Another year of difficult to controllight leaf spot (LLS) has probably left many growers pondering what to do thisseason, but SRUCs Dr Fiona Burnett says significant changes arent necessary.
She points to a more stable situationin Scotland where more LLS resistant varieties are grown. Control remainschallenging but severity data in monitored crops is no worse now than it wasfive years ago, despite the variable azole sensitivity we have noted in HGCAwork as far back as 2003, she says.
So why the problems south of theborder? Dr Burnett says its a combination of factors. Certainly the weatherhas been more favourable to the disease in recent seasons and with theintensity of rape grown, any trash-borne disease is bound to benefit. This isnot helped by agronomic factors. A focus on phoma means that fungicide timingand product choice can also be a factor, she warns.
With both phoma and LLS a significantthreat in many English counties, and growers unwilling or unable to make athird autumn pass, she says agronomic practice needs tweaking. In her viewcontrol comes down to variety choice and the effective use of fungicides. Theone thing we have learnt in Scotland is the importance of varietal resistance.OSR remains the most profitable break crop so rotation intensity or cultivationstrategies are unlikely to change, but which varieties you put in the groundcan.
In Scotland our general rule is lookfor a minimum of a six RL LLS score. English growers need to factor in RLscores when selecting varieties it might be less straight-forward but anyloss of performance is likely to be covered with better control of the disease.In addition, certain fungicides that are effective against phoma offer littleactivity against LLS.
With no alternative mode of actionfor prothioconazole and tebuconazole and little new product in the pipeline,selection pressure is inevitably intense. In cereals were relying on twoprimary azoles in mixtures yet in OSR we are relying on two azoles asstraights, or as a combination. Growers are heading the advice to use at aminimum of a rate in wheat, even when combined with an SDHI to get control ofseptoria, and we need to consider raising doses in oilseed rape as well.
When it comes autumn fungicideapplications split phoma treatments are typically too early for LLS, so thatability to delay applications to late autumn is useful. Late October to midNovember is standard there is a compromise to be made if land is likely tobecome unpassable much into November. You need to stay in a protectivesituation, and ideally see the crop through until you can get back in after thewinter. A third pass is probably impractical so the only real option growershave is to manipulate dose, and in my view you just cant get away with a lowrate anymore, regardless of when the application is made.
Dr Burnett hopes that in time betterforecasting will help, and highlights an HGCA funded project that aims to dojust that. This research should help us identify the timing of epidemics aswell as the risk. We can then pass this onto growers which will help themtarget application timing more effectively and remain in a protectivesituation, she concludes.
Bayers Gareth Bubb agrees and says that in somesouthern English counties where Phoma and LLS are a concern liftingrates to improve LLS control is a wise move. Where growers are opting forsplit Proline275 (prothioconazole) autumn treatments thenupping the second dose from 0.32 to 0.46l/ha is a sensible precaution. Eventhough youll want to get back in at early stem extension, or before, it canstill be a considerable gap so persistence is all important.
And when it comes to variety choiceshe suggests growers consider Harper. He says its nine for phoma provides abasis for phoma control and can pay dividends when it comes to LLS. It meansthat you might have a little bit more flexibility when it comes to fungicideapplications which might help better targeting of LLS infections, heconcludes.