PATTERNS OF ASCOSPORE DISCHARGE BY LEPTOSPHAERIA-MACULANS (BLACKLEG) FROM 9- TO 13-MONTH-OLD NATURALLY-INFECTED RAPESEED CANOLA STUBBLE FROM 1977 TO 1993 IN SASKATCHEWAN/

Ascospores of Leptosphaeria maculans (blackleg) often began to be prod uced on rapeseed/canola stubble in Saskatchewan in June, nine months a fter harvest. However, few ascospores usually were discharged before J uly 31, and rapeseed crops generally were flowering or podding by mid- July and were more resistant to infection. The number of samples disch arging ascospores was positively correlated with the number of days wi th measurable rainfall in April, June, and various combinations of mon ths from April to July. The mean number of ascospores caught per trapp ing date and maximum number of spores collected (most productive date) were also related to days with measurable rainfall in the April to Ju ly period. Total rainfall was less important than its frequency. Ascos pore numbers were negatively correlated with number of days from April to June or July having a maximum temperature of 30 degrees C or more. Strains of L. maculans differed in numbers of ascospores produced and in seasonal ascospore discharge patterns, with those from cruciferous weed hosts sporulating earlier and producing larger numbers of spores .