Disease Notes Overview The warm and humid growing season of the mid-Atlantic region presents challenges to grape growers, especially if diseases find their way into your vineyard and are allowed to spread. The adage that an ounce of prevention is worth a pound of cure is worth remembering when it comes to controlling these diseases, as many of them have the ability to overwinter in the vineyard. The cleaner (more disease-free) that you can keep your vineyard this year will save you much aggravation and $$ the following year. The following notes aim to help you to understand the conditions that favor the spreading of each disease. The better that you know your grapevine’s enemies, the better you’ll be able to recognize conditions that they favor. Some – like powdery mildew – thrive in warm-to-hot conditions and high humdity (but not leaf wetness), while others like downy mildew like raindrops for initial spread and then like at least 4 hours of overnight leaf wetness for their secondary spread. Another appropriate saying: Know Your Enemy! One of the best pathology resources was authored by Wayne Wilcox, Department of Plant Pathology, Cornell University Grape Disease Control (pdf). Explanations of each disease is well-written and easy to read, and while the pesticide recommendations are a little past their prime, his methodology in Spray Programs: Putting it All Together make this paper worth the read. Powdery Mildew Pictures The fungus overwinters as tiny fruiting bodies that form on leaves and clusters during late summer and autumn. Fall rains wash these onto the cordons and trunks where they survive all winter. In the following spring these fruting bodies produce spores that spread during the bud break to bloom stage to initiate the disease. This primary infection period produces millions of new secondary spores, which can spread the disease rapidly via multiple repeating cycles of infection throughout the season. Berries are very susceptible to infection during just before bloom through fruit set. Then the berries will begin to lose susceptibility and will eventually become resistant (in about 4 weeks ?). Keep in mind that early infections - even somewhat minor ones - can increase the likelyhood of Botrytis near harvest. Downy Mildew Pictures Downy spores can persist in the soil from the previous season’s infected leaves and mummies. Again, more infection last year, more this year. Then in the following spring when the temperature gets above 52F and the first 0.3”+ rainfall event occurs, the spores are splashed up into the canopy or suckers. Thus begins the primary infection. Then one of two things can happen. If the following weather is dry and sunny most of the spores will perish (not certain about the shaded spores not in direct sunlight). But if it stays cloudy until there’s a warm and humid night (4+ hrs of leaf wetness in darkness) the spores will germinate (secondary infection) and begin to develop. Within days this infection period can get crazy very fast on untreated vines. Black Rot Pictures Mummified berries are by far the major overwintering source of the black rot fungus. Mummies on the ground are much less likely to spread the disease so be sure to remove all that are still hanging when pruning! Early sprays are important for two reasons. 1)Berries become resistant about two to four weeks after bloom. Sprays after this time do very little. 2)The incubation period for black rot infections take several weeks. So it could take until July before a problem is noticed. Again, by then the damage has been done. Botrytis Pictures Botrytis attacks injured, dying and dead tissue. Any damage to the berries, be it from either insects (like grape berry moth larvae), or rain-splitting of tightly-bunched clusters, or early-season powdery mildew damage, gives botrytis a foothold. Evidence of this won’t be evident until the berries begin to ripen, which in our area coincides with periods of high humidity. Keeping the canopy open and airy helps greatly. Phomopsis Pictures Rachis infection by the Phomopsis infects the rachis early in the season when shoots are under 3 inches in length, around the time that the cluster are emerging. Early sprays offer the most important control of phomopsis infections. Ripe Rot Pictures Ripe rot is a bunch rot that occurs late in the season, similar in timing and appearance to bitter rot. Berries infected with ripe rot initially show brown, water soaked spots (on white cultivars), followed by the concentric growth of bumpy fungal structures on the berry surface, and lastly, shrivelling. The fungus can overwinter in mummified fruit, then in the following season, it can spread by splashing rain. Management methods for this disease include sanitation, pruning, and fungicides. The highest incidence of ripe rot occurs at temperatures between 27 to 32 C (80 to 90F) during long periods of leaf wetness. And as can be guessed by its name, as berries ripen they become more susceptible. So it appears that early season infections may not be a reason for concern.