Repeated soil applications of fungicide reduce activity against cavity spot in carrots

Farrar, J; Nunez, J; Davis, R

Repeated soil applications of fungicide reduce activity against cavity spot in carrots

Authors

James J. Farrar
J. Joseph Nunez
R. Michael Davis

Authors Affiliations

J.J. Farrar is Assistant Professor, Plant Sciences Department, California State University, Fresno; J.J. Nunez is Farm Advisor, UC Cooperative Extension (UCCE), Kern County; R.M. Davis is UCCE Specialist, Department of Plant Pathology, UC Davis.

Publication Information

Hilgardia 56(2):76-79. DOI:10.3733/ca.v056n02p76. March 2002.

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Abstract

In recent years, carrot growers in the San Joaquin Valley have suffered economic losses due to cavity spot, a soilborne disease, despite frequent applications of the fungicide mefenoxam. Although the pathogen remained highly sensitive to mefenoxam in laboratory studies, the effective dosage of the fungicide was apparently compromised in certain fields. Compared to its longevity in soils with no history of mefenoxam use, such as fields using organic production methods, the fungicide degraded rapidly in soil from fields with repeated mefenoxam use. Our research reveals that repeated applications of the fungicide to soil can increase the activity of microorganisms that degrade it, potentially reducing its efficacy against cavity spot. This is problematic in California since mefenoxam is the only fungicide available to carrot growers for cavity spot control. It may be prudent to practice long crop rotations and to limit use of mefenoxam, where possible.

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