SOME OF the UK’s best-loved potato varieties are under threat this year from a relatively new problem, as well as the usual round of late blight.

More commonly a threat in oilseed rape, alternaria – which causes symptoms in potatoes commonly known as early blight – has become an increasing problem over the last two years, according to Neil Thompson, the product manager for potato fungicides at Bayer CropScience.

He says this is due to more susceptible varieties and changes in fungicide use. “Limitation of mancozeb use has contributed and there’s been a knock-on effect from the development of more aggressive late blight strains. Recently introduced fungicides tackle them very effectively but have little or no alternaria activity.”

So far, the variety Markies has been worst affected, while Saturna and Maris Piper have shown susceptibility too. And, although disease outbreaks have generally been seen from late July onwards, primary infection can be spotted much earlier he says.

“Initial symptoms can be found two to four weeks from emergence on older leaves lower down plants. Look for irregular shaped dark brown to black spots. These develop into characteristic ‘target spot’ lesions, with dark concentric rings surrounded by a chlorotic halo.”

Agrovista’s technical manager, Dr Mark Palmer, advises adaptation of fungicide programmes for susceptible varieties to build in early protection against this primary infection. “The key thing is to keep ahead of alternaria risk by using protectant products with dual activity against alternaria and late blight.”

Consento is the most appropriate product to use during the rapid canopy growth phase he says, but mancozeb is another dual activity option when used at rates over 1500 g/ha.