Imazalil concentration for in vitro monitoring of imazalil resistant isolates of Penicillium digitatum in citrus packinghouses

The extensive use of imazalil (IMZ) in Uruguayan citrus packinghouses to control Penicillium spp. favored the selection and proliferation of resistant isolates. With the aim of detecting Penicillium digitatum biotypes that are not controlled by commercial doses of IMZ, the IMZ concentration within amended potato dextrose agar (PDA) plates was adjusted to 1.0 mg L−1 IMZ. This concentration allowed the detection of resistant isolates that were not controlled by commercial applications of 3.0 g L−1 IMZ. These isolates were able to grow in fruit with IMZ residues of 0.92 and 3.08 mg kg−1. Therefore, environmental monitoring of facilities where commercial dip applications containing 3.0 g L−1 IMZ are employed, should be done with 1.0 mg L−1 of IMZ in Petri dishes. We can conclude that use of IMZ in postharvest applications of 3.0 g L−1 remains effective to control wild Uruguayan P. digitatum isolates but not for resistant ones. A survey of 26 P. digitatum isolates collected in Uruguay indicated IMZ resistant isolates occurred in packinghouses and not in citrus groves.

Research highlights
► 1.0 mg L−1 IMZ concentration within amended PDA plates facilitated the detection of resistant isolates not controlled by 15 s commercial applications of 3.0 g L−1 IMZ.
► Resistant isolates were able to grow in fruit with IMZ peel residues of 0.92 ± 0.05 mg kg−1 and 3.08 ± 0.08 mg kg−1.
► Environmental monitoring where commercial immersion applications contain 3.0 g L−1. IMZ should be made using 1.0 mg L−1 of IMZ in media within Petri dishes.