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Effect of Leaf Wetness Duration and Air Temperature on Cercospora beticola Infection of Sugarbeet. Jack R. Wallin, Research Plant Pathologist, Crops Research Division, ARS, USDA, Ames, Iowa 50010; Daniel V. Loonan, Research Technician, Crops Research Division, ARS, USDA, Ames, Iowa 50010. Phytopathology 61:546-549. Accepted for publication 14 December 1970. DOI: 10.1094/Phyto-61-546.

Leaf wetness is important in the development of Cercospora beticola on sugarbeet. The number of spots produced after a 48- or 72-hr moist period was more than 30 or 80 times, respectively, the number produced after 24 hr. Favorable temperatures for leaf spot development were 24, 27, and 29 C, with 29 the most favorable temperature. Spots appeared 7 days after inoculation with 500, 1,000, and 2,500 spores/cc at moist periods of 48- and 72-hr duration and at 29 C. Spots appeared in 8 days on plants inoculated with 2,500 spores/cc, and held for a 24-hr moist period. Spots appeared in 9 days at the 12-hr moist period. There was no difference in the amount of leaf spot produced on 10 plants by spore concentrations ≥ 2,500/cc. Spore concentration, temperature, and duration of the wet period all influenced the effects of incubation period and disease severity.