Epidemiology and management of Leptosphaeria maculans (phoma stem canker) on oilseed rape in Australia, Canada and Europe

Phoma stem canker (blackleg), caused by Leptosphaeria maculans, is an impor tant disease on oilseed rape (canola, rapeseed, Brassica napus, Brassica ju ncea, Brassica rapa) causing seedling death, lodging or early senescence in Australia, Canada and Europe, but not in China. The two forms of L. macula ns (A group and B group) that occur on oilseed rape are now considered to b e separate species. The epidemiology and severity of phoma stem canker diff ers between continents due to differences in the pathogen population struct ure, oilseed rape species and cultivars grown, climate and agricultural pra ctices. Epidemics are most severe in Australia, where only the A group occu rs, and can be damaging in Canada and western Europe, where both A and B gr oups occur, although their proportions vary within regions and throughout t he year. Epidemics are slight in China, where the A group has not been foun d. Dry climates (Australia, western Canada) lengthen the persistence of inf ected debris and may synchronize the release of airborne ascospores (after rain) with seedling emergence. L. maculans spreads from cotyledon and leaf infections down petioles to reach the stem, with infections on cotyledons a nd leaves early in the season producing the most damaging stem cankers at t he stem base (crown). Development of both crown cankers and phoma stem lesi ons higher up stems is most rapid in regions with high temperatures from fl owering to harvest, such as Australia and Canada. Breeding for resistance ( genetic, disease escape or tolerance), stubble management, crop rotation an d fungicide seed treatments are important strategies for control of phoma s tem canker in all areas. Fungicide spray treatments are justified only in r egions such as western Europe where high yields are obtained, and accurate forecasts of epidemic severity are needed to optimize their use.