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Influence of Dew Period and Temperature on Infection of Onion Leaves by Dry Conidia of Botrytis squamosa. S. C. Alderman, Graduate research assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824-1312; M. L. Lacy, professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824-1312. Phytopathology 73:1020-1023. Accepted for publication 9 February 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1020.

A dry spore inoculation technique was used to examine the influence of dew period and temperature on infection of onion leaves by Botrytis squamosa. A linear relationship was found between mass of dry conidia used as inoculum and lesion numbers. Dry spores applied to leaf surfaces survived 2 days with no reduction in their potential to induce lesions, but lesion numbers declined when inoculated plants were held in a growth chamber at 60% relative humidity (RH) for more than 2 days prior to incubation in a dew chamber. Lesion production was optimal at 20 C, lower at 15 C, and greatly reduced at 25 C. Lesions were initiated after 6 hr of dew, and numbers of lesions increased sigmoidially through 32 hr of dew. On leaf surfaces, the percentage of conidia that germinated, and the proportions of germinated conidia that subsequently formed appressoria, formed infection hyphae, and resulted in lesions were greater at 20 C than at 25 C. Numbers of lesions averaging 62 ± 8% of dry conidia deposited on leaves resulted after 24 hr of continuous dew at 20 C. Hyphal development within lesions increased with time through 6 days of continuous dew at 20 C, but was restricted under conditions of no dew and moderate (60 ± 10%) RH.

Additional keywords: Allium cepa, Botrytis leaf blight, epidemiology.