Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Wayne Wilcox, Department of Plant Pathology & Plant-Microbe Biology,
Cornell University, NY State Agricultural Experiment Station, Geneva, NY
astern North America is the ancestral home of powdery mildew (PM), a disease that we fight and research intensively. There are obvious climatic differences between the eastern and western viticultural areas of the continent, but basic biological principles are universal. Applying them to local control programs, however, sometimes needs adjustment. The following summarizes some key points about the biology of this disease and its control in the eastern region, including some recent research findings.
1) As most growers know, the fungus overwinters either within infected buds (mild winter climates only) or as fruiting structures (cleistothecia) that form on infected leaves and clusters, from which they typically wash onto the bark of the vine and persist until initiating infection the next spring. Thus, the amount of fungus capable of starting disease this year is directly proportional to the amount of disease that developed in the previous year.
To illustrate the practical effect of this, we conducted an experiment in a Chardonnay vineyard where we either a) kept the vines squeaky clean
Pre-veraison (left) and pre-harvest (right) symptoms of Chardonnay clusters and young leaf severly infected with powdery mildew.
throughout the growing season; b) quit spraying midsummer to let a moderate level of mildew develop; or c) quit spraying in early summer, to allow rampant disease development.
When we intentionally applied a minimal spray program to these vines the following year (starting late and stopping early), the resulting cluster disease severities were a) 11%, b) 22%, and c) 48% of the cluster area infected, respectively, even though all were sprayed exactly the same during the second season. Conclusion: Higher disease in Year 1 led to more primary infections to start off Year 2, which meant that when the fruits were formed and highly susceptible to infection, there were many more new (“secondary”) spores. This resulted in much heavier disease pressure to “overwhelm” the fungicide spray program.
Several potential strategies can help those who had serious mildew in 2011. One is to apply lime sulfur during the dormant season to kill many of the cleistothecia on the vines. Unfortunately, this is an expensive and unpleasant task. In the eastern region,
it is generally more economical to instead put some of what the dormant lime sulfur application would cost into beefing up the spray program once vine growth begins. Do not push things — start a little earlier than you normally would, do not stretch intervals, do not cheat on rates. But every vineyard is different.
2) Temperature is the most important weather component affecting disease development, which is why it is the heart and soul of the successful UC Davis Risk Assessment model. But it is not the only one.
For example, even though PM can develop under an extremely wide range of humidity, high relative humidity (RH) increases disease severity. We found that disease severity doubled as RH increased from 40% to 80%, which was near the optimum level in our studies. German pathologists claim an optimum of >70% RH, those at UC Davis a little bit lower. We all agree on the basic concept that whereas mildew does not need humid conditions to develop, it does thrive under them.
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