Forecasting Incidence Thresholds of Cercospora Blight in Carrots to Initiate Fungicide Application. A. C. Kushalappa, Macdonald College of McGill University, Ste. Anne de Bellevue, PQ, H9X 1C0. G. Boivin, and L. Brodeur. Agriculture Canada Research Station, P.O. Box 457, St.-Jean-sur-Richelieu, PQ, J3B 6Z8; and Service de depistage, 539 Blvd. Edward 7, St. Jacques le Mineur, PQ, G0G 1Z0, Canada. Plant Dis. 73:979-983. Accepted for publication 24 June 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0979.

Observations of Cercospora blight, caused by Cercospora carotae, in carrots (Daucus carota var. sativa) during 1984–1987 were analyzed to formulate alternative methods for predicting the disease incidence threshold recommended to initiate fungicide application. Regression equations with days after cotyledon stage (DAY), plant growth stage (GS), and degree-day with a base of 7 C (DD₇), each as an independent variable, explained 83, 86, and 85%, respectively, of the variation in the rate of blight development; the disease incidence threshold of 50% was reached at 48 DAY, GS 8.6, and 557 DD₇. These alternative action thresholds could be used to time the first fungicide treatment for late carrots (those reaching cotyledon stage after the third week of May). The mean time after disease detection (TADD) until the threshold was reached was 21 days, but no regression equation was established. Predictive equations were not developed for early carrots because the disease developed late and few or no fungicide applications were needed. However, the mean observed values of DAY and TADD for a disease incidence threshold of 100% were 89 and 42 days, respectively, and these may be considered as alternative action thresholds to initiate fungicide applications on early carrots.

Keyword(s): expert system.