Managing Brassica Stem Canker Phase 2
Brassica stem canker is a disease complex of several fungi causing symptoms that range from superficial scurfing, russetting and discrete lesions on stems to complete stem rot and plant collapse. The primary fungal pathogens have been identified as Leptosphaeria maculans and Rhizoctonia solani AG 2.1, 2.2 and 4. These fungi are mainly soilborne and plants were infected early after transplanting into infected soil. Applications of fungicides such as azoxystrobin and fludioxonil prior to transplanting nursery seedlings suppressed stem canker, but none provided complete control. In 2009/10, 19 greenhouse and 9 field experiments were undertaken to improve the knowledge of stem canker and provide further management options. The main findings of this study were: There was no correlation between pre-planting soil amounts of L. maculans and severity of stem canker at harvest. L. maculans infects through leaves, roots and seed. The fungus was detected in spore traps near cauliflower plantings, showing potential for airborne dispersal. Infection through leaves was more severe when leaves were wounded and the pathogen moved systemically into the stem. Infected plants were detected in nurseries indicating seed infection. Brassica weeds (white mustard, wild rocket) and other Brassica crops (radish, rocket, kale and chinese cabbage) were all susceptible to L. maculans when inoculated leaves were wounded. Kale and chinese cabbage were also infected without wounding. The fungicides fluquinconazole and flutriafol, registered to control or suppress L. maculans in canola, reduced or eliminated stem canker when applied as pre transplanting drenches in combination with azoxystrobin. Fluquinconazole was also affective as a seed dressing. Greenhouse evaluations showed suppression of canker and increased plant growth when the biological agents Trichoderma (Trichoshield�) and Bacillus subtilis (Companion�) were used with fungicides as pre-transplanting drenches. Alternative plant growth products Rootpower� and Soil-Reviva� also reduced canker severity. Field evaluation showed the combination of azoxystrobin and flutriafol applied prior to transplanting reduced canker severity on both cauliflower and Brussels sprouts. There was no benefit in canker suppression by applying azoxystrobin, biological agents or plant growth products after transplanting. The increase in yield with the improved plant growth and suppression of stem canker may not provide economic benefit to warrant the application of the products. This needs to be further evaluated.