Identification of and in Citrus Packinghouses in California, Their Sensitivity to Cyproconazole, and Incomplete Cross-Resistance to Propiconazole.

High levels of sour rot on propiconazole-treated lemon fruit that was stored for extended times in some California packinghouses in 2020 and 2021 initiated surveys on fungicide sensitivity of the causal pathogen. In isolations from diseased fruit in 2020 to 2023, 157 isolates of spp. were obtained. Using species-specific primers, 143 were determined to be and 15 were . Isolates of were either sensitive (effective concentration of fungicide required for 50% growth inhibition [EC] 0.06 to 0.34 μg/ml), moderately resistant (EC 1.20 to 2.34 μg/ml), or highly resistant (EC ≥ 17.68 μg/ml) to propiconazole. There was incomplete cross-resistance to cyproconazole, another demethylation inhibitor fungicide, pending postharvest registration on citrus in the United States. Isolates sensitive to propiconazole were sensitive, isolates moderately resistant to propiconazole were sensitive, and isolates highly resistant were moderately resistant to cyproconazole (EC 0.11 to 0.63 μg/ml, 0.19 to 0.73 μg/ml, and 2.66 to 6.79 μg/ml, respectively). All except one isolate of were highly resistant to both fungicides (EC > 9.55). Isolates of both species were all considered sensitive to natamycin (EC 1.18 to 5.01 μg/ml). In lemon fruit inoculations with , the incidence of typical sour rot increased from 4.7 to 68.2% when inoculum was amended with 2 μg/ml or 100 μg/ml cycloheximide, respectively, a compound known to suppress host defenses. In coinoculations with 1:1 mixtures of the two spp., was only recovered from the centers of decay lesions, whereas was also obtained from the advancing margins. We conclude that is a secondary pathogen of lemons, and its presence was favored by extended late-season storage of senescent fruit with reduced defense mechanisms.