Issue link: https://read.dmtmag.com/i/56967
VINEYARD wo fungal dis- eases of pri - mary concern in global viticulture are powdery mildew, caused by Erysiphe necator, and Botrytis bunch rot, caused by Botrytis cinerea. Com- bating these diseases successfully takes a diligent, year-round strategy comprised of a variety of cultural and chemical practices. Powdery mildew is the most destruc- tive grapevine disease, and grapes cannot be grown in many regions of the world without effective control of this pathogen. Pow- dery mildew infections reduce yields, nega- tively impact grape quality, and cause severe economic loss. Erysiphe neca- Powdery mildew is one of the most destruc- tive grapevine diseases. Photo: Jack Kelly Clark, courtesy University of California Statewide IPM Program. PREVENTING POWDERY MILDEW tor overwinters as hyphae or as spore- bearing structures called chasmothecia in bark, canes, leftover fruit and leaves on the ground. Both hyphae and chasmothecia, through the release of sexual ascospores, can serve as inoculum for infection dur- ing the growing season. The fungus also produces conidia (asexual spores) that increase the severity of the disease on infected plants and spread the fungus throughout the season. Powdery mildew control is dependent upon reduc- Strategies for developing an effective program By Remi Cohen AND BOTRYTIS vention program start- ed in the spring, after bud break, is adequate. Most growers choose to begin spraying when foliage has grown to about 4-6 inches, and continue spraying throughout most of the growing season, until the grapes are no longer susceptible to infection. Grapes are reported to be suscep- tible until about 8 °Brix, but established colo- nies are reported to sporulate until 15 °Brix, so treatment in higher- risk areas should be applied until then. It is best to take a preventative approach to powdery mildew management, as eradi- cation can be difficult and costly. The most critical decisions to be made in the spray program are the intervals between sprays and the choice of materials. FUNGICIDE RESISTANCE Powdery mildew has been known to genetically ing spring inoculums and subsequent infection. Therefore, treatment must begin at the start of the growing season and be repeated throughout the season at appropriate intervals. Timing of the first treatment depends on the program you are using and the growth stage of the vines. Fre- quency of treatment thereafter depends on the fungicides you choose in your rotation, weather conditions and disease pressure. For most programs, getting the disease pre- 42 VINEYARD & WINERY MANAGEMENT MAR - APR 2012 AT A GLANCE Powdery mildew can negatively affect yields and grape quality. Treatment must start at the beginning of the growing season, and a preventative approach is best. It makes sense to incorporate Botrytis prevention into your powdery mildew prevention program. When applying any type of fungicide, adequate coverage is essential. mutate and develop resistance to fungicides, so resistance management is essential for an effective long-term strategy. There is an organization called the Fungicide Resistance Action Committee (FRAC) that is dedicated to provid- ing fungicide resistance management guidelines to prolong the utility of fungicides and prevent crop loss. FRAC has developed code num- bers (see www.frac. info) that correspond to the different classes of chemicals, based on their mode of action. This information can be used to select chemi- WWW.VWM-ONLINE.COM