APS Abstracts of Presentations

Using a reduced risk fungicide, copper and a disease forecaster to manage foliar blights on carrots. E. A. DORMAN and M. K. Hausbeck. Dept. of Plant Pathology, Michigan State University, East Lansing, MI. Phytopathology 93:S22. Publication no. P-2003-0155-AMA.

Alternaria blight (Alternaria dauci) and Cercospora blight (Cercospora carotae) incite disease on carrot leaves and petioles. A disease-forecasting model, Tom-Cast, was tested at thresholds of 10, 15, 20 disease severity values (DSV) to time fungicide sprays using a copper-based fungicide (copper hydroxide), a reduced risk fungicide (azoxystrobin), and a standard commercial fungicide (chlorothalonil), alone and in alteration. Based on AUDPC data, disease severity of petiole and leaf blight was similar for plants treated with fungicide every 7 days or according to Tom-Cast 10 or 15 DSV. The number of spray applications were reduced by 7 (2001) or 8 (2002) with the Tom-Cast 15 DSV program. In general, copper used alone or in alternation with chlorothalonil was not as effective as azoxystrobin alternated with chlorothalonil in limiting disease. Tom-Cast reduces grower reliance on fungicides to control foliar blight on carrots and has been implemented on some Michigan farms.