Lettuce black root rot pathogen is genetically distinct from other Japanese isolates of Berkeleyomyces rouxiae

Abstract Black root rot of lettuce caused by Berkeleyomyces rouxiae occurred in Japan recently. We analyzed the phylogenetic relationship between this pathogen and other Japanese non-lettuce isolates of B. rouxiae using microsatellite markers. The lettuce isolates were mutually closely related and were distinct from isolates from non-lettuce plants. The non-lettuce isolates were weakly virulent on lettuce and did not increase in virulence on lettuce even after several rounds of inoculation and reisolation in lettuce. Therefore, B. rouxiae isolates that cause disease in non-lettuce plants do not seem to be a cause of the sudden occurrence of lettuce black root rot in Japan.