Use of copper and stylet oil to control ice nucleating bacteria as a frost protection strategy in a Northern California vineyard

Abstract

Frost occurs in vineyards when temperatures fall below 0 degrees Celsius and ice nucleating bacteria are present to catalyze the formation of ice crystals on green tissues. It has been shown in the laboratory that in the absence of these bacteria, plant tissue can supercool to as much as 6 degrees C below freezing (0 degrees C). There are at least 6 genera of bacteria that are known to nucleate ice, but Pseudomonas syringae is most commonly associated with frost events in studies on other crops. In this study, we investigated the relative efficacy of fixed copper compounds alone or with paraffinic oil (Stylet Oil) to control ice nucleating bacteria on grape vine tissue and reduce the potential for freezing damage. A replicated trial covering 1.5 ha in a Syrah vineyard in Ukiah, Mendocino County, we compared sprays of cupric hydroxide (2.1 kg/ha); cupric hydroxide (2.1 kg/ha) plus “Stylet” paraffinic oil (6.0 l in 300 l water per ha) and an unsprayed control. These treatments were applied to grape vines in sub plots with the presence or absence of vineyard floor vegetation, a known source of ice nucleating bacteria that can contaminate emerging green grape vine tissue, for a total of 6 treatments replicated 3 times. In the laboratory, frost susceptibility of the plants was estimated by measuring the supercooling point of detached leaves and shoots in test tubes immersed in a chilled ethanol bath. Bacterial populations were determined for each treatment by washing leaves in a phosphate buffering solution and plated on a 10% TSA media. Grape shoots exhibited substantial supercooling ability compared to other crops such as almond and pear, and most likely are not a good substrate for ice nucleating bacterial growth. Ice nucleating bacteria carried by wind from weeds in adjacent areas probably contaminate new grape vine tissue as it emerges from dormancy. There were no frost events in the vineyard plots in 2014. In the lab tests, freezing temperature of the leaves treated with copper or copper/paraffin oil was significantly lower than the control treatments by between –