Phytophthora capsici is a destructive pathogen that affects the foliage, fruit, crown, and roots of cucurbit crops. To date, research has focused on cucurbit fruit rot caused by P. capsici, whereas little is known about foliar blight. Laboratory experiments were conducted to evaluate foliar blight incidence and severity associated with zoospore concentration (ranging from 1 × 103 to 1 × 106zoospores/ml), zoospore condition (encysted or motile), inoculation technique (mycelial plugs, zoospore suspension droplets, or zoospore suspension spray), P. capsici isolate, and leaf position (cotyledons, first or second true leaves). Incidence and severity of foliar blight in cucumber cotyledons were positively correlated (P < 0.0001) with zoospore concentration. Disease severity differed significantly for the interaction between inoculation technique and isolate. Pathogen isolates varied in their ability to cause disease on foliage of cucurbit crops tested. Cotyledons of cucumber, yellow squash, and green zucchini were more susceptible to P. capsici isolates than second true leaves and first true leaves showed an intermediate level of susceptibility. When the susceptibility of cotyledons and fruit of cucumber and yellow squash to select P. capsici isolates was compared, all P. capsici isolates caused disease on all samples, but virulence varied. Significantly (P ¡Ü 0.0025) larger lesions and more sporangia were observed on squash than cucumber cotyledons. Lesions and mycelial growth on fruit varied when inoculated with different isolates; significantly (P = 0.0034) more sporangia were produced on cucumber fruit than on squash fruit. Overall, cucurbit cotyledons exhibited a different susceptibility to P. capsici isolates than that observed in fruit, which has important implications for cucurbit growers.
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