Spread of Armillaria Root Disease in a California Vineyard

Abstract

The formation and expansion of groups of dead and dying grapevines (disease centers) caused by Armillaria root disease were tracked in a commercial vineyard in Sonoma County, California from 1998 to 2000. Approximately 50% of the vines that died in 1999 and 2000 were adjacent to vines that died in previous years, providing circumstantial evidence of vine-to-vine spread of root disease. To determine if symptomatic and dead vines were infected by vine-to-vine spread or by direct contact with partially decayed tree roots remaining from forest trees that inhabited the site prior to vineyard establishment, pneumatic soil excavation was used to expose the root systems of 30 vines within the oldest disease center. Root system excavation revealed infections on 26 of 30 excavated vines, 27 of which were in direct contact with decayed tree roots. No evidence of vine-to-vine spread was found and rhizomorphs were extremely rare. Therefore, the pattern of disease incidence was due to the patchy distribution of decayed tree roots belowground. Control efforts aimed at reducing vine-to-vine spread of Armillaria root disease in young vineyards, such as the one in this study, may be unnecessary, given the slow rate of spread of the pathogen.