Pyraclostrobin sensitivity of baseline and fungicide exposed isolates of Pyrenophora tritici-repentis

Abstract

Tan spot, caused by Pyrenophora tritici-repentis, is an important disease of wheat (Triticum aestivum L.) in the United States (U.S). Wheat growers sometimes rely on fungicide applications to manage foliar diseases. Pyraclostrobin is one of the recommended quinone outside inhibitor (QoI) fungicides and is commonly used to manage tan spot on wheat. Although P. tritici-repentis isolates has been reported as insensitive to QoI fungicides in Europe, little is known regarding the sensitivity of the P. tritici-repentis population to fungicides in the U.S. Twenty-eight isolates collected prior to 1997 and 136 isolates collected from pyraclostrobin treated fields in North Dakota were collected in 2007 and 2009 to determine if P. tritici-repentis isolates can use alternative respiration in the presence or absence of salicylhydroxamic acid (SHAM) in vitro and to determine the effective fungicide concentration that inhibited conidia germination by 50% (EC50). Six of 10 baseline isolates of P. tritici-repentis assayed had significantly higher (P ≤ 0.05) EC50 values when SHAM was not added to agar medium amended with pyraclostrobin, confirming that the fungus can utilize alternative respiration to overcome QoI toxicity in vitro. EC50 values of the baseline isolates ranged from 0.0012 to 0.0024 μg/ml (mean EC50 value of 0.0017 μg/ml and standard deviation is ±0.00039 μg/ml) while EC50 values of the 136 fungicide exposed isolates ranged from 0.0013 μg/ml to 0.0027 μg/ml (mean EC50 value of 0.0017 μg/ml and standard deviation is ±0.00030). These results indicate that the field exposed isolates have not shifted to reduced sensitivity to pyraclostrobin and that the baseline sensitivity values can be useful for a fungicide resistance monitoring program for P. tritici-repentis in the U.S.