Frequency of avirulence genes and race structure in a Leptosphaeria maculans population in Tunisia

Blackleg, caused by Leptosphaeria maculans, is a major oilseed rape disease present in many traditional growing regions worldwide and is also becoming increasingly important in Tunisia. Cultivation of oilseed rape varieties harbouring major blackleg resistance genes (R-genes) in an integrated approach represents the most effective means of controlling this fungal threat; knowledge of the pathogen population structure is a prerequisite for its efficient use. Leptosphaeria spp. isolates (145) were collected and characterized during 2018-2020 from oilseed rape stubble originating from seven northern Tunisian regions. As in most other growing areas, L. maculans and L. biglobosa occurred together on diseased plants. Most isolates (78.6%, n = 114) were L. maculans. For those, a combination of differential set phenotyping and PCR-based avirulence gene analyses was used to perform race typing. Overall, 30 races were identified, carrying at least two and up to eight of the 11 avirulence genes tested. The most predominant race (14.9%) carried AvrLm7, AvrLepR1 and AvrLepR2 and was detected each year and in six out of the seven regions sampled, indicating the effectiveness of major R-genes Rlm7 and LepR1 in Tunisia. Rlm2 was overcome by all Tunisian L. maculans isolates, but most were avirulent on Rlm6 and LepR2 (>50%), suggesting that using these R-genes would provide protection in Tunisia. Both mating types were identified, with MAT1-2 being twice as frequent as MAT1-1. These results endorse the need for regular race surveillance of the local pathogen population at temporal and regional scales to develop appropriate blackleg management strategies.