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The Influence of Dew Duration, Relative Humidity, and Leaf Senescence on Conidial Formation and Infection of Onion by Alternaria porri. K. L. Everts, Former graduate research assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824, Present address: USDA-ARS, Department of Plant Pathology, North Carolina State University, Raleigh, 27695-7616; M. L. Lacy, professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Phytopathology 80:1203-1207. Accepted for publication 5 June 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1203.

Formation of conidia by Alternaria porri was examined under variable dew duration and controlled relative humidity (RH). Viable conidia produced on lesions increased from 26% after 9 hr of dew to 72, 91, 93, 96, and 96% after 12, 15, 18, 21, and 38 hr of dew, respectively. Conidia formed during dew duration of 12 hr caused flecks when used to inoculate healthy plants, and those formed during dew duration of 16 hr caused typical lesions. Conidia were formed at all RHs tested (75?100%); numbers were very low at 75?85% RH but increased with increasing RH. Conidia formed on lesions on senescent leaves incubated in dew at 25 C. They formed repeatedly (up to eight cycles) on lesions exposed to alternating low (35?50%) and high (100%) RH in a dew chamber at 25 C. Conidial initials were seen after 9 hr of dew; septa developed after 12 hr; and conidia appeared fully mature after 15 hr of dew when developing conidia were examined with a scanning electron microscope.

Additional keywords: epidemiology, onion (Allium cepa L.).