Characterizing the aggressiveness of Leptosphaeria maculans strains by a petiole test in controlled conditions

Abstract

The blackleg of oilseed rape or phoma stem canker is a disease caused by a fungal pathogen called Leptosphaeria maculans. In the oilseed rape, two resistance types are known : qualitative and quantitative. The qualitative resistance determined by major genes acts in an « all or nothing » manner preventing the fungus to infect leaves. The quantitative resistance detemined in a polygenic manner does not prevent leaf infection but reduces fungal systemic growth from the leaves to the stem base and limits the severity of stem canker. Because the efficacy of the qualitative resistance decrases over time due to the adaptation of fungal populations, breeders search oilseed rape varieties with a polygenic resistance. Today the question is to know how the L. maculans populations will evolve when they are confronted to a polygenic resistance. How to characterize the fungus? Does the fungus become more agressive that is to say is it capable to do more damage on plants over time? As no method in controlled conditions was satisfactory to characterize the aggressiveness of the fungus on the stem, we implemented a new method of inoculation by cutting the petioles of the second leave. This article describes ou method in two steps: first, to determine the oilseed rape genotype allowing a better discrimination, with enough necrosis but not too much. Secondly, to validate the method confirming its capacity to discriminate the aggressiveness on the stem of two strains with contrasted aggressiveness on cotyledons. The Eurol variety allows to reveal the degree of aggressiveness between the two strains. The discrimination of the two strains, JN2 more aggressive than NzT4, corresponds to the results observed on cotyledons. We then used this method in various projects.