Abstract
The host range of Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew, includes many taxa of the Cucurbitaceae. Little is known of the interactions of this pathogen with wild and weedy Cucurbita species, the genus encompassing two important crops, pumpkin and squash. Interactions of 97 Cucurbita spp. accessions, mostly wild and weedy forms representing 10 species, with 11 P. cubensis isolates originating from Cucumis sativus were studied by using a leaf-disc method under controlled conditions. Among the 97 accessions, 57 distinct reaction patterns were observed. Fifteen accessions were resistant and 12 susceptible to all 11 pathogen isolates. Within-accession, heterogeneous reactions were observed in only nine interactions. Of the 15 accessions resistant to all pathotypes, 13 were wild and weedy C. argyrosperma. Reactions of Cucurbita spp. to P. cubensis are race-specific. The development of methodology for P. cubensis race determination and denomination should include a differential set of genotypes for each species separately. C. argyrosperma, which is partially cross-compatible with the economically important C. pepo, offers a potential source for breeding downy mildew-resistant squash and pumpkins.
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