Establishing the relationship of ascospore loads with blackleg (Leptosphaeria maculans) severity on canola (Brassica napus)

Abstract

A study was carried out to determine the relationship of blackleg (Leptosphaeria maculans) to ascospore loads from infested canola residue, and also to determine whether effects of chemicals on L. maculans development and ascospore discharge from residues would be reflected in subsequent disease on a freshly sown crop. Residues were dipped in a water-only control or solutions of flutriafol or glufosinate-ammonium and placed in the field prior to early winter rains where they remained through the growing season. Canola (Brassica napus cv. Dunkeld) was seeded adjacent to residues. Early seedling lesion and adult plant crown canker development were monitored. A significant exponential relationship between numbers of ascospores discharged from residues and seedling percentage disease index (SPDI; range 0–100%) and adult plant percentage disease index (APDI; range 0–100%) was evident. In particular, large variations in SPDI and APDI were evident in relation to changes in level of inoculum where less than 25 × 105 ascospores/stem (approx.) were discharged. A reduction in ascospore numbers discharged from 25 × 105 ascospores/stem to 5 × 105 ascospores/stem resulted in a significant reduction of SPDI and APDI. Above this ascospore/stem threshold, responses to inoculum increases were minimal, with relatively little further increase in SPDI or APDI. Our study established, for the first time, a clear response of disease severity to numbers of ascospores discharged. SPDI was lower in plots containing flutriafol- or glufosinate-ammonium-treated residues compared with water-only control plots. Nearly all seedlings remained symptom-free in flutriafol-treated plots compared with only 20% in the water-only control plots. Crown canker measurement on adult plants indicated significantly lower APDI in plots containing flutriafol- or glufosinate-ammonium-treated residues compared with the untreated control plots. SPDI was highly correlated to APDI.