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Development of inoculation methods to understand interactions of phoma stem canker and light leaf spot causal pathogens during leaf infection.

James Fortune: University of Hertfordshire

<div>UK oilseed yields have not increased in the last 10 years, in part due to yield losses from diseases, such as phoma stem canker and light leaf spot caused by <em>Leptosphaeria maculans</em> and <em>Pyrenopeziza brassicae</em>, respectively. Some cultivars resistant to <em>L. maculans</em> are susceptible to <em>P. brassicae</em>. This investigation aims to understand interaction between these pathogens during leaf infection. A robust inoculation method is needed so symptoms can develop reliably after inoculation. Conidial suspensions of <em>L. maculans</em> (10<sup>7</sup> spores/ml) or <em>P. brassicae</em> (10<sup>5</sup> spores/ml) were prepared with 0.05% Silwet wetting agent. Two oilseed rape cultivars, each with resistance to one of the pathogens, were inoculated using different methods. Infection rates (% sites for point or % of leaves for spray inoculation that showed symptoms) were 10%, 67% or 0% for <em>L. maculans</em> and 15%, 17% or 56% for <em>P. brassicae</em> when plants were inoculated with filter paper discs soaked with conidia suspension, drops of 10 µl of conidial suspension or sprayed with conidial suspension, respectively. Infection rates were low and there is a need to improve inoculation methods. Different resistance responses were observed for <em>L. maculans</em> and <em>P. brassicae</em>. <em>L. maculans</em> triggered a resistance response at the inoculation site and <em>P. brassicae</em> triggered a resistance response at the inoculation site and on the main vein. There is a need to further investigate the resistance response to inoculation with both pathogens.</div>