Strains of Leptosphaeria maculans with the Capacity to Cause Crown Canker on Brassica carinata are Present in Western Australia.

Abstract

Blackleg, caused by Leptosphaeria maculans (Desm.) Ces. et de Not., is the most important disease of canola (Brassica napus L.) in Australia, Europe, Canada, and North America (2). During the early 1990s, new cultivars of canola with resistance to blackleg were released in Australia. Despite good adult-plant resistance, these cultivars still suffered significant yield losses from blackleg under high disease pressure. Potential new sources of blackleg resistance such as B. nigra L., B. carinata L., and Sinapis alba L. are being evaluated. B. carinata is believed to be highly resistant to blackleg by virtue of its B-genome. However, some L. maculans isolates that can attack B. carinata have been reported from Germany (1). During the 2003 growing season, 22 isolates of L. maculans were collected from different canola-growing areas of Western Australia and tested for their reaction on 24 seedlings of each of various Brassica genotypes, including B. carinata, in a controlled environment chamber. Twenty-four seeds per genotype were sown in 100-ml plastic pots (12 seeds per pot) and both cotyledons of 10-day-old seedlings were wound inoculated with a conidial suspension (1 × 107 conidia/ml) of each isolate of L. maculans. Disease assessments were made 2 weeks after inoculation. The majority of isolates induced a noninvasive hypersensitive reaction on B. carinata without pycnidial development. However, four of the isolates caused lesions with abundant pycnidia on B. carinata cotyledons. The lesion size ranged between 3 and 7 mm and appeared similar to that on susceptible B. napus cultivars. B. carinata seedlings were grown for another 8 weeks in a glasshouse, and crown cankers were observed from plants inoculated with three of the four seedling virulent isolates. The severity of crown cankers as percent of stem circumference (percent disease index) ranged between 20 and 54%. Twenty-five stem pieces from mature B. carinata plants infected with one of the three isolates were plated on V8 juice agar and L. maculans was recovered from 70% of pieces. Abundant pycnidia were also observed on these stem pieces. These results have important implications for using B. carinata as a source of blackleg resistance in canola breeding. To our knowledge, this is the first report of L. maculans isolates with the capacity to induce crown cankers on B. carinata in Australia.