Potato growers should be targeting dumps now if they are to reduce the risk of early-season blight infecting emerging crops.

That is the warning from the Potato Council’s Rob Clayton, who says that wastage caused mainly by greening and cracking was much higher last season (15-20%, compared with 10-12% normally), so there are more potential sources of inoculum around.

“The worst-case scenario is to have green material on dumps as crops are coming through, so now is the time to get on and target them.”

Covering dumps with plastic is the best solution, he says. But where dumps are too big, using a granular herbicide, such as Casoron G (dichlobenil) will give good medium- to long-term control. “One application is unlikely to last the whole season, so you may have to go back in mid-July when the pile rots down, slumps and fresh growth starts on the newly exposed face.”

Spreading out the dump and covering it with soil will help maximise efficacy and reduce the need for follow-up treatments Peter Shakespeare of Certis advises. “You need the soil for the chemical to bind to. If you apply Casoron now, tubers in the dump will just be breaking dormancy and there should be enough moisture in the soil for the chemical to work.”

Mr Clayton says a residual and contact herbicide can be used but repeated applications will be needed as new growth emerges. Paraquat is one option and it is likely there will be many people keen to use up existing supplies before the final use date of 11 July 2008, he says. “It can be effective, but our trials at Potatoes in Practice found you’ll probably need 4-5 applications per season. You also need to make sure you don’t exceed the total dose allowed within a season in this sense, treat the dump as cropped area.”

Target dumps

  • Avoid green material on dumps to reduce blight risk
  • Best to cover with plastic
  • Consider granular herbicide if dump is too big
  • Casoron G main granular option (cost about £32/5kg pack). One pack will treat 160m2 at 30g/m2 rate
  • Paraquat can be used, but repeat applications needed
  • Do not exceed total paraquat dose
  • See www.potato.org.uk/blight