Factors Affecting Incidence of Sclerotinia Stem Rot of Rapeseed (Canola)

In a study of sclerotinia stem rot in commercial rapeseed fields in East and West Central Saskatchewan in 1983 and 1984, several factors relating to inoculum density were monitored when the crops were in bloom. Carpogenically-germinated sclerotia were counted in I II specific areas. The frequencies of Sclerotinia-infested live and dead petals, leafaxils and leaf bases were determined by plating on potato dextrose agar with added rose bengal and streptomycin. The final percentage of diseased plants was determined shortly before the crops were swathed. The results demonstrated a significant relationship between petal infestation at early bloom and final disease. Significant relationships between germinated sclerotia and disease were obtained only with intensive sampling. Infested petals and disease were regularly found when apothecia were absent, thereby demonstrating the infective potential of extrinsically produced ascospores. The possibility of using petal infestation to forecast stem rot and improve the economics of chemical control is discussed; however, several, refinements in the technique are required. Laboratory studies demonstrated the potential for petals to be colonized by. sclerotiorum before falling from the inflorescences onto plant surfaces. Laboratory and field studies of the persistence of water droplets in leaf axils, a major infection court of rapeseed, demonstrated that water sometimes persisted for relatively long periods even when ambient environmental conditions were generally unfavorable for infection. In view of the moisture requirements for infection, this may partially explain the occurrence of low to moderate levels of disease in some rapeseed fields during relatively dry years