Optimisation of the chemical control of ascochyta blight in chickpea

Ascochyta blight, caused by Didymella rabiei, is the most devastating foliar disease of chickpea in southern Australia. As part of an effort towards developing disease management practices for susceptible cultivars, programs for timing fungicide applications were developed. The efficacy of chlorothalonil and mancozeb in suppressing ascochyta blight was evaluated in five field experiments conducted over 4 years. The results were variable; in some experiments disease was adequately suppressed (control efficacy >89%) whereas in other experiments, control efficacy was insufficient (<32%). Not all of this variability could be explained by differences in the fungicides used or their concentrations. Analysis of the time of spraying in relation to rain events identified possible causes for most of this variability. Ascochyta blight was suppressed when fungicides were applied in time to protect the plants from infections that occurred during rain events, but whenever the plants were not protected during rains, disease suppression was insufficient and control efficacy was low. The coincidence between control efficacy values and the amount of uncontrolled rain was highly significant (P<0.01; R 2 =0.937). Data recorded in the field experiments were then used as input into a series of simulations aimed at quantifying how several management approaches could reduce fungicide use. Results were analysed using multiple regression with dummy variables. Compared with continuous protection of the crop throughout the season, which required eight mancozeb or five chlorothalonil applications, using rain forecast to time sprays may enable a reduction in the number of sprays by up to 5.5 and 2.7 per season, respectively, hence, vastly reducing production costs. Initiating sprays after disease onset (based on monitoring) may enable a further reduction of 0.6 sprays per season, on average. Validation of the threshold amount in Australia for local cultivars and implementation of these strategies awaits examination in field experiments.