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Influence of Environment on Conidial Concentration of Alternaria porri in Air and on Purple Blotch Incidence on Onion. K. L. Everts, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824, Current address: USDA-ARS, Department of Plant Pathology, North Carolina State University, Box 7616, Raleigh 27695-7616; M. L. Lacy, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Phytopathology 80:1387-1391. Accepted for publication 17 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1387.

A study was conducted to determine the influence of weather variables on conidial concentration of Alternaria porri in air above an onion field in Michigan. Temperature, humidity, leaf wetness, rainfall, solar radiation, wind speed, and concentration of airborne conidia were monitored at the Michigan State University Muck Experimental Farm in 1985 and 1987. Weather instruments and spore trap were placed in the center of an unsprayed 15- ? 30-m plot of field seeded Spartan Banner onions. In both years, the natural logarithm of numbers of airborne conidia sampled during the current day (D) was positively correlated with 1) the maximum hourly vapor pressure deficit (VPD) (= saturation-ambient vapor pressure) on D, and 2) the logarithm of the conidial concentration sampled on D-1. A regression equation was developed to predict relative conidial concentration on day D in 1987 that explained 59% of the variability. Purple blotch lesions were counted weekly at the same site in 1986 and 1987. In 1987, counts of lesions were also made on onion trap plants placed in the field plot at weekly intervals. Large concentrations of conidia of A. porri did not always precede increases in lesions.

Additional keywords: disease prediction, quantitative epidemiology.