A SEVERE infection of blackleg fungus has been discovered in a crop in western Victoria.

And there are fears the outbreak may only be the tip of the iceberg.

Marcroft Grains Pathology agronomist Steve Marcroft last week described the outbreak, in a paddock of canola variety 45y77 at Lake Bolac, as very severe.

The crop was sown 100m from thunder canola stubble, leading to the high disease pressure, he said.

"If the plants are dying now, then there will be major yield losses," Mr Marcroft said.

He said the average, or above-average rainfall in the Wimmera and Western District could result in high levels of disease.

"We're noticing there's more disease around this year," Mr Marcroft said.

"I haven't seen anything severe as this (at Lake Bolac) but I'm expecting to see more stem canker over the next couple of months."

He said farmers had become complacent because 10 years of drought meant blackleg had not been a major problem.

Mr Marcroft said the one first signs of blackleg infection were plants falling over.

"If plants are falling over now, there will be carnage by the end of the year," he said.

He said the plants could tolerate some blackleg disease, but once the disease level reached 50 per cent, there would be yield losses.

Early sowing could overcome the problem, but he advised against early sowing just to avoid blackleg.

"If you sow early in May, when it's reasonably warm, then the plants get through the susceptible stage before the weather gets too cold and wet," Mr Marcroft said.

He said in areas of high disease pressure, seed dressings could be effective

Mr Marcroft said stem canker at this time of year was the result of seedling infection.

"That's why you get an effect from seed dressings," he said.

Mr Marcroft said marlin canola stubble from two years ago had survived in the Lake Bolac paddock due to inter-row sowing.

He said fungicide applications were good insurance for canola in the Western District.

Farmer George Burdett, who planted the canola, said blackleg had not been a problem in previous years.

"I've got other paddocks with canola besides this one and it's not a problem (there)," he said.