Raisin growers start season early with early disease pressure

Raisin growers start season early with early disease pressure

It’s not only the vine growth that’s ahead of schedule. Zhuang expects disease and insect pressures will develop sooner than usual, too. What’s more, stress on the vines from the continuing drought along with the limited supplies of surface water and good quality groundwater that many growers are facing is likely to weaken the ability of vines to withstand disease pathogens and insect pests.

By the first of April, the shoots in Thompson seedless and other raisin-type vineyards in Fresno County, the heart of California’s raisin industry, had reached approximately 18 to 24 inches in length. This followed bud break in the first week of March, reports George Zhuang, the county’s University of California Cooperative Extension viticulture farm advisor.

“Because it’s been such a warm year so far, the grape crop is about two weeks more advanced at this time than normal,” he says.

It’s not only the vine growth that’s ahead of schedule. Zhuang expects disease and insect pressures will develop sooner than usual, too. What’s more, stress on the vines from the continuing drought along with the limited supplies of surface water and good quality groundwater that many growers are facing is likely to weaken the ability of vines to withstand disease pathogens and insect pests.

The most immediate disease concern in the raisin vineyards is powdery mildew, Zhuang notes. The stage of vine growth and warm temperatures had combined to heighten the risk of this fungal disease in the early days of April. Once initial infection occurs, ideal temperatures for growth of the fungus are between 70 degrees and 85 degrees F. Temperatures above 95 degrees F for 12 continuous hours or longer cause the fungus to stop growing.

“Some growers have already treated their vineyards for powdery mildew,” Zhuang says. On the third of April, the Risk Assessment Index, developed by UC IPM researchers, stood at 80.

“That’s a pretty high level,” he says. “It indicates that the fungus will reproduce every five days. In that situation, the UC IPM guidelines recommend applying sulfur every seven days to control powdery mildew.”

Any rain in early spring would increase the threat of Phomopsis cane and leafspot. The disease first appears three to four weeks after rain as tiny dark spots with yellowish margins on leaf blades and veins. However, with the advanced growth, its damage is less likely since the fungal structures are protected by the foliage from direct rain drop contact and splashing.

More information on these two diseases is available on the UC IPM website www.ipm.ucdavis.edu

Based on early-season conditions, Zhuang is advising raisin growers also to be on the lookout for higher-than-usual pressure from spider mites, which thrive in hot, dry and dusty conditions in vineyards. The vine mealybug is another pest to keep an eye on, he adds. The insect has posed problems for some growers in the past several years. Zhuang expects that to be the case, again, this season.

As California enters the fourth straight year of drought, irrigation management will continue to play a critical role in determining raisin grape yields and quality, Zhuang notes.

The UCCE and the California Department of Water Resources have teamed up in a trial program to help growers of grapes and other permanent and annual crops in Fresno, Tulare, Madera and Kings Counties to schedule irrigations in a more accurate, efficient and timely manner. Called ETc Etc., this service will use data collected from the network of California Irrigation Management Information System (CIMIS) automated weather stations in these counties to provide real-time regional estimates for crop evapotranspiration (ETc) in alfalfa, almonds, cotton, grapes, tomatoes, tree fruit and walnuts.

Growers will receive weekly crop water use updates via email that include estimates, in inches, of ETc from the prior week and recommended water application amounts based on field application efficiency levels, as determined by the growers. During the year, growers will also receive articles related to water use by individual crops to supplement the weekly ETc estimates.

To register for these email/smart phone updates email your county viticulture advisor and indicate whether you wish to receive updates for permanent crops and/or annual crops and your location, either west side and/or east side region (west or east of Kerman).

For more information about ETc Etc. contact:

-Fresno County: George Zhuang [email protected] and Dan Munk [email protected]

-Madera County: Lindsay Jordan [email protected]

-Kings and Tulare Counties: Allison Ferry-Abee [email protected]

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