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A rapidly evolving situation is how ADAS plant pathologist Dr Peter Gladders views the current Phoma threat in OSR crops.

He points out that the dry spell came to an end during the later weeks of August and warns that in many areas the 20 day rain threshold could already have been reached.

“Some parts of the east had eleven days where rain occurred in August, topped up with nine already this month. The twenty rain day threshold from August 1^st is a guide that ascospores could be maturing on rape stubbles.”

What this means he says is the possibility of an earlier Phoma threat this season. “Following all that fine weather many growers are well on top of autumn drilling. We already have some crops at the four leaf stage. It could be that our Phoma threat is more forward this season, and I would suggest growers be on the lookout for the disease from early October,” he notes. Phoma is easier to manage on larger plants. Check crops regularly so fungicide is applied when 10-20% of plants have phoma leaf spots.

But he says that with Phoma could come with Light Leaf Spot (LLS) spores too.” This season could be very variable. Last season we saw many Winter OSR crops lost to the wet conditions and pigeons which could reduce inoculum this season. However, that comes against a background of intensive OSR over the last six years, poor varietal resistance and weather impact on farming operations last autumn.”

His advice is to plan for split pre-stem extension applications. “With the possibility of a forward Phoma threat the chances are you’ll be coming back in with a second application. It makes sense to use this as another opportunity to manage LLS by picking a product with activity against both. And management is the key – if it gets into the leaf you can’t kill it.

For those tempted to come back in at stem extension he urges caution. “Light Leaf Spot can cycle even when temperatures are close to freezing. With relatively mild winters over recent seasons cycling has been reduced from eight to six weeks to more like four to five,” he concludes.

Bayer’s Tim Nicholson reminds growers that when it comes to autumn applications product choice is just as important as dose. “For Phoma although Proline²⁷⁵ (prothioconazole) remains the most active fungicide, a number of products do provide respectable activity. However, when it comes to LLS and combined activity on both of these important diseases, Proline²⁷⁵ is the gold standard.

“In most crops a split-spray Proline²⁷⁵ programme of two 0.32 L/ha applications with no more than a 4-6 week gap will set the crop up for the winter, but growers might need to get back in at early stem extension. Should some PGR activity be required on larger crops, Folicur can be added to Proline²⁷⁵ as necessary. Where LLS is a serious concern this autumn, then growers should consider a late autumn treatment of 0.46 L/ha Proline²⁷⁵ with a spring follow up.  

Regardless of dose, going for split treatments would appear sensible this season given that Phoma thresholds have already been reached and to help with cycling LLS throughout autumn and early winter,” he adds.

Mr Nicholson also reminds growers to think forward to the Sclerotinia spray timing in their OSR fungicide programme planning, as the total allowable Proline²⁷⁵  dose per crop is 1.26L/ha.