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Isolate Source and Daylight Intensity Effects on the Pathogenicity of Verticillium dahliae in Watermelon Seedlings. Y. Ben- Yephet, Division of Plant Pathology, Agricultural Research Organisation, The Volcani Center, Bet Dagan, Israel; Phytopathology 69:1069-1072. Accepted for publication 15 January 1979. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-1069.

Watermelon (cv. Sugar Baby) seedlings, which were root-dip inoculated with Verticillium dahliae spore suspensions developed wilt symptoms usually starting with cotyledon wilt, continuing with leaf wilt, and ending in seedling death. These symptoms were observed in greenhouses with low daylight intensities of 60320 μE m-2 sec-1 and temperature of 1930 C. More plants developed symptoms at the lower daylight intensities of 100 or 180 than at 320 μE m-2 sec-1 at temperatures of 22 2 C. As inoculum concentration increased the percentage of seedlings with disease symptoms also increased with both root-dipping and hypocotyl spore-injection techniques. Disease developed more quickly after root-dip than after hypocotyl inoculation. The influence of growth by the pathogen on three media and for different incubation times on pathogenicity to watermelon seedlings was compared. No differences were found for the three media after 4 days of incubation. Spores obtained after 14 and 21 days growth on yeast extract agar clearly had lost pathogenicity to watermelon seedlings. The longer incubation periods on potato dextrose agar or synthetic agar did not significantly influence pathogenicity, as compared with that of spores collected after 4 days. Loss in pathogenicity was due to virtually complete loss of spore viability. In pathogenicity tests, 15 isolates from seven hosts (cotton, melon, avocado, watermelon, peanut, potato, and tomato) caused typical disease symptoms in watermelon seedlings. One isolate from olive was nonpathogenic to watermelon.