Alternaria (early blight) outbreaks in potatoes could be earlier and more aggressive this year, especially in the processing variety Markies, says John Sarrup, agronomist in North Yorkshire.

Cases have already been reported in Lincolnshire, Warwickshire, Norfolk and at Askam Bryan College in Yorkshire.

Alternaria causes lesions to develop on leaves, resulting in early leaf loss and leading to reduced tuber yield and quality. The disease usually appears on senescing leaves at the end of the season and doesn’t cause too much of a problem, but recently cases have been coming in earlier, says Mr Sarrup.

“I have seen a significant increase in cases of alternaria over the past couple of years, especially in varieties such as Markies that achieve good growth with relatively low nitrogen inputs.”

Normally Markies would have another two months until burn-down, but experience last year showed disease can develop from patches to a completely destroyed crop in two weeks during August, he says.

Markies is generally the worst affected variety, but Hermes, Maris Piper, Maris Peer and Saturna have also shown increased levels of infection, he added.

Mr Sarrup suggests cuts in nitrogen application on these low-demand crops could be contributing to the problem.

“As growers apply less nitrogen to crops they are put under some nutritional stress and are more susceptible to Alternaria – once infected, crops are also less likely to grow through the disease.”

Manganese deficiency also contributes to the problem along with certain weather patterns, he adds. “Dry weather during sporulation and wet weather during inoculation increases disease risk, and this has been the case this year.”

Growers should look out for the disease when the crop is at full canopy and if signs are found apply Olympus (azoxystrobin and chlorothalonil), which gained a specific off-label approval (SOLA) this year, says Mr Sarrup.

“I recommend two applications of Olympus a fortnight apart after first signs of the disease appear.”

Some believe mancozeb can give moderate protection, but Mr Sarrup reckons benefits are minimal and the high cost means it has been taken out of his spray programmes this year.

“You need a huge amount of mancozeb to achieve the same results as Olympus, the high cost is prohibitive and the product is hard to get hold of.”