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Influence of Interruptions of Dew Period on Numbers of Lesions Produced on Onion by Botrytis squamosa. S. C. Alderman, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824-1312, Present address of senior author: Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh 27695-7616 ; M. L. Lacy(2), and K. L. Everts(3). (2)(3)Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824-1312. Phytopathology 75:808-810. Accepted for publication 21 February 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-808.

Onion plants inoculated with dry conidia of Botrytis squamosa and placed in a dew chamber for 6 hr, followed by a dry period of variable (0.3- 24 hr) duration, and returned to the dew chamber for the remainder of the incubation period, had progressively fewer lesions as duration of the dry interruption period increased. Lesion numbers also tended to decline as relative humidity decreased from 90 to 30% during a relatively short 20-min dew interruption period. Plants similarly inoculated showed increasing spore germination, appressorium formation, numbers of lesions, and visible infection hyphae as dew periods were increased from 2 to 24 hr. Plants were inoculated, given an initial dew period of from 2 to 12 hr, a 2-hr dry period, and a subsequent dew period of 12- 22 hr for a total incubation period of 26 hr. Since these plants received 24 hr of dew in a 26 hr period, the only variable was the timing of the 2-hr dew interruption. Conidial germination was near 100% after 26 hr, regardless of timing of the dry period. However, appressorium formation and numbers of lesions were both reduced more by dew interruptions occurring after 6 hr of initial dew than by interruptions occurring after 2, 4, 8, 10, or 12 hr of initial dew, presumably because germinating conidia were most vulnerable to drying at this time. Numbers of visible infection hyphae were lowest when dry periods occurred after a 6- or 8-hr initial dew period.