ROLE OF PYTHIUM SPECIES IN CAVITY SPOT DEVELOPMENT ON CARROTS IN BRITISH-COLUMBIA

One hundred and twenty isolates of Pythium species were recovered from cavity spot lesions on carrots grown in the Fraser Valley of British Columbia during 1989 to 1992. Pathogenicity tests performed in vitro o n carrot seedlings, on mature carrot roots, or on carrots planted in s oil, showed that 83 isolates belonging to eight species were pathogeni c and were capable of causing cavity spot symptoms. These included Pyt hium violae, P. sulcatum, P. ultimim var. ultimum, P. irregulare, P. s ylvaticum, P. acanthicum, P. paroecandrum, and P. manillatum. The firs t five species have been reported to cause cavity spot in other carrot -growing regions of the world, while the latter three represent new re cords. In greenhouse tests to determine the effects of soil moisture, temperature, and carrot age on development of cavity spot, high soil m oisture (near field capacity) or flooding, and a temperature of 15 deg rees C were found to be optimal. Carrot age (1 to 3 months) did not in fluence cavity spot development. Thirty-eight isolates, representing a ll Pythium species recovered, were compared for pectolytic enzyme acti vity in a cup-plate assay and for virulence on mature carrot roots in vitro. The highly virulent isolates were found to produce significantl y higher (P<0.03) enzyme levels (up to 11.6 units per mg dry weight of mycelium) compared with the the moderately or weakly virulent isolate s. The most virulent isolates were P. violae, P. sulcatum, and one iso late of P. ultimum var. ultimum. A histopathological study of P. viola e on carrot revealed the disintegration of host cells and development of hyphae beneath the epidermis, ultimately resulting in the collapse of the lesioned area to form a cavity. Using an in vitro mature carrot root inoculation procedure, 37 carrot cultivars were evaluated over a 2-year period for resistance or susceptibility to cavity spot. The mo st resistant cultivars included Panther, Caropride, Fannia, and Navajo .