<i>In planta</i> growth of <i>Leptosphaeria maculans</i> as a measure of quantitative resistance to blackleg in canola

Quantitative resistance (QR) plays an important role in managing blackleg of canola caused by Leptosphaeria maculans in Canada. However, QR is traditionally assessed through multi-year field trials that are time-consuming and influenced by environmental variability. This study evaluated fungal growth kinetics in canola stem tissues as a quantitative proxy for QR. Fifty-six commercial canola varieties representing a range of field resistance ratings were first evaluated under greenhouse conditions by quantifying relative L. maculans DNA abundance in stem tissues using droplet-digital PCR (ddPCR) following cotyledon or petiole inoculation. The isolates used carried complementary virulence profiles designed to evade major R-gene resistance, enabling assessment of QR. Both inoculation methods produced consistent stem infections, and relative fungal DNA abundance generally corresponded with field resistance ratings collected previously. Fungal DNA levels were also strongly correlated between cotyledon and petiole inoculation assays and with disease severity observed in multi-year inoculated field trials (2019–2022). Using the petiole inoculation, a total of 80 commercial hybrid cultivars from Canadian breeding programmes were subsequently evaluated. Over 90% of hybrids exhibited strong QR, limiting stem fungal DNA accumulation by >70% relative to a susceptible control. These results demonstrate that ddPCR-based quantification of pathogen biomass provides a rapid and biologically meaningful measure of QR in canola. This approach provides a practical tool for quantifying QR and supports integrating quantitative and major R-gene resistance to improve the sustainability of blackleg management in Canadian canola production.