Grape growers use weather data to protect their vines
“Dry soil makes freeze damageworse,” Zhuang said. Spring frost can cause mild damage to leaves or shoots, or severe damage that kills shoots and results in yield loss. Zhuang said the extent of dam- age depends on locat ions of vine- yards, and can be most extensive in low spots. Temperatures above 100 degrees can result in heat stress, leaf sunburn and berry sunburn. The stress can reduce yield and fruit quality. It’s important to
manage irrigation with drip or in-can- opy mist prior to heat events, Zhuang said; sunblock can also be sprayed prior to heat events. In advance of the baking days of sum- mer, Zhuang touched on the importance of heat illness prevention to protect farm employees, and the need to take protec- tivemeasures based on air temperatures and relative humidity. He said he hopes todevelop anup-to-the-minute, easy-to- use UC IPMHeat Index Alert.
ByDennis Pollock Tracking the weather can be a key to fighting vineyard pests, calculating water needs and avoiding crop dam- age—and weather stations scattered around the state play a significant role in that process. TwoUniversity of California viticultur- al farm advisors discussed the value of information those stations provide, and pointed the way to accessing key data without having to resort towrestlingwith semi-complicated formulas. George Zhuang, UC Cooperat ive Extension farm advisor for Fresno County, opened a San Joaquin Valley Winegrowers Association virtual work- shop by saying the benefits of weather stations include their role in following disease andpestmodels, tracking chilling hours andproviding alerts for freeze, frost and heat stress. They also, of course, track precipi- tation and can be used to prevent heat illness, alert growers to impacts from wildfire smoke and track growing de- gree days. Each station tracks wind direction and speed, air temperature, precipitation, light intensity, leaf wetness, relative hu- midity and canopy temperature. Theweather stationshavebeenaround for some 30 years, Zhuang said, and all of those in Fresno and Madera counties were upgraded last year with new data- loggers and new batteries. A key source of information the sta- tions contribute to is the UC Davis PowderyMildewRisk Assessment Index. It is developed by following a formula that takes into account canopy tempera- tures between 70 and 85 degrees, which allow for rapid growth, and above 95 de- grees, which inhibit spore germination. Temperatures above 105 degrees can kill the fungus. The index is based on disease pressure and shows the likelihood of a pathogen being present and reproducing. It sug- gests treatment intervals based on the mildew risk assessment. But the index is not “a silver bullet,” Zhuang said. “It’s not a substitute for field observations.” It is, however, a tool for understanding the biology of powdery mildew and its growth in a vineyard, and it is a tool to help control powderymildewby defining optimal times to apply fungicides. Zhuang said following the index “will save you time and money, but it takes time to learn.” The index can be reached online at ipm.ucanr.edu/WEATHER/index.html. At that page, the user can select “Grape powdery mildew index.” From there, the user can click on a county and find a weather station within it, and then will have access to the current index. Zhuang said grapevines require cold temperatures to break bud dormancy and approximately 150 chilling hours.
“Not enough chilling hours results in erratic and non-uniform bud break and uneven cluster ripening at harvest,” Zhuang said. At the same time, dormant freeze dam- age can also cause delayed and erratic budbreak, stunt shoot growth and dam- age vascular tissues.
See WEATHER, Page 14
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May 12, 2021 Ag Alert 13
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