Sensitivity of Cochliobolus heterostrophus to three demethylation inhibitor fungicides, propiconazole, diniconazole and prochloraz, and their efficacy against southern corn leaf blight in Fujian Province, China

Southern corn leaf blight (SCLB) caused by Cochliobolus heterostrophus is a fungal disease that impacts production of corn in China. Fungicides have been the main strategy to manage SCLB. In this study, 276 isolates of C. heterostrophus from seven locations in Fujian Province of China were tested for sensitivity to three demethylation inhibitor (DMI) fungicides. The results indicated that most of the isolates of C. heterostrophus tested were exceptionally sensitive to the three DMI fungicides. Correlation analysis revealed positive association between propiconazole and diniconazole (r=0.8145, P<0.0001), propiconazole and prochloraz (r=0.6190, P<0.0001), and diniconazole and prochloraz (r=0.5784, P<0.0001). However, there was no cross-resistance between these three DMI fungicides and the other six fungicides tested, which included carbendazol, chlorothalonil, mancozeb, iprodione, fluazinam, and pyraclostrobin. In a preventive pot experiment, one spray of 25% propiconazole emulsifiable concentrate (EC) with 250g active ingredient (a.i.) mL(-1) applied 12 and 24h before inoculation at the seedling (V6) stage reduced severity of SCLB by 85.60-89.21%. Nevertheless, the curative activity of propiconazole was much weaker (P<0.05) than its preventive efficacy. In greenhouse pot assays, one dose of propiconazole at 250g a.i. mL(-1) was the most efficacious for controlling SCLB at the seedling and tasseling (VT) stages of corn, decreasing severity by 80.31%-84.85%, which was higher (P<0.05) compared to diniconazole, prochloraz, and other reference fungicides. Therefore, propiconazole appears to be very effective in reducing SCLB and should be applied as a preventive rather than therapeutic fungicide. Our findings provide essential information on the evolution of DMI resistance in C. heterostrophus in Fujian Province of China and may serve as a guide for early resistance monitoring in the future.