Sensitivity of Exserohilum turcicum to demethylation inhibitor fungicides

Abstract

Exserohilum turcicum, the causal agent of northern leaf blight (NLB) of corn (Zea mays), can cause significant yield reductions in Illinois. Metconazole, propiconazole, and prothioconazole are demethylation inhibitor (DMI) fungicides often applied as solo active ingredients or mixed with other fungicide groups to control NLB. Monitoring for shifts in DMI fungicide sensitivity in E. turcicum populations is important for making management decisions and maintaining fungicide efficacy. Sensitivity to metconazole, propiconazole, and prothioconazole were determined for E. turcicum isolates collected prior to DMI use on corn (baseline group collected prior to 1992) and E. turcicum isolates collected between 2009 and 2014. An in vitro mycelial growth assay was used to determine the effective fungicide concentration at which 50% of the fungal growth was inhibited (EC50) for each isolate-fungicide combination. Baseline EC50 value least-square means (lsmeans) for metconazole, propiconazole, and prothioconazole were 0.032, 0.060, and 0.254 μg/mL, respectively. When lsmeans of EC50 values for 2009, 2010, 2011, 2012, 2013, and 2014 E. turcicum isolates were compared to the lsmean of the baseline E. turcicum EC50 values, no significant (P ≤ 0.05) shift towards reduced sensitivity was observed for metconazole, propiconazole, or prothioconazole. Three E. turcicum isolates had EC50 values significantly higher (P ≤ 0.05) than the least sensitive baseline isolate for metconazole, and one isolate had an EC50 value significantly higher (P ≤ 0.05) than the least sensitive baseline isolate for propiconazole. These isolates will require further evaluation to determine if they demonstrate reduced field sensitivity. Isolate prothioconazole EC50 values were positively correlated (P ≤ 0.05) with those of metconazole and propiconazole (r = 0.1690 and r = 0.3269, respectively); however, no correlation was found between metconazole and propiconazole EC50 values. Positive correlations between EC50 values of prothioconazole and the other fungicides suggest the potential for cross-resistance between these DMI fungicides. To date, no loss of NLB control has been observed with the use of metconazole, propiconazole, and prothioconazole in the field.