HOST-SPECIFICITY OF PHYTOPHTHORA-INFESTANS ON TOMATO AND POTATO IN ECUADOR

Sixty Ecuadorian isolates of Phytophthora infestans from potato and 60 isolates from tomato were compared for dilocus allozyme genotype, mit ochondrial DNA haplotype, mating type, and specific virulence on 11 po tato R-gene differential plants and four tomato cultivars, two of whic h contained different Ph genes. Restriction fragment length polymorphi sm (RFLP) fingerprints of subsamples of isolates from each host were c ompared by using RG57 as the probe. All potato isolates had the allozy me genotype, haplotype, and mating type of the clonal lineage EC-I, wh ich had been previously described in Ecuador. With the same markers, o nly one isolate from tomato was classified as EC-I; all others belonge d to the globally distributed US-1 clonal lineage. RFLP fingerprints o f isolate subsets corroborated this clonal lineage classification Spec ific virulence on potato differentials was broadest among potato isola tes, while specific Virulence on tomato cultivars was broadest among t omato isolates. Some tomato isolates infected all tomato differentials but no potato differentials, indicating that specific virulence for t he two hosts is probably controlled by different avirulence genes in P . infestans. In two separate experiments, the diameters of lesions cau sed by nine isolates from potato and 10 from tomato were compared on t hree tomato and three potato cultivars. All isolates produced larger l esions an the host from which they were isolated. No isolates were fou nd that were highly aggressive on both tomato and potato. We conclude that there are two different populations of P. infestans in Ecuador an d that they are separated by host.