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

item Perez, Frances
item Deahl, Kenneth

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2008
Publication Date: 2/2/2009
Citation: Chen, C., Sheu, Z., Wang, T., Black, L.L., Perez, F.G., Deahl, K.L. 2009. Phenotypic and Genotypic Changes in the Phytophthora infestans Population in Taiwan - 1991 to 2006. Journal of Phytopathology. 157:248-255.

Interpretive Summary: A severe outbreak of late blight on potato occurred in central Taiwan in 1997, spreading quickly to potato and tomato fields over the entire island. Since that time, late blight has become prevalent and poses a significant threat to potato and tomato production throughout Taiwan. To understand why the causal agent, Phytophthora infestans, caused such severe epidemics of late blight in Taiwan, diseased plant samples were collected from the major potato- and tomato-growing areas of Taiwan from the 1997 outbreak to 2006, and compared with isolates collected prior to the 1997 outbreak. These results suggested that during 1997-98 native strains of the population of P. infestans were rapidly displaced by a new immigrant strain, possibly introduced into Taiwan with potato tubers from overseas. These results explained many previously unanswered questions about the population biology of P. infestans in Taiwan, brought quarantine control failures to the attention of Taiwan officials, and caused plant pathologists to revise late blight control strategies.

Technical Abstract: 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, were collected from the major tomato and potato production areas during 1991 to 2006 in Taiwan. Isolates were characterized on their pathogenicity, mating type, in vitro metalaxyl sensitivity, and molecular genotype, which includes allozyme pattern, mitochondrial genomic haplotype, and DNA fingerprint analyses for monitoring the population shift of P. infestans. The population of P. infestans in Taiwan underwent a significant genetic shift in 1997-1998 growth seasons. Isolates collected from tomato before 1997 were aggressive to tomato but not potato; however isolates obtained after 1998 were aggressive to both hosts. Metalaxyl sensitivities were changed from sensitive/intermediate to resistant since 1998. Similarly, the isolates obtained before 1997 were all US-1 clonal lineage, including variants US-1, US-1.1, US-1.2, US-1.3, and US-1.4; however, new US-11 clonal lineage, including variants US-11, US-11.1, and US-11v, appeared during the 1997-1998 growth season possibly introduced on imported table potatoes. The new lineage spread rapidly and displaced the old population in Taiwan since 1999. All isolates were A1 mating type, indicating that the A2 mating type had not established in Taiwan. Although sexual recombination was not yet detected, increasing percentage (up to 42.3% in 2006) of the US-11 variants implied that genomic diversity of the pathogen is changing quickly. Therefore, it is important to monitor the population shift of the P. infestans continuously and develop the integrated management strategy for the disease control.