Grapevine trunk disease and the suite of pathogens that cause it are known to be problematic in California and the Pacific Northwest. So much so that wine grape growers in some cases have elected to remove and replant vineyards because of the yield loss.

Alix Whitener, a technical services manager with FMC who covers the Pacific Northwest, says the disease symptoms result from complexes of systemic wood-infecting fungi, bacteria, and even viral pathogens. It's not just wine grapes that are affected, but fruit and nut trees are also susceptible to damage.

Addressing grape vines specifically, Whitener pointed to several pathogens, including Eutypa lata, an aggressive fungal pathogen, that researchers in California say is responsible for upwards of $260 million per year in losses.

In a webinar designed to educate growers on the disease, Whitener, and a colleague used the information to report on an FMC fungicide that university testing reveals works well against the various pathogens responsible for systemic wood diseases.

Rhyme fungicide by FMC is registered in grapes as a Group 3 fungicide. According to Issa Qandah, technical services manager with FMC in central California, research trials report the effectiveness of the fungicide when applied at label rates.

Qandah touts the efficacy of Rhyme fungicide by reporting that its active ingredient properties differ from other products. He said this makes the active ingredient more mobile in the xylem of the plant. Oregon State data suggests that the active ingredient is six times more mobile in the xylem than the next-leading active ingredient.

Understanding the problem

George Zhuang, viticulture farm advisor with the University of California Cooperative Extension, said the diseases including Botryosphaeria and Phomopsis dieback are widespread in California. They enter the wood through wounds that typically come from pruning.

In grapes, Zhuang suggests growers employ late pruning of vines in February or March, when the vines can more quickly recover from those wounds and heal from the pruning cuts. Moreover, spur pruning tends to lead to bigger issues, while cane pruning can be less problematic.

As growers employ mechanical pruning, Zhuang says this technology tends to be problematic as the rough way the machines cut the vines – almost ripping them versus the clean cuts of manual pruning techniques – leaves more points of entry for pathogens to attack vines.

For this reason, the University of California recommends fungicide spray applications after pruning activity to protect vines against pathogens. Zhuang says growers looking to trim costs by employing mechanical pruning should not skip their post-pruning fungicide spray.

Data show that vines as they age tend not only to be more susceptible to yield-robbing pathogens but can dramatically decrease yields over time. Zhuang has seen this in vineyards once they hit about 10 years of age. While minimal yield loss may be acceptable in coastal regions where wine grape prices are high, Zhuang says Central Valley growers must maximize production if they are to be profitable. Even a minimal loss in tonnage can make kill any profit margins a grower might receive.

"My take-home message here is the high tonnage grape yields are vital for Central Valley growers," he said.

Where seven to eight tons of grapes per acre could yield a profitable crop, he now says growers must exceed 12 tons per acre because of rising costs due to inputs and regulations.

He recommends growers remain vigilant and act early when discovering symptoms that could lead to production losses.

"Prevention is the best way to control and minimize disease pressure," he said.