Detection of Soilborne Alternaria radicina and Its Occurrence in California Carrot Fields

Alternaria radicina, causal agent of black rot disease of carrot, was recovered from soil by plating dilutions on a semi-selective medium, A. radicina semi-selective agar. The efficiency of this soil assay was 93% based on recovery of the fungus from non-infested field soil amended with A. radicina conidia. Soilborne A. radicina was recovered from five of six carrot-growing areas in California, but was only commonly found in the Cuyama Valley, where the fungus was detected in 83% of sampled fields. Over a 3-year period of sampling, A. radicina soil populations in Cuyama Valley fields prior to carrot planting ranged from 0 to 317 CFU/g. There was a positive correlation between A. radicina soil populations in these fields and the incidence of black rot disease at harvest. A. radicina was recovered from dry soil after 4 years of storage, and the fungus survived in this soil as solitary conidia or as conidia associated with organic debris.