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 effect-ive 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 ap-plications of the fungicide to soil can increase the activity of microorganisms that de-grade 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 mefenox-am, where possible.