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Propiconazole Sensitivity in Populations of Geotrichum candidum, the Cause of Sour Rot of Peach and Nectarine, in California

The sour rot pathogen of peach (Prunus persica var. persica) and nectarine (P. persica var. nectarina) fruit, Geotrichum candidum, can cause significant postharvest losses in California fruit production. Harvested peach and nectarine fruit, treated with fungicide at the packinghouse but culled after inspection because of disease and defects, were collected for further assessment and pathogen isolation. The incidence of fruit with sour rot was 3.4 ± 1.0 to 26.1 ± 2.3%. Culled fruit that had been treated with postharvest fungicides from five different orchards had a significantly higher incidence of sour rot when compared with nontreated fruit. Since August 2006, propiconazole has been used as a postharvest treatment to protect peach and nectarine fruit against sour rot. The mean effective concentration that inhibits 50% of mycelial growth (EC₅₀) value of 57 isolates of G. candidum to propiconazole collected before and during 2006 was 0.072 μg/ml. However, 61 isolates from propiconazole-treated, diseased fruit collected from 2007 to 2009 had a mean EC₅₀ value for mycelial growth of 0.378 μg/ml, a fivefold shift in mean sensitivity. Propiconazole applied as either a protective or curative treatment significantly reduced disease severity and decay development in fruit inoculated with a propiconazole-sensitive isolate. The fungicide was significantly less effective when applied as a preventive or a curative application to fruit that were inoculated with a less-sensitive isolate of G. candidum. Under laboratory conditions, isolates of the pathogen less sensitive to propiconazole were stable over multiple transfers on fungicide-free medium. The potential for the development of G. candidum populations with reduced sensitivity to propiconazole and the increased risk of crop loss due to the practice of returning culled fruit to production orchards are discussed.