New Outbreaks of Verticillium Wilt on Hop in Oregon Caused by Nonlethal Verticillium albo-atrum

Abstract

In 2006 and 2007, outbreaks of Verticillium wilt on hop were detected on two farms in Oregon. Some Verticillium pathogens cause minor damage to hop but others can kill susceptible cultivars: strains of lethal V. albo-atrum are not known to occur in Oregon. Studies were conducted to determine the identity of the Verticillium species associated with these outbreaks and their virulence to hop to devise an appropriate response to the outbreaks. Isolates were recovered from wilted plants, and identified as V. albo-atrum based on morphological characters and ITS region sequence. Pathogenicity assays conducted on a set of differential cultivars indicated all isolates had a low to intermediate level of virulence. Inspection and mapping of all symptomatic and dead plants in a hop yard on an affected farm indicated that 29.3% and 19.7% of plants had symptoms of Verticillium wilt in 2006 and 2007, respectively. However, the number of dead or missing plants did not increase from 2006 to 2007 (0.26% in both years). There was a clear directionality to the pattern of symptomatic plants among rows running north-south vs. east-west in 2006, although directionality was less pronounced in 2007. These findings indicate that the recent outbreaks of V. albo-atrum on hop in Oregon were caused by nonlethal strains, which has key implications for devising management strategies on the affected farms. Accepted for publication 30 March 2012. Published 21 May 2012.