UC Riverside

The two major citrus postharvest decays green mold and sour rot of citrus are caused by Penicillium digitatum and Geotrichum citri-aurantii, respectively, and result in significant crop losses in California citrus if not properly managed. Several postharvest fungicides are registered to manage green mold, including imazalil, thiabendazole, azoxystrobin, fludioxonil, pyrimethanil, propiconazole, and natamycin, but only the latter two are highly effective against sour rot. Cyproconazole is a demethylation inhibitor (DMI) triazole fungicide currently undergoing registration for citrus postharvest use. My results on vitro toxicity and efficacy for management of these decays provide support for its registration, particularly in the presence of DMI resistance in sub-populations of the causal pathogen with EC50¬ values > 0.5 µg/ml. Cyproconazole was effective in reducing mycelial growth of P. digitatum isolates sensitive or resistant to imazalil, and EC50¬ values ranged between 0.05 and 0.90 µg/ml. For G. citri-aurantii isolates sensitive or moderately-resistant to propiconazole, EC50 values ranged between 0.10 and 0.83 µg/ml. Inoculated citrus fruits treated in laboratory or experimental packingline studies with cyproconazole alone or in mixtures with other registered fungicides reduced decay by sensitive or resistant isolates of both pathogens by 78 to 99% compared to untreated controls. Cyproconazole was compatible with storage and packing fruit coatings when applied as mixtures using a low-volume spray system. Cyproconazole was compatible with inorganic salts like sodium carbonate and sodium bicarbonate in high-volume applications, and with sanitizers like sodium hypochlorite and peroxyacetic acid without reducing efficacy. The combined in vitro sensitivity and efficacy studies demonstrate incomplete cross resistance between cyproconazole and the registered DMIs imazalil and propiconazole that can improve efficacy in integrated pest management programs in the presence of resistant pathogens. In G. citri-aurantii, point mutations in the DMI target sites of CYP51A result in moderate or high resistance to propiconazole without conferring fitness penalty costs, whereas mutations in CYP51B do not have an effect. The genes encoding the two target molecules were identified and characterized using whole genome sequencing and classical molecular cloning techniques, and they can be distinguished using allele-specific diagnostic PCR primers. Lastly, G. candidum was identified as being a weak pathogen of citrus, causing sour rot on senescent fruit with compromised host defenses.