Evidence that Cercospora carotae Causes Leaf Spot on Carrot in Western Washington

During 1999, a leaf spot on carrot (Daucus carota L. subsp. sativus [Hoffm.] Arcang.) was observed on nearly every plant in a 20-ha field of carrots (cv. Red Chantenay) grown for processing in western Washington. Circular to elongate, light brown lesions surrounded by chlorosis were present on leaflet margins and petioles of affected plants. Conidia of Cercospora carotae (Pass.) Solheim were present in the lesions. Small pieces of surface-sterilized leaf tissue were placed onto potato dextrose agar plates and incubated at room temperature to obtain fungal isolates. Koch's postulates were completed by atomizing the upper and lower leaves of carrot seedlings at the three to four leaf stage with sterile water or C. carotae at 1.0 × 104 conidia/ml in sterile 0.01% Tween 80. Treatments were replicated five times using single plants. The plants were bagged in clear plastic and placed in a greenhouse at 25°C for 72 h. Disease symptoms developed within 10 days as light brown lesions on leaflet margins and petioles, and were similar to those found in the field. The fungus was reisolated as described above. Symptoms did not develop in control plants sprayed with water. Farr et al. (1) report that C. carotae occurs in several states but not Washington, and Shaw (2) lists C. carotae only from British Columbia and Oregon. To our knowledge, this is the first report of Cercospora leaf spot on carrot in Washington. References: (1) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN. (2) C. G. Shaw. W.S.U. Agric. Exp. Sta. Bull. 765, 1969.