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 on carrot seedlings, on mature carrot mots, or on carrots planted in soil, showed that 83 isolates belonging to eight species were pathogenic and were capable of causing cavity spot symptoms. These included Pythium violae, P. sulcatum, P. ultimum var. ultimum, P. irregulare, P. sylvaticum, P. acanthicum, P. paroecandrum, and P. mamillatum. The first five species have been reported to cause cavity spot in other carrot-growing regions of the world, while the latter three represent new records. In greenhouse tests to determine the effects of soil moisture, temperature, and carrot age on development of cavity spot, high soil moisture (near field capacity) or flooding, and a temperature of 15 degrees C were found to be optimal. Carrot age (1 to 3 months) did not influence cavity spot development. Thirty-eight isolates, representing all Pythium species recovered, were compared for pectolytic enzyme activity in a cup-plate assay and for virulence on mature carrot roots in vitro. The highly virulent isolates were found to produce significantly higher (P < 0.03) enzyme levels (up to 11.6 units per mg dry weight of mycelium) compared with the moderately or weakly virulent isolates. The most virulent isolates were P. violae, P. sulcatum, and one isolate of P. ultimum var. ultimum. A histopathological study of P. violae 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 most resistant cultivars included Panther, Caropride, Fannia, and Navajo.