Sclerotinia stem alert

Canola growers - especially those with early flowering crops - have been warned to be on guard for the fungal disease Sclerotinia stem rot.

Recent rain, combined with cool conditions, has elevated the risk of the disease to early sown crops that may be or about to flower.

Department of Agriculture and Food research officer Ravjit Khangura said as the fungal fruiting structures of the disease, apothecia, were minute and often overlooked, growers were unlikely to observe any infection or symptoms until it was too late to treat.

Dr Khangura said it was important for growers to understand the impact of weather conditions on the Sclerotinia risk to canola crops, particularly just before and during flowering.

"If wet and humid conditions prevail a few weeks before flowering and the forecast is for continuous showers for the next two to three weeks, growers are advised to spray their crops at 20-30 per cent bloom if there is a history of Sclerotinia in their paddock or adjacent paddocks," she said.

"Where the conditions are dry or only average rainfall is expected until 30 per cent flowering and the forecast is for heavy rain afterwards, growers can still spray their crop until 50 per cent bloom."

The flowering or bloom stages are categorised as: 10pc bloom - 10 open flowers on the main stem; 20pc - 11-15 open flowers; 30pc - 16-20 open flowers; and 50pc - more than 20 open flowers on the main stem.

At 30pc bloom the crop attains an intense, yellow colour as the maximum number of flowers are open at this stage.

If dry weather is forecast during the entire flowering duration, the Sclerotinia risk is reduced.

Department research has shown a well-timed single spray can successfully control Sclerotinia and minimise crop losses in most cases.

Two treatments should be considered if growers have high risk paddocks, such as those with tight rotations, heavy soil types, a dense stand of canola and in high rainfall areas where the forecast is for continuous wet conditions during flowering.

Dr Khangura said growers could determine their Sclerotinia risk by considering the paddock history, proximity to 2013-14 Sclerotinia affected paddocks, likelihood of frequent rain events coupled with high relative humidity in the lead up to flowering and wet and humid conditions during flowering.

"In a normal year, disease will usually be severe in heavier soil parts of the paddock but in a wetter year disease can be as bad on all soil types," she said.

"Although the onset of disease epidemics is unpredictable, weather conditions a few weeks before and during flowering play a major role in driving the disease epidemic."