VIEW ARTICLE

Ecology and Epidemiology

Seasonal Patterns of Ascospore Discharge by Leptosphaeria maculans in Relation to Blackleg of Oilseed Rape. D. C. McGee, Plant pathologist, Department of Agriculture, Victorian Plant Research Institute, Victoria, Australia, Present address of senior author: Department of Botany and Plant Pathology, Iowa State University, Ames 50011; G. A. Petrie, Research Station, Research Branch, Agriculture Canada, 107 Science Crescent, Saskatoon, Saskatchewan, Canada S7N 0X2. Phytopathology 69:586-589. Accepted for publication 12 December 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-586.

A consistent seasonal pattern of Leptosphaeria 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. Exceptions occurred in 1977, when ascospores were discharged in May from residues of four 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 third strain of L. maculans 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 C than at 18 C. Symptoms of blackleg could not be detected on rape in the field until September. This supported the contention that 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.