One of these days, when spring- time temperatures threaten to slip below the freezing mark, instead of turning on their overhead sprinklers or fans, some California growers may be protecting their vines from frost damage with the help of a beneficial bacteria.
Glenn McGourty, University of California Cooperative Extension farm advisor for Lake and Mendocino counties, and Professor Steve Lindow, plant pathologist at the University of California–Berkeley, have set up field trials this spring as part of their research on the role of bacteria in fending off frost in the vineyard.
This works stems from Lindow’s research, dating back to the 1980s, on the use of bacteria to protect pears and citrus trees from frost and fire blight.
A vineyard or orchard with bare soil is less likely to freeze than one where a cover crop is growing. Soil moisture allows the soil to absorb and hold heat. However, during the day, any vegetation on the vineyard or orchard floor reflects more sunlight from the surface and increases evaporation. This reduces the amount of heat energy stored in the soil, leaving less energy available to radiate heat up to the vine or tree and help protect it from frost damage. The taller the vegetation, the greater the risk of injury, In fact, that’s why McGourty advises growers to mow any cover crop in their dormant vineyard very short or disk it in prior to budbreak.
But there’s another reason vines or trees are more prone to frost injury in the presence of a cover crop – ice nucleating bacteria on the leaves of the vegetation, which act as nuclei for formation of ice crystals from moisture in the air.
McGourty and Lindow are studying the behavior of one species of such ice-nucleating bacteria, Pseudomonas syringae, This bacterium, found throughout the environment, ranges from high up in the atmosphere, where they cause snow and rain to form, to the earth’s surface, where, blown around by the wind, they move about. Some plants, particularly grasses, harbor more than others, such as grape vines. The surfaces of some plants can contain upwards of 10 million bacteria per gram of tissue, McGourty notes.
“In previous lab and field trials, we’ve reduced the number of these ice-nucleating bacteria on grape vines by spraying them with copper,” he report. “As a result, we’ve been able to supercool the vines to withstand temperatures as much 3 degrees below freezing without any frost damage.”
To put this 3 degrees of frost protection in perspective, consider this:
McGourty has examined vineyard frost damage reports in the Ukiah Valley of Mendocino County over the past 10 years. In 90 percent of cases, frost damage to vines occurred at temperatures no lower than 29.5 degrees.
McGourty’s trials this spring are designed to test the effectiveness of killing Pseudomonas syringae using a copper spray for protecting commercial vineyards from frost damage.
This is the first year of the two-year project, which is being funded by a specialty crops grant from the California Department of Food and Agriculture.
The research is being conducted in a vineyard in the Anderson Valley of Mendocino County and one in the Red Hills area of Lake County. Neither has sprinklers or fans for frost protection.
A block in each vineyard has been divided into six one-acre plots. A cover crop of self-seeding legumes, Blando brome grass and Zorro fescue has been cut short in half of each plot and allowed to grow taller in the other half. In addition, each plot will either be sprayed with copper or left untreated. Four data loggers in the trellis of the vines in each plot track temperatures during the experiment.
Those plots sprayed with copper were treated shortly after the leaves had opened. Since then and until the end of the frost season, the researchers have been testing vines from the plots and in the lab. Copper will be applied about every 5 to 10 days depending on frost risk and shoot growth. If a freeze occurs in the field, vines will be evaluated for damage.
If no frosts occur, data can still be generated in the laboratory. Shoots collected from the vineyard are placed in equipment that simulates a freeze, and the researchers check the temperature at which frost begins to form. They also collect and count the Pseudomonas syringae bacteria from each shoot by plating on sterile agar in the laboratory.
At the end of the season, the researchers will use these numbers to determine any effect of cover crop management and copper treatment on the ability of the shoots to withstand freezing temperatures. Presumably, vine shoots from the copper-treated plots where the cover crop was mowed should withstand colder temperatures before freezing than the others.
Although copper is relatively non-toxic to humans, it can be very toxic to fish, McGourty notes. So, as another part of their frost-protection research, he and Lindow are also looking at a different species of bacteria as an alternative to using copper to control the ice-nucleating Pseudomonas syringae.
Lindow studied the frost-protecting ability of this naturally-occurring bacterium, Psuedomonas fluorescens A5O6, in pears and citrus. This bacterium, which is safe in the environment and protects against frost formation, out-competes Pseudomonas syringae for space to grow, McGourty notes.
In their trials, which are being conducted at the University of California’s Hopland Research and Extension Center in Mendocino County, the researchers are diluting pure cultures of Pseudomonas fluorescens A506 with water and spraying it on cover crops in vineyards to test its effectiveness as a frost-protectant for the vines.
“If we can get A5O6 to colonize and displace Pseudomonas syringae to become the dominant bacterium in a vineyards, growers would have a very sustainable approach to limiting frost damage,” McGourty says.
“This way, growers, particularly those unable to use sprinklers or fans for frost protection, would have two options. They could treat their vineyards with copper sprays to control Pseudomonas syringae. Or, if they’re rather not use copper, they could go with A506.
“Based on our results so far, we think A506 is a good example of a harmless biopesticide. We hope to discover more of them in the future as we continue to develop the technology to work with beneficial bacteria in the field.”