Alternative methods for the control of a resistant isolate of Penicillium digitatum to Imazalil on Citrus unshiu fruits

Postharvest control of green mold (Penicillium digitatum) in citrus packinghouses in Peru is mainly accomplished with the use of imazalil (IMZ) fungicide. The use of IMZ has increased the population of biotypes that are resistant to this fungicide. From a previous investigation, of 15 total isolates collected in W. Murcott and Nova discarded mandarin fruits from a packinghouse from Huaral NDASH Lima. Based on a survival rate at 5 mL L^‑1 of IMZ, sporulation rate, high germination performance, and decay degree in fruits, the most resistant strain was isolated. The objective of the present study was to evaluate the most effective method to control this highly resistant strain of P. digitatum to IMZ using in vitro and in vivo essays. P. digitatum grown on potato dextrose agar (PDA) was treated with “Impala” (Imz) at 5 ppm and 500 ppm, “Timorex Gold” (TG, essential oil of Melaleuca alternifolia tea) at 5 mL L^‑1, “Serenade” (BS, Bacillus subtilis) at 7.5 mL L^‑1 and “Tricho D” (TD, Trichoderma harzianum) at 0.75 g L^‑1. For the in vivo trial, Imz at 500 ppm, TG at 5 mL L^‑1, BS at 7.5 mL L^‑1 and TD at 0.75 g L^‑1 was used. Murcott mandarin fruits were sprayed with a drench treatment for 30 s, once the fruits were dry, 10 μL of suspended conidia solution (1×10⁶ conidia mL^‑1, 1 mm deep) were used to inoculate each fruit. The best treatment for the in vitro trial was IMZ at 500 ppm, followed by Serenade with a control percentage of 88% and Timorex Gold with 61%. The best treatment for the in vivo trial was Timorex Gold, which delayed mold progression with a 35% control of sporulation on the 6^th day and with a 45% control of decay on the 8^th day. Results showed a way to design control trials using Timorex Gold (M. alternifolia) as an alternative method to chemical fungicides in order to develop an anti-resistance management of P. digitatum.