Alternaria, the trigger for early blight, has become a bigger problem because of the increasing popularity of susceptible potato varieties and changes in fungicide use.
Worst hit in the past two years was the Markies variety, while Saturna and Maris Piper had also been affected, said Bayer CropScience product manager Neil Thompson. Disease usually strikes from late July, but primary infection could be spotted earlier.
"Limitation of mancozeb use has contributed and there's been a knock-on effect from the development of more aggressive late blight strains," he said. "Recently introduced fungicides tackle them very effectively but have little or no Alternaria activity."
Initial symptoms could show two to four weeks from emergence. Tell-tale signs are irregular-shaped brown or black spots, developing into lesions with dark rings.
Agrovista technical manager Dr Mark Palmer advised growers to adapt fungicide programmes for susceptible varieties to protect early against this primary infection. "The key is to keep ahead of the Alternaria risk by using protectant products with dual activity against Alternaria and late blight," he said.
Palmer added that Consento was the most appropriate product to use during the rapid canopy-growth phase. "From canopy complete onwards, susceptible varieties still need to be protected against secondary Alternaria infection but the focus must be on late blight control," he explained.
"This is the time when programmes need to switch to the premier league late blight products - Infinito (fluopicolide + propamocarb), Ranman (cyazofamid) and Revus (mandipropamid) - to combat the more aggressive strains that are now dominating."