Efficacy of fungicides against Alternaria late blight and their effects on resistance levels

Field trials were performed during the 2021 and 2022 seasons to evaluate the efficacy of several commercial fungicide products in controlling Alternaria late blight (ALB) and their effects on the frequency of mutations associated with resistance to SDHI (H134R, S135R, and H277Y) and QoI (G143A) fungicides. Treatments consisted of three applications using: Luna Sensation, Regalia, Vacciplant, Luna Sensation + Serenade, Adament, Cevya, Luna Experience, Luna Experience + Serenade, Miravis Duo, Miravis Prime, Merivon, Mibelya. The severity was evaluated using a 0-to-5 rating scale and the frequency of each mutation was determined using qPCR methods. As expected, the non-treated control showed the highest severity scores. Except for Regalia, all fungicide treatments reduced the severity of ALB. The fungicide treatment using Miravis Duo resulted in the lowest severity scores. Six fungicide products showed a similar performance to the above-mentioned treatment. The remaining fungicides showed an intermediate efficacy in controlling the disease. The frequency of mutation H134R was higher on trees sprayed with Miravis Duo, Miravis Prime, Merivon, and Mibelya. Mutations H277Y and S135R showed low frequency values and were not influenced by the fungicide treatment. Regalia, Vacciplant, and Cevya were the only products that did not increase the frequency of mutation G143A. Fungicide products that provide the best control of ALB may increase the frequency of mutants associated with SDHI and QoI resistance. SDHI and QoI fungicides should be used in alternation or combination with other chemical classes to minimize the risk of resistance development.