Glenn McGourty,* Stephen Lindow, Mark Battany and Ryan Keiffer
*UCCE Mendocino County, 890 North Bush Street, Ukiah, CA 95482 (gtmcgourty@ucanr.edu)

Grape vine tissue can super-cool to -5°C without damage in the absence of ice-nucleating bacteria, which are catalysts for the formation of frost. Ice-nucleating bacteria are ubiquitous in herbaceous and woody vegetation adjacent to vineyards. They readily colonize seedlings germinating on the vineyard floor in the fall and winter. As green grape tissue emerges in the spring, bacteria migrate from cover crops to the vines. We investigated the use of copper sprays, chemical and mechanical mowing of cover crops to suppress ice-nucleating bacteria as a strategy to prevent frost in two 6-ha plots during the 2017 and 2018 growing seasons. No frost occurred in the field during the trial years. One plot was in a Chardonnay vineyard in the Anderson Valley AVA, and the other was a Cabernet Sauvignon vineyard in the Lake County Red Hills AVA. Bacterial populations were very different between sites. Chemical mowing (glyphosate at 1.2 l/ha) and four weekly copper sprays (copper hydroxide  at 1.6 kg/ha.) significantly reduced bacterial populations on emerging grape foliage, averaging 1000 colony forming units (cfu) in copper-treated plots compared to 1,000,000 in the control. Vines growing in areas of the vineyard that were chemically mowed had a similar effect in suppressing foliar bacteria on vines. Under temperature-controlled conditions in the laboratory, fresh grape foliage sampled from copper-treated plots could super cool almost -3°C lower than samples from the control plots without freezing. By contrast, in the Red Hills plot, bud break occurred mid-April and total bacterial populations were much lower (100 fold) in all treatments. There was no significant treatment effect. Our results suggest that copper sprays and suppressing vineyard floor vegetation can be an effective strategy for frost protection where budbreak is early.

Funding Support: California Dept. of Food and Agriculture Specialty Crop Block Grant