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This study was conducted to determine the host range of Phytophthora capsici isolates from pumpkin and virulence of the isolates on pumpkin cultivars. The pathogenicity of P. capsici isolates from pumpkin was evaluated on 45 species of herbaceous plants, including 36 species of crops grown in rotation sequences with pumpkin and nine species of weeds that commonly grow in pumpkin fields in Illinois. Plants were grown in the greenhouse, and 4-week-old seedlings were inoculated by adding 5 ml of a zoospore suspension (2 × 105 spores per ml of water) onto the soil surface around the stem of each plant in the pot. Twenty-two crop species and two weed species became infected with P. capsici and developed symptoms. P. capsici was reisolated from all of the symptomatic plants by culturing tissues onto a semiselective medium (PARP). Also, P. capsici was detected in 87.5% of symptomatic plants by a polymerase chain reaction (PCR) method using PCAP and IT5 primers. Cucurbits and pepper were the most susceptible hosts of P. capsici. Five crop species or varieties, beet (Beta vulgaris), Swiss-chard (Beta vulgaris var. cicla), lima beans (Phaseolus lunatus), turnip (Brassica rapa), and spinach (Spinacia oleracea), and one weed species, velvet-leaf (Abutilon theophrasti), were found to be hosts of P. capsici for the first time. Six isolates of P. capsici were inoculated onto six pumpkin cultivars (three processing and three jack-o-lantern pumpkins) in the greenhouse and resulted in significant interactions between pathogen isolates and pumpkin types. P. capsici isolates were more virulent on jack-o-lantern pumpkins than on processing pumpkins.

This study was conducted to investigate pathogenic, morphologic, and genetic variations among Phytophthora capsici isolates from processing pumpkin (Cucurbita moschata) fields in Illinois. Random amplified polymorphic DNA (RAPD) markers were employed to assess genetic variation among 24 isolates of P. capsici from 10 individual fields at six locations. Unweighted mean pair group analysis clustered isolates into six groups. The genetic distances ranged from 0.03 to 0.45. Inoculation of pumpkin seedlings in the greenhouse revealed that the isolates belonged to six distinct genetic groups differing significantly (P = 0.05) in virulence. Isolates tested exhibited four growth patterns in culture: cottony, rosaceous, petaloid, and stellate. P. capsici isolates, including an ATCC isolate (ATCC-15427), with cottony growth pattern did not grow at 36°C. The mean oospore diameter of A1 mating type isolates was greater than that of A2 mating type isolates. Nine of 24 isolates tested produced chlamydospores in V8-CaCO3 liquid medium.

Apron XL LS (mefenoxam) and Allegiance FL (metalaxyl) were highly inhibitory to growth of mycelium of Phytophthora capsici in vitro. Effective dose (ED50) of mefenoxam and metalaxyl for 50% inhibition of mycelial growth, for all five isolates of P. capsici tested, was 0.98 and 0.99 μg a.i./ml of culture medium, respectively. For mefenoxam at 200 μg a.i./ml, sporangium and zoospore germination were reduced by 92 and 96%, respectively, and 21 and 24%, respectively, for metalaxyl. In greenhouse studies, seed treatment with mefenoxam (0.42 ml of Apron XL LS/kg of seed) and metalaxyl (0.98 ml of Allegiance FL/kg of seed) significantly reduced pre- and post-emergence damping-off of seedlings caused by P. capsici in three pumpkin cultivars (Dickinson, Hybrid-401, and Hybrid-698) tested. Thirty-one days after seeding, at inoculum levels of 0, 90, 600, 1,400, and 4,000 CFU/g of soil, the average seedling stands for mefenoxam treatment were 98.4, 93.8, 88.3, 77.8, and 64.8%; for metalaxyl, 99.1, 85.3, 85.8, 73.5, and 59.3; and for the untreated control, 97.5, 55.2, 45.7, 37.0, and 22.9%, respectively. In field trials, the average seedling stands 35 days after seeding were 76.7, 74.7, and 44.9% for mefenoxam, metalaxyl, and untreated control, respectively. Seed treatment with mefenoxam or metalaxyl did not have any significant effect on either seed germination or seedling vigor.

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