Temperature affects the effectiveness of R gene mediated resistance against Leptosphaeria maculans (phoma stem canker) in Brassica napus (oilseed rape)

Phoma stem canker (Leptosphaeria maculans) is an economically important disease on oilseed rape (Brassica napus) in Europe, Australia and North America. Effective control of this disease relies on the use of two types of resistance: major resistance (R) gene-mediated qualitative resistance and quantitative resistance (QR). Plant–pathogen interactions are known to be affected by environmental factors, including temperature. To investigate effects of temperature on durability of R gene-mediated resistance to L. maculans, oilseed rape cultivars or breeding lines with different R genes in backgrounds with/without QR were inoculated at 20°C and 25°C. Cotyledons of 12-day old plants were wounded and a drop of 10µl of 107 spores/ ml conidial suspension placed over the wound. There were differences in temperature sensitivity between the ten different R genes (Rlm1, Rlm2, Rlm3, Rlm4, Rlm5, Rlm6, Rlm7, LepR1, LepR2 and LepR3) tested and there were differences in response to temperature for the same R gene in different cultivars. Background QR affected the temperature sensitivity of R genemediated resistance. To avoid the effects of background QR and investigate plant defence responses, near isogenic lines of Topas with R genes Rlm4 or LepR3 were used. Cotyledons of 12-day old plants were infiltrated with 10 µl of 10⁶ spore/ml conidial suspension at 20°C and 25°C. There were differences in defence responses between Rlm4 and LepR3, with Rlm4 responding more quickly and more strongly than LepR3 at the higher temperature. Understanding effects of temperature on interactions between hosts and pathogens will help to breed cultivars with durable, temperature-resilient resistance