University of California researchers are investigating whether a devastating pest is combining with a fungus to threaten premium vineyards. Vines in four vineyards in Napa, Sonoma, and Mendocino counties in northern California have stopped bearing usable amounts of fruit. Researchers fear that a fungus and an aphid-like insect called phylloxera, which ravaged the Northern California wine industry in the 1980s, are responsible.

Two decades ago, growers in coastal areas ripped out 70% of their vines and replaced them with rootstock thought to be resistant to the pest.

Now researchers worry that some rootstock could be susceptible to a combination of the bug and a fungus it spreads.