Identification of Bremia lactucae races on Lactuca sativa in Kagawa Prefecture and assessment of host range in wild Asteraceae in Japan

Abstract Two Bremia lactucae isolates were obtained from infected lettuce (Lactuca sativa) plants cultivated in fields in Kagawa Prefecture, Japan. The two isolates exhibited distinct reaction phenotypes compared to those of races previously characterized by the International Bremia Evaluation Board and detected in Hyogo Prefecture, suggesting the presence of previously unreported races in Japan. Commercial lettuce cultivars with resistance genes showed minimal sporulation, confirming their effectiveness for disease control. Cross-inoculation experiments with B. lactucae on wild Lactuca serriola collected from three regions reveled sporulation only on plants from Hiroshima Prefecture. Furthermore, L. sativa cotyledons inoculated with conidia from L. serriola also developed sporulation, providing evidence of cross-infection of B. lactucae between L. sativa and L. serriola in Japan. The detection of new B. lactucae races, as well as the regional susceptibility of L. serriola to B. lactucae, may be influenced by factors such as weed management and crop rotation practices, including paddy rice cultivation in Japan, compared to field crops in other countries.