Symptoms of blackleg (Leptosphaeria maculans) on the roots of canola in Australia

Abstract

The fungus Leptosphaeria maculans (anamorph Phoma lingam) causes blackleg in canola (Brassica napus). Infection of all above-ground parts of the plant has been described, but there are no reports of infection of roots. In a recent review West et al. (1999) described the common plant symptoms, but did not mention root infection. Canola plants at the flowering and pod-filling stages, with foliar symptoms of blackleg, were collected from commercial crops in South Australia, Victoria and New South Wales, Australia in November and December 2000. Some plants had lodged and had the basal cankers commonly described for blackleg, however, many plants had died prematurely during late pod fill. Cankers and other external symptoms were not present on the lower stems of these plants and many could be pulled easily from the ground. The lateral roots had blackened areas. Taproots had extensive internal tissue necrosis. In some plants, the pith of the stem was also necrotic, although there were no external symptoms. In another case, a distinct black band was seen around the circumference of the taproot below the crown. Samples of discoloured tissue were surface-sterilized and cultured on quarter-strength potato dextrose agar. Phoma lingam was isolated from all samples, identified as described by Punithalingham & Holliday (1972). All isolates were virulent (A-group) according to PCR-based tests carried out by the Root Disease Testing Service, South Australian Research and Development Institute (Sosnowski et al., 2001). Phytophthora and Rhizoctonia spp. were not isolated from the discoloured tissues. Pycnidiospore suspensions (106 spores mL−1) were prepared from two isolates of P. lingam (66/97 and 208/00) obtained from leaves and roots, respectively, of diseased plants. These suspensions were used to inoculate (i) lateral roots severed with surface-sterilized scissors; and (ii) hypocotyls wounded with sterile toothpicks on otherwise healthy canola seedlings in the glasshouse. Three weeks after inoculation, stem and crown cankers appeared on plants inoculated with either isolate on either roots or hypocotyls and root tissue was necrotic, whereas control plants, wounded as described above and inoculated with sterile water, remained healthy. Phoma lingam was the only pathogen isolated from diseased root tissue. This confirms the occurrence of the blackleg fungus in the roots of canola plants, and reveals its ability to infect through wounds in the roots. This is the first record of symptoms of blackleg on canola roots in Australia. However, there is anecdotal evidence that such symptoms have occurred in the past (D. Ballinger, Dovuro Pty Ltd, Horsham, Victoria, Australia, personal communication; M. Barbetti, Agriculture Western Australia, South Perth, Western Australia, Australia, personal communication). Initially it was assumed that the infection had spread from the crown into the roots, but new evidence suggests that the disease can also be soilborne. This may have important ramifications for understanding the blackleg disease cycle. This aspect will be investigated further in the current project entitled ‘Investigating resistance and management of blackleg in canola’, which is funded by the Australian Research Council, the South Australian Grains Industry Trust Fund and the South Australian Research and Development Institute.