Researchers aren’t exactly sure why leafroll virus (LRV) is showing up with increasing frequency in California vineyards, but they are worried about the trend.

“I think all growers need to be aware of this and be on the alert for it in their vineyards,” says Deborah Golino, director of Foundation Plant Services at UC Davis. “One of the problems is that leafroll virus is not caused by one disease or pathogen.”

There are at least eight identifiable strains of LRV, she notes, and more are being isolated through laboratory analysis.

In the field, fruit symptoms include delayed maturity; red leaves in the fall on red fruit varieties; rolled under leaves; and reduced vigor. Foliage symptoms on white fruit varieties are much more difficult to distinguish. To make visual detection even more complicated, not all leafroll viruses will cause the same symptoms.

“There are differing sensitivities to various rootstocks as well,” Golino says.

Leafroll virus can be transmitted in different ways. “We know some of them are transmitted by mealybug. Others are possibly transmitted by aphids. We’re very concerned about (the spreading) vine mealybug and the effect it may have on spreading leafroll virus around the state.”