Ag Alert Nov. 3, 2021

Cold injury harms grapes, but recent rains may help

ByDennis Pollock As fall drifted into winter in 2020, a se- ries of unfortunate events set upadelayed punch in the gut for grape farmers in the San JoaquinValley. Byspringof 2021,UniversityofCalifornia viticulture advisors found their ears burn- ingwithcalls fromvalleygrowers frustrated by the sputtering growthof their vines. Matthew Fidelibus, a UC Cooperative Extension specialist in viticulture, ex- plained that theculpritwasaperfect storm — combined with the lack of storms—in the autumn of 2020. Factors that played into theearly-spring problems in 2021 included excessive late-season growth in 2020, enabled by warm fall temperatures. That recipe is ripe for winter cold inju- ry, a crop impact that ismost severewhen freezing temperatures are preceded by warmdays. Fidelibus, with UC Davis and the Kearney Agricultural Research and Extension Center, recently advisedmem- bersof theSanJoaquinValleyWinegrowers Association on steps to avoid having a re- peat of this year’s delayed spring growth. Days after his talk, anatmospheric river coursed throughNorthern California and

the upper Central Valley. The long-over- due showers set a single-day rain record of 5.44 inches in Sacramento on Oct. 24, following 212 days without a drop. “That ’s def initely going to help,” Fidelibus said. The rain that fell on the much of the valley floor was a consider- able improvement over last fall, when no rainwas recorded for October. Fidelibus said, “It’s welcome and good to have soil moisture going into dorman- cy.” But he remained cautious. He said growers should track future rainfall and “ina fewweeks get out anauger andcheck soil moisture then.” California’sdamaging scenario last year came when warmer-than-normal tem- peratures in October were followed by a freeze inearlyNovember. Youngvines and vines thatwerecane-prunedwereparticu- larly prone to this type of cold injury. Fidelibus said the issue was more no- ticeableonyoungvines andcertainvariet- ies and rootstocks, with cane pruning and with overcropping. When meeting with the San Joaquin Valley wine growers, he showed a pho- tograph of a young, cane-pruned vine of Freedomrootstock thathadavirtuallybare cane for a stretch. The same was true of a Sunpreme vine on Freedomrootstock.

Delayed spring 2021 growth is seen in the long open cane on a Sunpreme vine on Freedom root- stock. Behind that vine, a Sunpreme vine on its own rootstock is flourishing with abundant foliage.

But Fidelibus said that vine, which is a prized dried-on-the-vine variety, was less likely to face problems if raised on its own rootstock. “The rainy season has been starting lat- er, and lately we’ve been having fewer big storms,” Fidelibus said. “California’s rainy season is startingnearlyamonth later than it did 80 years ago.” In 2020, he said, October rainfall was zero.Averagefor thatmonthis0.67inch.For November, itwas0.24 inch, comparedtoan average of 1.42. For December, it was 0.63, compared to an average of 2.05. Fidelibus saidgrowers should take stockof soilmois-

ture in late fall and early winter and see if winter irrigation is necessary. He said vari- etiesof grapes susceptible todelayedspring growth include Thompson seedless, crim- son seedless, Redglobe, SelmaPete, Fiesta, Sunpreme, chardonnay,merlot, grenache, cabernet sauvignonandsauvignonblanc. Freedom and Harmony are especially susceptible rootstocks. Fidelibus explained that normal growth fromdormant buds requires rehydration and “vascularization.” He said dormant buds are dehydrated and have relatively weak vascular connections with the vine.

See GRAPES, Page 11

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