With the right fungicide, at a reasonable dose, it is possible to get away with just one autumn disease control treatment in winter oilseed rape, say agronomists.
A one-hit strategy, timed somewhere between late October and mid-November, is often effective against both phoma and light leaf spot, and gives a good yield response, says Essex-based independent agronomist Jamie Mackay.
“It was certainly the case last year. The autumn fungicide went on with the Kerb at the November timing, and was then followed up in the spring,” he recalls.
Mr Mackay says that even varieties such as Charger, with its greater disease susceptibility, were managed well with this approach, and there was no need for two autumn fungicides.
However, every year is slightly different, which is why it is so important to monitor crops closely, he cautions.
Prothioconazole is considered one of the best fungicide choices for oilseed rape disease control. For a one-hit strategy, it should be used at between half and two-thirds rate, says agronomist Jamie Mackay.
He suggest if growers need to spray twice, then use a cheaper option such as difenconazole first; then prothioconazole should be used for the second application.
Hutchinsons’ Dick Neale is of a similar opinion. “The prothioconazole-based treatments have proved themselves in trials and in the field. They have been totally reliable,” he says.
He opts for Prosaro (prothioconazole + tebuconazole), so that the growth regulation effect can be exploited too.
“There are cheaper products. But if you’re planning to spray once, you won’t get the same activity,” he adds.
Otherwise, there’s the SDHI-based Refinzar (penthiopyrad + picoxystrobin), which looks good for an early application.
“You get the disease control and the rooting benefit from an early timing. But remember it can only be applied once and it has a 5m buffer zone,” Mr Neale adds.
“The exception to this situation is where phoma comes into crops very early. If this happens, there can be a need to spray once thresholds are reached, which may be ahead of the optimum light leaf spot timing,” he says.
Although there are fungicides which are active on both light leaf spot and phoma, the application timing can be a compromise. It all depends on how any early disease infection develops, he says.
The number of days of rain from 1 August is often used to give an indication of when phoma is likely to be found, with 20 days considered to be the trigger.
“Given that we’ve had 13 days of rain in that time so far this year, the chances are that by mid-September we will have had enough. If that’s the case, we will start seeing phoma in early October,” says Mr Mackay.
Dick Neale, technical manager at distributor Hutchinsons, also warns against complacency, following similar weather patterns for the past four years with dry September months.
“Up until now, we’ve had plenty of wet weather, so there’s potential for phoma to be around. What we don’t yet know is whether the disease will get going or not,” he says.
That’s where the Rothamsted Research phoma forecasting service will be useful.
“It’s a very reliable tool which uses temperature and rainfall data to gives a prediction of when we expect 10% of plants to show symptoms,” Mr Neale says.
He agrees that it is possible to reduce fungicide spend and use just one autumn spray.
“But it requires some planning and forethought. You need to use a robust dose of one of the better fungicides, and ideally have one of the more resistant varieties in the ground,” he says.
Chosing varieties with good disease resistance can have an influence on spray costs, he adds.
“With phoma, a variety with a resistance rating of 4 often requires two autumn sprays. But those with a 6 or 7 will usually only require one application,” he adds.
What growers have to remember is that fungicides are applied to reduce the risk later in the season.
“Fungicides are insurance packages and should be viewed as such. Everything that we do in the autumn is pre-emptive, so the weather and disease history are relevant, he says.
Crops that aren’t treated in October and November can be badly affected by disease in the spring. With phoma, for example, the cankers don’t form in the stems until then, he says.
Mr Neale notes that autumn fungicides also have a role in regulating plant survival through the winter.
“They are part and parcel of overwintering the crop, which is why it is always sensible to use one autumn fungicide,” he says.
|Disease resistances of a selection of oilseed rape varieties from the AHDB Recommended Lists|
|Light leaf spot||Stem canker|
|Varieties are rated on a 1-9 scale where 1 is highly susceptible to the disease and 9 is highly resistant.|