Identification of Geotrichum citri-aurantii and G. candidum in Citrus Packinghouses in California, Their Sensitivity to Cyproconazole, and Incomplete Cross-Resistance to Propiconazole.

Abstract

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 Geotrichum spp. were obtained. Using species-specific primers, 143 were determined to be G. citri-aurantii and 15 were G. candidum. Isolates of G. citri-aurantii were either sensitive (effective concentration of fungicide required for 50% growth inhibition [EC₅₀] 0.06 to 0.34 μg/ml), moderately resistant (EC₅₀ 1.20 to 2.34 μg/ml), or highly resistant (EC₅₀ ≥ 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 (EC₅₀ 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 G. candidum were highly resistant to both fungicides (EC₅₀ > 9.55). Isolates of both species were all considered sensitive to natamycin (EC₅₀ 1.18 to 5.01 μg/ml). In lemon fruit inoculations with G. candidum, 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 Geotrichum spp., G. candidum was only recovered from the centers of decay lesions, whereas G. citri-aurantii was also obtained from the advancing margins. We conclude that G. candidum is a secondary pathogen of lemons, and its presence was favored by extended late-season storage of senescent fruit with reduced defense mechanisms.