A Simple Day-Degree Model for Initiating Chemical Control of Potato Early Blight in Colorado. G. D. Franc, Department of Plant Pathology and Weed Science, Colorado State University, Fort Collins 80523. M. D. Harrison, and L. K. Lahman. Department of Plant Pathology and Weed Science, Colorado State University, Fort Collins 80523. Plant Dis. 72:851-854. Accepted for publication 14 April 1988. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1988. DOI: 10.1094/PD-72-0851.

A simple day-degree (DD) model was developed to predict appearance of the first early blight (Alternaria solani) lesions on potato foliage in two potato production areas of Colorado. The model, based upon accumulated day-degrees above 7.2 C (45 F) from the date of planting, effectively predicts the beginning of secondary spread of the pathogen. Accumulated day-degrees-centigrade (DDC) required for appearance of first lesions is 361 in the San Luis Valley, a high mountain valley, and 625 DDC in northeastern Colorado, an area with lower elevation. Growers use the DD information to time the initial fungicide application to minimize the cost of early blight control. Subsequent applications are made according to label directions. Different day-degree thresholds may need to be established if the model is to be used in other potato production areas. In Colorado, where growers do not routinely apply fungicides for the control of late blight (Phytophthora infestans), the model effectively reduces fungicide use by as much as four applications.

Keyword(s): epidemiology, spore dispersal.