High resistance levels to pyrimethanil and fludioxonil among fourteen Penicillium spp. from pome fruits in the U.S. Pacific Northwest

In this study, 162 Penicillium isolates, i.e., 31 P. expansum isolates and 131 isolates from 13 other Penicillium spp. referred to as “non-expansum” were collected from apples and pears from multiple packinghouses in Washington State and Oregon. The sensitivity of the isolates to the postharvest fungicides pyrimethanil (PYR) and fludioxonil (FDL) was assessed in vitro. The mean EC₅₀ value for PYR was 0.75 μg/mL in P. expansum compared to 1.63, 3.47, 6.95, 7.06 and 32.21 μg/mL in P. solitum, P. palitans, P. commune, P. roqueforti and P. carneum, respectively. For FDL, the mean EC₅₀ value was 0.04 μg/mL in P. expansum compared to >0.80, 1.00, 10.40, 13.99, and 158.10 μg/mL in P. commune, P. palitans, P. roqueforti, P. solitum, and P. paneum, respectively. Overall, > 40 % of isolates from five “non-expansum” species showed dual resistance to PYR and FDL versus 9.6 % in P. expansum. The recommended rates of PYR and FDL failed to control isolates of six Penicillium spp. on detached apples after five months at 1.5 °C. Sequencing of the Mdl1, NikA, and Os1 genes from different isolates of eight species revealed a high polymorphism in the Mdl1 and NikA of several “non-expansum” species. Three and two concurrent mutations, in addition to a G409R and S959, were detected in the Mdl1 and NikA, respectively, that potentially confer resistance to PYR and FDL. The high level of resistance and the control failure observed on fruits highlight the potential risk posed by several “non-expansum” Penicillium species to pome fruit packers.