By Darin Allred, Ph.D.; Technical Sales Support; Chemtura AgroSolutions
There is no magic to managing against vineyard disease challenges. It takes time and attention to achieve success. That means when it comes to staying on top of powdery mildew, there’s no substitute for regular scouting.
Powdery mildew is the most serious and widespread disease of grapes in California in terms of expense to control, reduction in quality and losses in yield. Infection of the rachis or grape stem shortens storage life of table grapes. Wine quality can be negatively affected when as few as 3% of the wine berries are infected. Severe mildew can crack berries, which allows rot organisms to enter and then spread to other grapes in the bunch which reduces yield.
Symptoms of powdery mildew on dormant canes are visible as red blotchy areas. Initial symptoms on leaves appear as pale, yellow or yellow-white patches about 0.25 inches in diameter on the upper leaf surface The pathogen appears as a white, web-like mat of mycelium shortly thereafter. The infected areas take on a white, powdery or dusty appearance as spores are produced. The pathogen appears as white, powdery masses on fruit and stems, and 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 about 20 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.
During rapid shoot growth look for flagging (wilting) shoots or shoot tips. If flagging is found, try to determine if it is due to branch and twig borer, Botrytis, or powdery mildew.
Newly emerging shoots displaying malformed foliage, stunting, and a white powdery or dusty appearance indicate an overwintering bud infection by powdery mildew.
Watch this video for more insight into managing powdery mildew in your vineyard this season.
Experts suggest application of control and management tools should begin promptly at the start of the season and repeated at appropriate intervals.
Be sure to include Viticure® fungicide, a systemic sterol inhibitor fungicide that’s labeled for protective and curative control for powdery mildew in your management plan. It works by inhibiting development of cell walls in the target organism, which, without cell walls, cannot survive. Viticure can be applied in rotation throughout the season, and is an excellent tool to help prevent fungicide-resistance issues. It also suppresses Botrytis bunch rot.
Also be sure to visit with your PCA or Chemtura AgroSolutions™ representative for additional ways to ensure a successful spraying season:
• Mike Ansolabehere, Southern San Joaquin Valley: (661) 304-3023; email Mike.Ansolabehere@Chemtura.com
• Matt Loftus, Central San Joaquin Valley: (559) 960-1112; email Matthew.Loftus@Chemtura.com
• Carson Conover, North Central California: (530) 906-1504; email Carson.Conover@Chemtura.com
• Eric Leer, North Central California: (209) 531-6478; email
• Eduardo Garcia, California Coast: (805) 625-4101 or email Eduardo.Garcia@Chemtura.com