The discovery of a new blackleg strain with the ability to attack the previously resistant varieties (varieties with Western Australia Blackleg Resistance Rating of 8+) means growers need to reconsider their blackleg management strategies.

Western Australia Department of Agriculture canola pathologist Ravjit Khangura said blackleg is known to be both seed and stubble-borne.

"For the existing strains, seed-borne inoculum is considered to be of negligible importance compared with the vast amount of inoculum carried over on the stubble," Dr Khangura said.

"However, the possibility of carry over of the new strain on the seed and its subsequent spread into new areas can not be disregarded," she said.

It is not known how widespread, or to what degree the new strain will affect the 2003 canola crop. There are now three confirmed reports of the evolution of these new strains including one from Mt Barker and two from eastern Australia.

The new strain was first isolated from Mt Barker by Dr Li Hua of the University of Western Australia and was shown to affect a previously highly resistant variety Surpass 400, in subsequent tests.

In a small blackleg survey in Western Australia in 2002, mild cankers were observed in some of the Surpass 501TT crops and moderate cankers, measuring up to a quarter of the stem diameter, were observed in a few Surpass 501TT crops from Northam and Cunderdin.

During the 2001 blackleg survey, some cankers on 8+ varieties were in canola crops from Scadden. It is yet to be determined if the strain(s) detected at these other sites are similar to that detected at Mt Barker by Dr Li Hua.

Dr Khangura said canola sowing was in progress in many areas, but those who had not sown yet, should consider some of the management options.

"If sowing 8+ varieties, growers should try to isolate their current canola paddock from previous year’s residues 8+ resistant varieties. An isolation distance of more than 500 metres is best. Upwind is better," Dr Khangura said.

"In line with the Canola Association of Australia recommendations and if feasible, consider using varieties with blackleg resistance with ratings lower than 8+. As suggested by the Canola Association of Australia, if 8+ varieties were planted last year, as much residue as possible should be destroyed," she said.

"Consider applying fungicide treatments (seed dressing or fertiliser treatment) while seeding to minimise the risk of adverse effects if resistance breaks down.

"However, it should be noted that applying fungicides with 8+ varieties is recommended as a precautionary measure if resistance breaks down, otherwise Western Australia Department of Agriculture trial data suggests that there is no economic benefits of applying fungicide with 8+ varieties under existing strains of blackleg."

Growers who see any blackleg leaf lesions on 8+ varieties early in the season or crown cankers late in the season, should send samples to Ravjit Khangura at the Western Australia Department of Agriculture in South Perth or through their local District Offices.