First report of Phytophthora cactorum causing root rot of processing carrots (Daucus carota) in Michigan.

In the fall of 2005, processing carrot fields in Mason, Newaygo, and Oceana counties, Michigan, were surveyed for Phytophthora spp. Carrot roots were sampled from areas of fields that exhibited patches of chlorotic, blighted, or wilted foliage. Dark brown, firm, water-soaked lesions occurred near the middle and crown areas of diseased carrot roots. In the advanced stages of disease, carrot root tissue readily collapsed and a soft rot developed while petioles turned black. The internal portions of the diseased carrot roots were brown and rubbery. Roots with these symptoms are not suitable for processing. Carrot roots were washed with tap water and the tissue excised from the edge of developing lesions and plated aseptically onto BARP-amended (25 ppm of benomyl, 100 ppm of ampicillin, 30 ppm of rifampicin, and 100 ppm of pentachloronitrobenzene) regular V8 juice agar. Plates were incubated at 23 to 25°C for 7 days. Phytophthora sp. was isolated from carrot root samples from all surveyed areas. Ten represent...