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Molecular mechanisms of mutation to virulence in Leptosphaeria maculans populations in the UK.

Lakshmi Harika Gajula: University of Hertfordshire

<div><em>Leptosphaeria maculans</em>, the cause of phoma stem canker disease of oilseed rape, develops gene-for-gene interactions with its host plant resistance genes. Pathogens develop new effector proteins to overcome recognition by the host and plants evolve new recognition proteins to target novel <em>Avr </em>proteins. In this study, the regional distribution of the <em>L.</em> <em>maculans</em> races in the UK was monitored and the molecular mechanisms of mutation to virulence were investigated. Field experiment sites were set up at different sites in the UK: from leaf spot lesions on Drakkar (susceptible cultivar, trap crop) and other cultivars (with <em>Rlm7</em> resistance gene), 64 and 88 <em>L. maculans</em> isolates, were obtained in the 2015/2016 and 2016/2017 cropping seasons, respectively. Changes in frequencies of avirulent <em>AvrLm1</em>, <em>AvrLm4 </em>or <em>AvrLm7 </em>alleles were investigated by testing isolates on cotyledons of a differential set of cultivars. Isolates virulent towards <em>Rlm1</em>, <em>Rlm4</em> or <em>Rlm7</em> were investigated for molecular events. There were variations in the frequencies of avirulent<em> AvrLm1</em> and <em>AvrLm4 </em>alleles between cropping seasons. All the isolates from different sites were avirulent against <em>Rlm7 </em>in the 2015/2016 season. In the 2016/2017 season, 6.8% of isolates were virulent towards <em>Rlm7</em>. The molecular mechanism of mutation to virulence in <em>AvrLm1</em> was observed to be whole gene deletion in 86% of isolates. The other 13% were sequenced to investigate events leading to virulence. Whole gene deletion was observed in 6% or 50% of isolates carrying the virulent alleles of <em>AvrLm4 </em>or <em>AvrLm7</em>, respectively. The others need to be sequenced for further investigation.</div>