Investigation of mechanisms underlying blackleg mitigation in canola through three novel integrated pest management approaches
Padmathilake, K. Rasanie E.
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Blackleg disease caused by the fungus Leptosphaeria maculans is one of the most economically significant diseases of canola (Brassica napus L). Three approaches that can be used in mitigating this devastating disease were investigated in this study. Introducing new resistant genotypes is pivotal as host resistance plays the leading role in managing the disease and allows gene rotation. Since AvrLm7 is one of the most abundant avirulence genes in the pathogen population and Rlm7 has been reported as a stable host genotype in other countries, this study focused on evaluating the potential of B. napus genotype, ‘01-23-2-1’, which carries Rlm7. ‘01-23-2-1’was inoculated with L. maculans isolate, carrying AvrLm7, and the CRISPR/Cas9 AvrLm7-knockout mutant of the same isolate to make incompatible and compatible interactions, respectively. A dual RNA-seq study explored differential gene expressions in both interactions. An upregulation of defense-related genes was observed in the incompatible interaction, and in contrast, the same gene expressions were started late in compatible interactions. High expressions of virulence-related pathogen genes-CAZymes, merops, and effector proteins after 7-dpi in compatible interactions but not in incompatible interaction—confirmed that the pathogen was actively virulent only in compatible interactions. Pydiflumetofen is a broad-spectrum seed treatment fungicide in the succinate dehydrogenase inhibitor (SDHI) group. Pydiflumetofen was evaluated compared to Vibrance® Flexi (current company recommendation) under greenhouse conditions using moderately resistant canola cultivar SY4135, and susceptible cultivar Westar. Both SY4135 and Westar plants developed from pydiflumetofen treated seeds showed significantly lower disease severity in canker formation under controlled conditions. Moreover, both host genotypes exhibited lower disease severity in Green fluorescent protein (GFP) tagged L. maculans inoculated seedlings demonstrated control of mycelial growth in planta. Under field conditions, SY4135 and Westar plants treated with pydiflumetofen demonstrated lower disease severity than another three field recommendations, but the difference was not significantly different. However, pydiflumetofen treated SY4135 produced significantly more seed yield than untreated SY4135. The results revealed pydiflumetofen protects the plant at the critical window for infection. Therefore, pydiflumetofen is a worthy tool that farmers can add to the blackleg control. In addition, co-inoculation and pre-inoculation of less virulent L. biglobosa into canola cotyledons showed successful control of the pathogen at morphological, fluorescence microscopic, and transcriptomic levels. The observations confirmed the potential of L. biglobosa as effective biological control of the pathogen L. maculans. Taken together, the knowledge gained from this dissertation will facilitate to add new tools into the blackleg management toolbox.
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