Phenotypic and Genotypic Changes in the Phytophthora infestans Population in Taiwan-1991 to 2006

Late blight, caused by Phytophthora infestans, is one of the most destructive diseases of tomato in Taiwan. A total of 655 isolates of P. infestans, including 29 isolates from potato, was collected from major tomato and potato production areas of Taiwan during 1991 to 2006. Isolates were characterized for their pathogenicity, mating type, in vitro metalaxyl sensitivity and molecular genotype (including allozyme pattern, mitochondrial genomic haplotype and DNA fingerprint) to monitor population changes in P. infestans. The population of P. infestans in Taiwan underwent a dramatic genetic shift in the 1997-1998 cool growing season. Isolates collected from tomato before 1997 were aggressive to tomato but not potato; most isolates obtained after 1998, were aggressive to both hosts. Metalaxyl sensitivity of isolates changed from sensitive/intermediate before 1997 to resistant since 1998. Similarly, the isolates obtained before 1997 were all US-1 clonal lineage (including variants US-1.1, US-1.2, US-1.3 and US-1.4). During the 1997-1998 cool growing season, the US-11 clonal lineage and the TW-1 genotype appeared, possibly introduced on imported table potatoes. The US-11 lineage spread rapidly and since 1999 has almost completely displaced the old population in Taiwan. Mating type determined by pairing with A1 and A2 reference isolages of P. parasitica, showed all isolates were of the A1 mating type, suggesting that the A2 mating type had not become established in Taiwan. The increasing percentage (up to 42.3% in 2006) of the US-11 variants (including US-11.l, US-11.2, US-11.3 and US-11.4) implied that genomic diversity of the pathogen is changing quickly. Therefore, it is important to continuously monitor the population changes of P. infestans and develop an integrated management strategy for this disease.