Characterization of sensitivity of grove and packing house isolates of Penicillium digitatum to pyrimethanil

• Baseline sensitivities for pyrimethanil against green mold were determined.
• No resistant isolates were found in the field where the fungicide was not used.
• One isolate, originated from a packing house, was considered resistant.
• Fruit decay by sensitive isolates was reduced 80% by fungicide at 500–600 mg L−1.
• The resistant isolate was not controlled by pyrimethanil at 1000 mg L−1.

In most northeast Argentinean citrus packing houses, postharvest fungicide treatments are based on the use of thiabendazole and imazalil. However, these fungicides have been used in a manner highly conducive to the selection and proliferation of resistant biotypes of Penicillium digitatum, the main fruit decay fungus in the area. Recently, a new fungicide, pyrimethanil (PYR), was introduced to control molds. Aims of this study were to determine the baseline sensitivities for PYR against isolates of P. digitatum considering its use in the region is not yet widespread and to evaluate the control of the fungus in vivo. One hundred and nine (109) P. digitatum isolates were collected from diseased fruit within citrus groves (43 isolates) and packing houses (66 isolates). EC50 was determined for each isolate by measuring colony diameters on different agar dilutions of the fungicide. The mean EC50 value of the green mold isolates collected from the groves was 0.14 ± 0.03 mg L−1 while the mean EC50 of those collected from packing houses was 0.13 ± 0.05 mg L−1. No resistant isolates were found in the field where the fungicide is not used, while one isolate originated from a packing house showed an EC50 of 3.40 mg L−1, 26-fold higher than the mean level. This isolate was collected from lemons stored in cool rooms of a packing house where PYR had not been used. Fruit decay by sensitive isolates was reduced approximately 80% by PYR applied at 500–600 mg L−1 by immersion for 60 s at room temperature to inoculated oranges and mandarins. In contrast, the resistant isolate was not controlled by PYR applied at 1000 mg L−1. Thus, the introduction of PYR applied into packing houses should be done carefully and control strategies should be implemented in order to minimize the development of resistant isolates.