Keep late blight protection in place as rain returns

Potato growers should maintain late blight protection through the final stages of the season as wetter conditions return and crops remain vulnerable to infection.

While blight pressure has generally remained low through the dry season, Agrii Lead Technical Advisor for Potatoes Ed Maule says experience from previous years shows how quickly the situation can change.

“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.”

That makes maintaining appropriate protection particularly important where crops still have green leaf to protect and during the period when tuber blight becomes an increasing consideration.

EU_46 detected in two outbreaks

Fight Against Blight monitoring has not detected the resistant EU_43 strain this season. However, EU_46 has been identified in two separate outbreaks.

Dr David Cooke, Senior Research Development Scientist at the James Hutton Institute, says the outbreaks were found in Shropshire and Lancashire shortly before conditions became particularly dry.

“We aren’t entirely sure of the origins, but the evidence points to ingress from airborne spores because, in one case, the infection spread across the field.”

The outbreaks were successfully controlled.

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

Fight Against Blight is run by the James Hutton Institute with support from across the potato industry.

Volunteers remain a potential source of infection

Recent rainfall means attention should return to late blight even where only relatively small amounts of green leaf remain.

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

The risk from volunteers was illustrated last autumn when EU_43 was first detected on a volunteer potato plant.

Managing volunteers and maintaining appropriate crop protection therefore remain important parts of limiting the opportunity for blight populations to develop and spread.

Maintaining the canopy through a dry season

For many potato crops in eastern England, disease management earlier in the season has been less about late blight and more focused on Alternaria and maintaining green leaf area.

Ed says this has been particularly important in high-yielding varieties where retaining the canopy is needed to support late-season bulking.

“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.”

This has influenced the products used within programmes.

Voyager, containing valifenalate and fluazinam, and Privest, containing ametoctradin and potassium phosphonates, have both featured in programmes where their wider characteristics have suited the crop and disease situation.

Ed has also used Innocul8, a peptide-based plant defence elicitor containing zinc and manganese.

“Innocul8 also provides the plant with zinc and manganese. Foliar feeds have been another important part of spray programmes this year to maintain a healthy canopy.”

 

Refocus programmes on late and tuber blight

As the season moves towards its final few weeks, the emphasis now changes.

Where green leaf remains, programmes need to reflect renewed late blight risk while also providing appropriate protection against tuber blight.

Spray intervals may also need to tighten according to crop condition, weather and disease pressure.

“Where there is still green leaf that needs protecting from blight, Lieto, containing cymoxanil and zoxamide, will form the backbone of programmes from now on.”

“We will alternate with Ranman Top, containing cyazofamid, Shinkon, containing amisulbrom, or Infinito, containing fluopicolide and propamocarb, for tuber blight protection.”

Product choice and sequencing should continue to take account of resistance management as well as the disease risk in individual crops.

Keep programmes flexible until the crop is finished

The relatively low late blight pressure experienced through much of the season should not encourage growers to finish protection prematurely where susceptible green material remains.

“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.”

The final weeks therefore remain a period for adapting programmes to the crop in front of you, rather than relying on how disease pressure has behaved through the earlier part of the season.

Speak to your Agrii agronomist about late-season potato disease management and how current conditions may affect programme choice.

To report a suspected late blight outbreak, visit the Fight Against Blight website.