I’m fed up with talking about the weather, but I can console myself with the fact we have grabbed every opportunity so far and progress is not too bad.Balance PGR activity with need for control
OILSEED rape crops in the West are likely to be showing the first signs of phoma in the next few days and growers are advised to be alert to the disease particularly with such a variation in crop size this season.
Although the onset of phoma is already significantly later than last year on account of the relatively dry weather experienced in August and September, big differences in crop growth stages across the region means spray strategies must be carefully thought through.
Leaf stage
That’s according to Bayer CropScience’s commercial technical manager Tim Nicholson who says some crops in the West are at the 7-8 leaf stage while others are at 2-4 leaves and some have still to germinate.
Based on the 20-day ‘rain events’ threshold, Rothamsted Research’s phoma prediction model is suggesting crops in the Plymouth area will see the disease this weekend, while at Shawbury in Shropshire the first signs of phoma will occur next week.
“For those where the rain has come their problem is not the number of sprays but the nature of them. It will be a balance between autumn disease control and PGRs,” says Mr Nicholson.
“Some growers are talking of putting on a PGR-type fungicide if the crops are very advanced, but these treatments are usually weak on phoma and will need a follow up spray to focus on controlling disease,” he adds.
In this situation, he advises a Proline (prothioconazole)-based treatment within four weeks of the PGR spray on phoma-susceptible varieties with a rating of 5 or less for the disease.
“With more resistant varieties there is a little more leeway as to when you do the follow up spray,” he says.
Threshold
If a PGR isn’t needed the phoma spray should be applied when the 10 per cent disease threshold is reached.
The phoma threshold will also soon be reached on small crops, where the danger of the disease spreading is greater because of the reduced distance between leaf and stem.
These crops should also be treated when the 10 per cent disease threshold is achieved and should then be monitored continually as new growth will be unprotected which could mean a second spray is required four to six weeks later. “But be prepared to treat foliage if it becomes infected,” says Mr Nicholson.
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