Late blight risk threatens potato crops as rain returns

Potato growers are being warned against cutting fungicide programmes too soon after resistant late blight was detected at two sites and wetter weather increased the risk of late-season infection.

The EU_46 strain was identified in Shropshire and Lancashire before conditions turned dry. Although disease pressure has remained unusually low most of the season, rain and surviving green leaf is an opportunity for blight to develop before harvest.

“We learned last year that even in a dry year it can still turn wet late in the season and then, all of a sudden, EU_43 or EU_46 appears,” said Agrii lead technical advisor for potatoes Ed Maule.

Monitoring has found no EU_43 this season. But the two EU_46 outbreaks underline the need to maintain chemistry alternation and protect against resistance, said David Cooke, a senior research development scientist at James the Hutton Institute.

Resistant strains

“Now we have had some rain, it’s good to put some focus back onto blight programmes,” said Dr Cooke. “It can still infect low levels of green leaf late in the season, and volunteers will need managing.”

The warning is particularly relevant because EU_43 was first identified on a volunteer last autumn. The origin of this season’s EU_46 outbreaks remains uncertain. Evidence points towards airborne spores because infection spread across the field at one site.

“Fortunately, these outbreaks have been controlled by good programmes, alternating chemistry, and a bit of luck because of the weather becoming so dry,” said Dr Cooke.

Disease priorities have been different across eastern England, where Alternaria and maintaining green leaf have demanded more attention.

Mr Maule said protecting the canopy had been important in varieties such as Nectar, where maintaining green leaf allows crops to continue bulking late in the season.

“We have some varieties like Nectar that can produce big yields, but we have really had to focus on canopy retention if we are going to hit that potential with late season bulking,” he said.

Programmes have therefore included products with elicitation or plant-stimulus effects.

Mr Maule has used Voyager, containing valifenalate and fluazinam, and Privest, containing ametoctradin and potassium phosphonates. Innocul8, a peptide-based plant defence elicitor containing zinc and manganese, has also featured.

Final protection

Foliar nutrition has played a greater role as growers sought to maintain healthy canopies during difficult growing conditions. Attention is now shifting back towards late and tuber blight with crops in their final weeks.

Mr Maule said spray intervals may need shortening where green leaf remains, with programmes adjusted towards products offering appropriate late-season protection.

“Where there is still green leaf that needs protecting from blight, Lieto (cymoxanil + zoxamide) will form the backbone of programmes from now on,” he said. “We will alternate with Ranman Top (cyazofamid), Shinkon (amisulbrom) or Infinito (fluopicolide + propamocarb) for tuber blight protection.”

For growers, the lesson from another dry season is that low disease pressure does not remove the threat. “Even though this season will largely be known for very low pressure, we knew at the start that it would still need a flexible approach,” said Mr Maule.

The Fight Against Blight website relies on reports of outbreaks to monitor blight strains circulating in Britain. It is run by the James Hutton Institute with potato industry support. For details, visit blight.hutton.ac.uk