Effective Alternatives for the Postharvest Control of Imazalil Resistant Penicillium digitatum Strains

Postharvest diseases control in Citrus is essential to maintain fruit quality during storage, overseas transport and selling. Green mold caused by Penicillium digitatum is one of the most important postharvest diseases. In Uruguay it has been controlled by synthetic fungicides. Fungicide resistant strains make it neccesary to develop new strategies to control postharvest decay in Citrus. In view of this situation, we attempted to develop an effective alternative management to control Penicillium digitatum imazalil (IMZ) resistant strains. For this purpose 'Valencia' oranges were inoculated with imazalil sensitive SS-22 and resistant RS-20 strains selected previously according to their aggressiveness on lemon fruits. From 18-22 hours before treatments, each fruit was inoculated by wounding (depth 2 mm, breadth 1 mm) and inoculating with a P. digitatum spore suspension (1,0 x 10(6) spores/ml). Incidence and sporulation was evaluated after 7, 11 and 15 days at 20 degrees C. Similarly, an experimental packing-line was used to evaluate chemical alternatives and application methods. Imazalil with pyrimethanil (Philabuster (TM)-PI, 750 ppm) mixture or combined with potassium sorbate (KS, 2%) resulted in a significant control of the incidence and sporulation of the most aggressive P. digitatum resistant strain. The incidence was reduced from 75-80% in controls to 0-8% in treatments. IMZ applied at 1500 ppm did not control the incidence or sporulation of the resistant strain. The application of KS reduced strains incidence and sporulation by 50-70%. No difference (p <= 0.05) was found in the pathogen control comparing both application methods. All treatments were effective on reducing sensible strain sporulation, being only PI alone or combined with KS, effective for the resistant strain. We concluded that PI (750 ppm) alone or with KS at 2% can be applied as an alternative management to control Uruguayan Penicillium digitatum sensitive and resistant IMZ strains.