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Relation of Air Temperature to Development of Verticillium Wilt on Cotton in the Field. R. H. Garber, Plant Pathologist, Crops Research Division, ARS, USDA, Shafter, California 93263; John T. Presley, Plant Pathologist, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 61:204-207. Accepted for publication 23 September 1970. DOI: 10.1094/Phyto-61-204.

Two cotton strains grown in two seasons had different levels of field tolerance to Verticillium wilt. Neither Cal 7-8, the more susceptible, nor Acala 4-42-77, the more tolerant, were seriously affected by the disease in a warm summer. In a cool summer, the relative disease tolerances of the strains were readily separated. In a warm summer, V. albo-atrum was isolated from plants with symptoms in early summer and fall. A decline in isolations in midsummer was attributed to a prolonged hot period from late June through August. In the cool summer, the fungus was isolated from most plants throughout the summer. In both seasons, the organism was present in many symptomless plants. High temperatures can mask or obscure the influence of pathogen population and virulence in determining disease severity.