Septoria leaf blotch is a devastating wheat fungal disease caused by the fungus Mycosphaerella graminicola that, under severe epidemics, can cause up to 60% yield loss. Even though resistant varieties do exist, the possibility that M. graminicola will overcome resistant varieties is very high due to its genetic diversity and rapid evolution upon selection pressure. M. graminicola control is therefore, heavily dependent on fungicide treatment. Modern agrochemicals compounds with varying modes of action are successful in controlling a wide variety of fungal diseases that threaten crop yield potential. Nonetheless, in order to provide the most appropriate protection and, hence, secure crop productivity, appropriate application timing is of great importance. For this reason, the present study aimed at evaluating the effects of application timing and mode of action of the Pyraclostrobin, Epoxiconazole and Fluxapyroxad fungicides on controlling septoria leaf blotch and the corresponding development of wheat yield. Three different experimental setups were tested, ranging from controlled conditions in a glasshouse, semi-control conditions in micro-plots to natural growth environments in the field. Firstly, by employing molecular biology techniques such as quantitative real time PCR (qPCR) and microarray DNA chips, it could be demonstrated that preventative fungicide application not only provides the most appropriate disease control, it also avoids the activation of costly defense responses that would have a negative impact on wheat yield. Furthermore, changes in plant hormone metabolism, which may have additional impacts on plant growth and development, after the application of Pyraclostrobin, Epoxiconazole and Fluxapyroxad under disease pressure and disease-free conditions, were observed. Additionally, a transient induction of photosynthesis- and energy-related genes after the preventative application of the cellular respiration inhibitors Pyraclostrobin and Fluxapyroxad was also observed. Based on these results, yield trials under semi-controlled and natural conditions were carried out in order to evaluate if higher yields would be obtained when a product is sprayed preventatively, thereby avoiding the activation of plant defense responses, than when applied after the infection has already occurred. Even though no significant differences between the application timings could be identified due to the high variability of the data, a tendency for higher yields were detected in the micro-plot trials under semi-natural conditions after the preventative application of Pyraclostrobin and Fluxapyroxad. These results could not be transferred to the field, however, most likely due to the fact that in nature deciding the point of time at which a preventative application is needed is more complicated as M. graminicola infection occurs throughout the growing season. Nonetheless, it could be demonstrated that an approximately 20% yield increase can be obtained by performing a single fungicide application to treat the septoria leaf blotch disease in wheat.
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