Arapidly evolving situation is how ADAS plant pathologist Dr Peter Gladdersviews the current phoma threat in OSR crops.
He points out that the dry spell came to anend during the later weeks of August and warns that in many areas the 20 dayrain threshold could already have been reached.
Some parts of the east had eleven dayswhere rain occurred in August, topped up with nine already this month. Thetwenty rain day threshold from August 1st is a guide that ascosporescould be maturing on rape stubbles.
What this means he says is the possibilityof an earlier phoma threat this season. Following all that fine weather manygrowers are well on top of autumn drilling. We already have some crops at thefour 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 earlyOctober, he notes. Phoma is easier to manage on larger plants. Check cropsregularly so fungicide is applied when 10-20% of plants have phoma leaf spots.
But he says that with phoma could come withlight leaf spot (LLS) spores too. This season could be very variable. Lastseason we saw many winter OSR crops lost to the wet conditions and pigeonswhich could reduce inoculum this season. However, that comes against abackground of intensive OSR over the last six years, poor varietal resistanceand weather impact on farming operations last autumn.
His advice is to plan for split pre-stemextension applications. With the possibility of a forward phoma threat thechances are youll be coming back in with a second application. It makes senseto use this as another opportunity to manage LLS by picking a product withactivity against both. And management is the key if it gets into the leaf youcant kill it.
For those tempted to come back in at stemextension he urges caution. Light leaf spot can cycle even when temperaturesare close to freezing. With relatively mild winters over recent seasons cyclinghas been reduced from eight to six weeks to more like four to five, heconcludes.
Bayers Tim Nicholson (above) reminds growers thatwhen it comes to autumn applications product choice is just as important asdose. For phoma, althoughProline275 (prothioconazole) remains the most active fungicide, anumber of products do provide respectable activity. However, when it comes toLLS and combined activity on both of these important diseases, Proline275is the gold standard.
In most crops a split-spray Proline275programme of two 0.32 L/ha applications with no more than a 46 week gapwill set the crop up for the winter, but growers might need to get back in atearly stem extension. Should some PGR activity be required on larger crops,Folicur (tebuconazole) can be added to Proline275 as necessary.Where LLS is a serious concern this autumn, then growers should consider a lateautumn treatment of 0.46 L/ha Proline275 with a spring followup.
Regardless of dose, going for splittreatments would appear sensible this season given that Phoma thresholds havealready been reached and to help with cycling LLS throughout autumn and earlywinter, he adds.
Mr Nicholson also reminds growers to thinkforward to the sclerotinia spray timing in their OSR fungicide programmeplanning, as the total allowable Proline275 doseper crop is 1.26L/ha.