Potato growers face a high risk of Sclerotinia attack in crops this season, warns leading disease pathologist Dr Peter Gladders of ADAS. Oilseed rape crops have suffered a second successive season of very high infection, with repeated rain events creating ideal conditions for further spore release to infect other susceptible crops, he adds.
"Continued spore release from soil-borne sclerotinia fruiting bodies, along with the potential for secondary spread from plants already infected, poses a significant threat for other susceptible crops this season.
"Carrot, potato and pea growers will have to be especially vigilant and take necessary measures to protect crops from infection," advises Dr Gladders.
Whilst blight control is obviously the number one priority for foliar protection, growers are increasingly turning to Shirlan (fluazinam) to help protect against both Sclerotinia and blight. Field trials in Norfolk last year highlighted the value of mid-season Shirlan applications – where including the fungicide in the mid-season blight programme also resulted in an average 65% reduction in sclerotinia infection.
On untreated plots Sclerotinia infection levels in the three trial fields reached over 80 plants per 150m2, equivalent to more than 5000 plants/ha. But a programme of Shirlan at 0.4 l/ha, alternated with Revus, gave a 68% reduction in the worst incidence and cut the number of infected stems from 80 to 25. Even in the least affected plots, the level of infection was reduced by over 50%, reports Syngenta Area Manager, Andrew Curtis.
"The trials have confirmed US research and advice for Shirlan application. It supports the traditional view that early application before the canopy closes is important to coat the lower leaves before they senesce and provide food sources for sclerotinia, as well as application during flowering, or early stable canopy, as required by blight programme timing," he advises. In all instances the level of blight control from the Shirlan and Revus programme was exceptional, he added.
The trials were undertaken on the Elveden Estate, where Vegetable Manager, Andrew Francis, reports up to 60% of plants affected in the worst fields. "It is the long season crops with big leafy canopies which suffer most from sclerotinia attacks. Affected plants are dying off up to four weeks early, which can seriously compromise bulking up of the crisping and set-skin baker varieties," he says.
Maris Piper has repeatedly been the hardest hit, with damaging effects also seen in Estima and Russett Burbank, although the Saxon maincrop appears relatively unaffected. He notes that Piper is also a free flowering variety, which may be increasing levels of attack if infected petals fall and stick onto leaves. Estima has a similar growth habit but generally far fewer flowers. The effects are not seen in the farm's 80 hectares of salad crops.
Mr Francis is now focusing Shirlan applications before and during petal fall to reduce the risk of spores developing on the leaf. The higher 0.4l/ha rate has proven to give the all important required level of blight control, even in last season's high risk periods, along with the reduction in Sclerotinia. Now, it is planned to use Shirlan as a standard in the mid-season programme, with up to five applications where sclerotinia is a risk.
"In our field trials you could see to the line where the Shirlan applications had been made," he reports. "Clearly the early senescence would be having an effect on tuber size and consistency. But we are also very keen to reduce the return of sclerotia into the soil and maintain a sustainable rotation for the potatoes and the carrots."
