Baseline Sensitivity of Ascochyta rabiei to Azoxystrobin, Pyraclostrobin, and Boscalid

Ascochyta rabiei, causal agent of Ascochyta blight on chickpea (Cicer arietinum), can cause severe yield loss in the United States. Growers rely on applications of fungicides with site-specific modes of action such as the quinone outside inhibiting (QoI) fungicides azoxystrobin and pyraclostrobin, and the carboximide fungicide boscalid, to manage disease. In all, 51 isolates collected prior to QoI fungicide registration and 71 isolates collected prior to boscalid registration in the United States were tested in an in vitro assay to determine the effective fungicide concentration at which 50% of conidial germination was inhibited (EC₅₀) for each isolate-fungicide combination. The effect of salicylhydroxamic acid (SHAM) on conidia of A. rabiei in the presence and absence of azoxystrobin also was assessed to determine whether the fungus is capable of using alternative respiration. Five of nine A. rabiei isolates tested had significantly higher (P ≤ 0.05) EC₅₀ values when SHAM was not included in media amended with azoxystrobin, indicating that A. rabiei has the potential to use alternative respiration to overcome fungicide toxicity in vitro. EC₅₀ values of azoxystrobin and pyraclostrobin ranged from 0.0182 to 0.0338 μg/ml and from 0.0012 to 0.0033 μg/ml, with mean values of 0.0272 and 0.0023 μg/ml, respectively. EC₅₀ values of boscalid ranged from 0.0177 to 0.4960 μg/ml, with a mean of 0.1903 μg/ml. Establishment of these baselines is the first step in developing a monitoring program to determine whether shifts in sensitivity to these fungicides are occurring in the A. rabiei pathogen population.