Several young vineyards in Napa and Sonoma counties have been ailing from a disease that has thus far stumped viticulturists.

Typical leaf symptoms include a wide range of patterns of veinal and interveinal chlorosis. In some cases, the leaves lose their green coloring and appear bleached. Marginal burning, usually in sections of the leaf, often accompanies the chlorosis. Shoot growth stops prematurely and the vines are usually stunted.

Having ruled out herbicide injury and common viruses as causes, Napa County Farm Advisor Ed Weber is planning to conduct trials this spring. According to Weber, a lack of mycorrhizal fungi in fumigated soil may be the root of the problem. These fungi play an important role in the uptake of micronutrients in grapevines' root systems, so he theorizes an unusual nutrient deficiency may be causing the disease.

Have you seen these symptoms?

“We will be innoculating mycorrhizae into symptomatic vineyards and to new vineyards planted this spring,” Weber said.