LONG-TERM SURVIVAL AND SPORULATION OF LEPTOSPHAERIA-MACULANS (BLACKLEG) ON NATURALLY-INFECTED RAPESEED CANOLA STUBBLE IN SASKATCHEWAN/

In semi-arid Saskatchewan, Canada, peak ascospore discharge by Leptosp haeria maculans from. blackleg-infected rapeseed/canola (Brassica napu s and B. rapa) stubble residue generally occurred in the second year f ollowing the year of crop growth. Over 90% of the original stubble had disappeared by that time, leaving infected crowns and taproots that d eteriorated slowly under the prevailing dry surface soil conditions. A scospores of L. maculans continued to be discharged from this residue for a further 3-5 years, greatly exceeding the mean 3.3 year length of rotations out of rape followed by producers. Ascospore discharge from stubble residue could be intermittent, occasionally missing entire ye ars. Burial or flooding of infected 19-month-old stubble for 10 days a lmost entirely eliminated the production of ascospores. Rape stems wit h severe basal cankers produced L. maculans ascospores earlier and in greater numbers than did stems with extensive superficial lesions take n from the same field.