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Pathogenic Specialization in Phytophthora infestans: Aggressiveness on Tomato. D. E. Legard, Department of Plant Pathology, 334 Plant Sciences Building, Cornell University, Ithaca, NY 14853, Current address: Department of Plant Pathology, 13138 Lewis Gallagher Road, University of Florida, Dover 33527; T. Y. Lee, and W. E. Fry. Department of Plant Pathology, 334 Plant Sciences Building, Cornell University, Ithaca, NY 14853. Phytopathology 85:1356-1361. Accepted for publication 8 August 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-1356.

Isolates of Phytophthora infestans from the USA, Canada, Mexico, and the Netherlands were evaluated for aggressiveness on tomato in three types of growth chamber experiments and one field experiment. In the first type of growth chamber experiment, fitness components (lesion area and sporangia per lesion) of the four predominant clonal lineages in the USA were measured in growth chamber detached leaflet assays. Isolates of two of the lineages, predominantly associated with potatoes, produced lesions on tomato that were significantly smaller with less sporulation than isolates from two lineages that were found associated with both potato and tomato. The second growth chamber experiment involved isolates from northwest Mexico, as well as those from the USA and Canada; and the third experiment involved isolates from central Mexico and the Netherlands, in addition to those from the USA. Again, two categories were confirmed: genotypes that produced larger lesions with more abundant sporulation on potato than tomato (tomato-nonaggressive genotypes) and genotypes that were equally aggressive on potato and tomato (tomato-aggressive genotypes). With few exceptions, isolates with the same allozyme genotype had similar host reactions. Central Mexican isolates from potato and wild Solanum spp. were all tomato-nonaggressive. Since this is the center of origin for the pathogen, aggressiveness to tomato may be a recently acquired trait. A field experiment with one tomato-aggressive isolate and one tomato-nonaggressive isolate was conducted on one potato and two tomato cultivars. The tomato-nonaggressive isolate caused significantly less disease on tomato than potato. The tomato-aggressive isolate caused severe disease on both potato and tomato, but there was significantly more disease on tomato than potato. These results support the hypothesis that host specialization to tomato occurs in P. infestans.