Vine Lines: California’s Weather In 2011
January 10, 2012 In 2011, California had a grape growing season much like 2010. Both years were characterized by below-normal seasonal temperatures, which resulted in very healthy appearing grapevines with crops that matured later than “normal.” Our winter and spring rainfall provided us with good early season soil moisture levels, and cooler springtime temperatures set the stage for vigorous vine growth (just like in 2010). The vines responded with great growth and you frequently heard growers say, ”that vineyard hasn’t looked this good in 20 years” (just like in 2010).
When we look at the weather data for the 2011 season, it supports the appearance and development of the vines. The season started off with reasonably high levels of soil moisture. Both 2010 and 2011 followed several years of drought in California’s grape growing regions.
Bud Break To Bloom To Veraison
Bud break occurred two to three days later than normal in 2011. Following bud break, temperatures were mostly cooler than normal, which resulted in vigorous vine growth. When we reached the time when bloom would normally occur, we were eight days later than a normal year in the San Joaquin Valley, seven days behind on the North Coast, and 13 days behind on the Central Coast (not quite as far behind as in 2010). The days that we were behind are based on cumulative growing degree-days, which are a good overall measure of vine physiological progress during a season.
Bloom occurred eight to 10 days later than normal in the San Joaquin Valley and 10 to 15 days later on the Central Coast. These delays in physiological development carried through to veraison.
Veraison To Harvest
From mid-summer through the fall, temperatures were closer to normal in the San Joaquin Valley, which resulted in a crop that was only about 10 days behind a normal year and about 15 days behind in the North Coast. However, very cool fall temperatures pushed the Central Coast much further behind. In addition to our problems with cooler than normal temperatures, many vineyards had larger crop sizes, which further delayed maturity.
So in the end, as we look back at 2011 in the San Joaquin Valley, because our temperatures were a bit higher than in 2010 we ended up with a slightly higher growing degree-day total. However, 2011 had a degree-day total that was one of the lowest of the last 20 years.
Powdery Mildew In 2011
As if a delayed and extended growing season weren’t enough for California growers, most areas had temperatures that favored the development of powdery mildew. A measure of powdery mildew pressure is the powdery mildew index (PMI). The PMI is calculated based on the vine in-canopy temperature residing at 70ËšF to 85ËšF for six hours or longer on a given day. The calculated PMI is a cumulative number during the season that can be rated as resulting in high, intermediate, or low powdery mildew pressure.
A “high pressure” PMI rating means that growers must treat for powdery mildew on a high frequency — with dusting sulfur at a maximum of seven days or if a product lists a treatment interval of 12 to 14 days, you do not exceed 12 days. Missing a day or two can have consequences and missing by three or four days can result in a significant infection.
One way to look at the potential for powdery mildew to be a problem for growers is to examine the number of consecutive days with “high pressure” and the last three years have been troublesome for growers in the San Joaquin Valley. During each of these three years, growers had to be on a rigid schedule for powdery mildew treatment for approximately 60 consecutive days. The only real break for some growers was if they happened to be growing a grape variety that is less susceptible to powdery mildew.
For the 2012 season, if you want to monitor the progress of degree-day accumulations you can go to our web site at www.CalAgQuest.com. Degree-day information is updated each Monday.
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