Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/24/2007
Publication Date: 2/1/2008
Citation: Wise, K.A., Bradley, C.A., Pasche, J.S., Gudmestad, N.C., Dugan, F.M., Chen, W. 2008. Baseline Sensitivity of Ascochyta rabiei to Azoxystrobin, Pyraclostrobin, and Boscalid. Plant Disease.92:295-300. Interpretive Summary: The fungus causing Ascochyta blight of chickpea is named Ascochyta rabiei (sexual state: Didymella rabiei). This study used isolates of the fungus collected prior to the introduction of certain fungicides in order to determine the resistance such isolates might display to these fungicides. This level of resistance ("baseline" resistance), once determined, can be compared to any subsequent level of resistance to these same fungicides among contemporary populations of the pathogen. In essence, the study measured the dose of fungicide which killed half the spores in a given germination test. In order to effectively accomplish this objective, a form of respiration ("alternative" respiration), known to occur only when the fungus grows in pure culture, had to be measured and blocked. This alternative respiration can, if not blocked, give false impressions for the results of germination tests. This study tested for alternative respiration, quantified its impact on germination tests when present, and supplied data for baseline resistance to important, modern fungicides.
Technical Abstract: 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. Fifty-one isolates collected prior to QoI fungicide registration and 71 isolates collected prior to boscalid registration in the U.S. 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 was also assessed to determine if 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 if shifts in sensitivity to these fungicides are occurring in the A. rabiei pathogen population.