California Agriculture

In the 1930s and 1940s, little was known about viruses, and information on plant diseases caused by viruses was just beginning to appear in the scientific literature. Problems with grapevines in California, first referred to as “red leaf,” were initially attributed to inexperience in viticultural techniques and poor growing conditions. However, the problem was later identified as leafroll disease, which causes red leaves, and poor yields and fruit quality. We evaluated its rate of spread for 5 years in a Napa Valley vineyard, and found an average rate of more than 10% per year. Leafroll disease can be vectored by low-level populations of grape mealybugs, and is now spreading rapidly in at least one Napa Valley vineyard for unknown reasons. Using stock for planting vines that is certified as virus-free is a key strategy in preventing the spread of grapevine leafroll disease.