Quantitative blackleg resistance in the host differentially interacts with L. maculans isolates

Dr Susie Sprague1, Dr Luke Barrett1, Dr Wendelin Schnippenkoetter1, Dr Angela Van de Wouw2, Dr Steve Marcroft3 

1CSIRO Agriculture And Food, Canberra, Australia, 2University of Melbourne, Parkville, 3010, 3Marcroft Grains Pathology, Horsham, 3400

Blackleg, caused by Leptosphaeria maculans, is a constant threat to canola production in Australia with growers relying on disease resistance for control. Australian canola cultivars have two types of resistance to blackleg: complete or major gene resistance (MGR) and partial or quantitative resistance (QR). MGR can be rapidly overcome while QR is considered a more sustainable resistance and is generally assumed to be partially effective against all blackleg isolates. We conducted controlled experiments in which cultivars differing in QR were inoculated with homogeneous (single spore isolates) or heterogenous (populations of ascospores) to determine the interaction between host and pathogen genotype by assessing severity of leaf lesions produced on cotyledons and crown canker severity at maturity. There were significant host x isolate interactions at different stages of disease development. These interactions were significant for the size and severity of lesions on the cotyledons and in the crown at maturity but were inconsistent between experiments and seasons. Ascospore populations appear to produce a more consistent level of disease amongst cultivars, probably because they are highly heterogeneous. QR does not give partial resistance to all isolates, but instead reacts with individual isolates differently. This finding is highly significant, particularly in the context of developing a robust phenotyping method and needs to be considered in the context of ascospore or pycnidiospore inoculum.


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