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Title: POPULATION CHANGES IN PHYTOPHTHORA INFESTANS IN TAIWAN ASSOCIATED WITH THE APPEARANCE OF RESISTANCE TO METALAXYL

Author
item Deahl, Kenneth
item COOKE, LOUISE - AGR. RURAL DEV. - N.I.
item BLACK, LOWELL - AVRDC, TAIWAN
item WANG, TIEN - AVRDC, TAIWAN
item Perez, Frances
item Moravec, Brian
item QUINN, MICHELLE - AGR. RURAL DEV. - N.I.
item Jones, Richard

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2002
Publication Date: N/A
Citation: N/A

Interpretive Summary: Although late blight is always a problem in Taiwan, P.O.C., severe epidemics occurred throughout the island between 1997 and 2001 on tomato and potato, raising concerns that the feared mating type that once was found only in Mexico had found its way to Asia. Often associated with the introduction of the new mating are an increase in aggressiveness and resistance to a fungicide widely used against late blight. Although the new epidemics displayed these characteristics, effective control strategies for late blight management must be based on critical data on the exact pathogenic strains that are present. Therefore, 139 pathogenic strains isolated from 1991 to 2001 were sent to the Vegetable Lab at BARC for genetic and molecular characterization. Results showed that in 1997 a 'new' strain completely displaced the 'old' strain. Although this 'new' strain was not the feared mating type, it was extremely aggressive, fungicidal resistant, pathogenic on both potato and tomato hosts, and never before isolated in Asia. This information is extremely beneficial to pathologists and tomato and potato farmers trying to deal with these epidemics. Models for predicting late blight epidemics should be modified or at least verified against the new strains.

Technical Abstract: In Taiwan, late blight has been endemic on outdoor tomato crops grown in the highlands since the early 1900s, but recent epidemics have been more damaging. To ascertain the present status of the Taiwanese population of P. infestans, 139 isolates of the pathogen were characterized using mating type, metalaxyl sensitivity, allozyme genotype, mitochondrial haplotype and RFLP fingerprinting. Up to 1997, all isolates were found to belong to the old clonal lineage of P. infestans (US-1 and variants), but in isolates from 1998 a new genotype appeared and by 2000 this had apparently completely displaced the old population. This new genotype was an A1 mating type and has the dilocus allozyme genotype 100/100/111, 100/100 for the loci coding for glucose-6-phophate isomerase and peptidase, respectively. These characters, together with RG57 fingerprinting, indicated that these isolates belonged to the US-11 clonal lineage, a minority (11%) being a previously unreported variant of US-11. Whereas metalaxyl-resistant isolates were not detected in the old population, 96% of the new genotypes proved resistant, with the remainder being intermediate in sensitivity.