The Crop Doctor
Grains Research & Development Corporation

Keeping canola in the rotation means having a disease management strategy in place. We've seen yields either plateau or decline over the last five years, and while we still lack conclusive scientific proof, circumstantial evidence says disease is the culprit.

Initially the trend was most obvious in the higher rainfall areas but as the area under canola has increased and, more importantly, the rotations tightened, it becomes more tempting to point to a disease build-up as the reason for declining yields.

While it's still hard to be definite, farmers and researchers working in the high rainfall area of southern NSW have shown that where disease is absent or controlled, canola will yield to its water-limiting potential.

Looking for the best bet for canola, they've been conducting trials since 2000 and it seems that blackleg and sclerotinia are a bit like Mohammad Ali's fists, 'if the right one doesn't get you then the left one will'. In 2001, fungicide treatment for sclerotinia produced yield responses as high as 0.9 tonnes/ha at sites throughout the area. Blackleg didn't present as a problem that season, but the following year preventative fungicide treatments for blackleg resulted in as much as 0.7 tonnes per ha increase in yields.

Conventional wisdom says that to be safe from blackleg you need to sow canola at least 500 metres from last year's stubble and with the amount of canola now being grown right across the grain belt that paddock is hard to find so, the risk of the disease is widespread. Lodging caused by the stem canker form of the disease is obvious.

Over the last couple of seasons there have been increasing reports of the root form of blackleg showing up. This is a more insidious symptom and we still don't fully understand it's impact, but you've only to see the damage done to the plant's root system to think that it must be affecting yield. Managing risk comes at a cost - success is about weighing the potential risks and the cost of control against potential rewards. Picking the right variety is the first step in managing that risk.

The collapse of resistance in the sylvestris lines in the last two seasons rules those varieties out of contention. The risk associated with growing them far outweighs the relatively small potential increase in yield. In fact it's worth checking on the resistance rating of all varieties - some have slipped back a little over the last year.

If you're in the high rainfall area, say east of the Newell Highway in New South Wales, and you're sowing a canola variety with a resistance rating of seven or eight, it's probably worth an application of Jockey® at about $6/ha as an insurance policy. If the variety you're sowing has a lower rating or, you're in a high risk area, then Impact® on fertiliser at $20/ha is the way to go.

That's a high cost strategy, so what's the potential reward? Working under commercial conditions researchers have demonstrated increased yields of about 0.25 tonnes/ha from strategic applications of Jockey® and 0.5 tonnes/ha for Impact® on fertiliser applications.

Sclerotinia presents a different problem. Once again, because so many plants host this disease, the risk is widespread. We know that the right weather conditions are needed for an outbreak, and in a bad sclerotinia year there are significant yield benefits from an application of Rovral® liquid fungicide. The less predictable winter rainfall pattern in the northern growing areas makes strategic planning difficult, in the south we know that a spray, when 20-50% of the crop is flowering and rain is promising, gives the best protection - but at a price.

A preventative application like this costs about $60/ha and we still don't understand all of the factors that turn the risk of a sclerotinia outbreak into a serious outbreak. With such a high cost involved, the potential yield loss is the deciding factor. Losses from sclerotinia have been as high as 30% or as little as 5%. When the potential yield is up around three tonnes/ha, it's worth the cost of protecting it. Otherwise, until we have a better understanding of what drives the sclerotinia disease cycle, it's a case of wait and see.