Seasonal Patterns of Ascospore Discharge byLeptosphaeria maculansin Relation to Blackleg of Oilseed Rape

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Abstract
A consistent seasonal pattern of L. maculans ascospore discharge, which started in July of the year after crop growth, was detected from residues of 40 fields of oilseed rape (Brassica napus and B. campestris) grown during 3 yr in Saskatchewan [Canada]. Exceptions occurred in 1977, when ascospores were discharged in May from residues of 4 rape fields grown in 1976. Ninety-five percent of the L. maculans isolates obtained from one of the latter groups of residues were the normally infrequent, virulent strain of the pathogen, while 98% of the isolates from residues of another crop grown in 1976, from which the more common discharge pattern was detected in 1977, were the prevalent avirulent strain. A distinct seasonal pattern of ascospore discharge also was detected from a 3rd strain of L. marculans which is pathogenic to stinkweed (Thlaspi arvense L.), but not rape. Severe blackleg only developed in rape plants inoculated at the crown before the 6-leaf stage of growth, because younger tissues were more susceptible to infection than older ones. Disease progress was slower in plants grown at 12.degree. C than at 18.degree. C. Symptoms of blackleg could not be detected on rape in the field until Sept. Ascospore discharge of the prevailing strain on rape residues, which starts after developing crops are past the 6-leaf stage, occurs too late to cause severe loss in Canada. A similar relationship between the times of ascospore discharge and disease development on stinkweed was found for the stinkweed strain of L. maculans.