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Characteristics of Phytophthora infestans Isolates and Development of Late Blight on Tomato in Taiwan. G. L. Hartman, USDA/ARS, University of Illinois, Urbana 61801-4709; Y. H. Huang, AVRDC, Shanhua, Tainan, Taiwan, ROC. Y. H. Huang, AVRDC, Shanhua, Tainan, Taiwan, ROC. Plant Dis. 79:849-852. Accepted for publication 4 May 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0849.

Thirteen isolates of Phytophthora infestans were grown on various solidified media. Growth of all isolates was best on rye A agar. Mean sporangia length and width ranged from 23.5 to 41.7 μm x 13.9 to 23.4 μm, respectively, with a length/width ratio of 1.69 to 1.92. All isolates were Al mating type and all isolates, except one isolate from potato, were classified race Tl because blight developed on three tomato lines with the Phl resistance gene. Area under the disease progress curve (AUDPC) differed significantly (P < 0.05) by isolate on both detached tomato leaflets and pot-cultured seedlings. Isolate Pill from potato had the lowest AUDPC values on detached leaves and seedlings. Late blight occurred on tomato seedlings transplanted monthly in the central highlands of Taiwan with greatest disease severity from March to June. Field-grown tomatoes sprayed with metalaxyl once and twice per week reduced late blight AUDPC values, decreased yield losses, and increased fruit numbers, compared with the results from fewer or no sprays. Disease, measured as AUDPC, correlated negatively (P < 0.05) to yield (r = -0.82) and number of fruit (r = -0.76).