Risk assessment of resistance to prochloraz in Phoma arachidicola causing peanut web blotch

Peanut web blotch (PWB) caused by Phoma arachidicola, is one of the most serious foliar diseases of peanut. Although prochloraz is an active fungicide with broad anti-fungal spectrum, it has not been registered for the control of PWB in China. The activity of prochloraz against P. arachidicola and the risk of resistance to prochloraz in P. arachidicola are still unclear. In current study, the inhibitory activity of prochloraz against 96 P. arachidicola strains was determined with the average EC₅₀ value of 1.2700 ± 0.7786 μg/mL. Prochloraz exhibited excellent protective and curative effect on detached peanut leaves, and the effect was obviously better than that of carbendazim and difenoconazole at the same concentration. After prochloraz treatment, the mycelium of P. arachidicola contorted, shrunk and ruptured, with shrinking of cell wall and membrane, enhanced cell membrane permeability, and reduced ergosterol content. Totally 80 prochloraz-resistant mutants were obtained by fungicide adaptation with the frequency of 6.7 × 10^-3. All the selected 12 prochloraz-resistant mutants lost their resistance to prochloraz after 10 transfers on PDA plates. And these mutants exhibited decreased biological fitness in mycelial growth and pathogenicity. Moreover, there was positive cross-resistance between prochloraz and other demethylation inhibitor (DMI) fungicides, such as tebuconazole, triflumizole and difenoconazole, but no cross-resistance was found between prochloraz and other classes of fungicides, such as carbendazim, pydiflumetofen or fludioxonil. Overexpression of PaCYP51 and PaAtrB genes were detected in the resistant mutants. All the above results demonstrated that prochloraz has a great potential in management of PWB. The risk of P. arachidicola developing resistance to prochloraz is relatively low-to-medium. Overexpressing of PaCYP51 and PaAtrB might be linked to prochloraz resistance in P. arachidicola.