 basal leaves from 20 or so vines at random. Then examine the undersurface for powdery mildew spores. If spores are found, monitor disease development by using the powdery mildew risk assessment index from the University of California found here.
Experts suggest application of control and management tools should begin promptly at the start of the season and repeat at appropriate intervals. Viticure® is a systemic sterol inhibitor fungicide that’s labeled for protective and curative control for powdery mildew. It works by inhibiting development of cell walls in the target organism, which, without cell walls, cannot survive. The product can be applied in rotation throughout the season. Viticure also suppresses black rot and Botrytis bunch rot.
Also be sure to visit with your PCA or Chemtura AgroSolutions representative for additional ways to ensure a successful season:
• Mike Ansolabehere, Southern San Joaquin Valley: (661) 304-3023 or email Mike.Ansolabehere@Chemtura.com
• Matt Loftus, Central San Joaquin Valley: (559) 960-1112 or email Matthew.Loftus@Chemtura.com
• Eric Leer, North Central California: (209) 531-6478 or email Eric.Leer@Chemtura.com