Carrot (Daucus carota) leaf blights can be induced by both Alternaria dauci and Cercospora carotae fungi. In an attempt to establish a spray advisory system for such disease, the frequency of each pathogen was determined in carrot fields located in Passo Fundo (RS), from August/98 to December/99. The incidence of each fungus on seeds, seedlings, and adult plants (leaves and petioles), as well as the number of airborne propagules captured by spore trapping devices, were quantified. The seed-borne inoculum in nine carrot cultivars ranged from zero to 5.75% for A. dauci and from zero to 1.25% for C. carotae. Only A. dauci was found in greenhouse grown seedlings of the carrot cultivars Forto (3.7%) and Gigante Flakker (17.1%). The fungus A. dauci occurred during the whole year in the field, and was isolated from 81% of foliar lesions and 81.9% from petioles. On the other hand, C. carotae was found from June to December in 40.4% of diseased leaves and 33.4% of petioles. The number of airborne spores of A. dauci reached 207/week/cm² at the harvest. Alternaria dauci was shown to be the main pathogen causing carrot leaf blight in local conditions. Therefore, adoption of control strategies should be A. dauci oriented.
