Saturday, August 24, 2019

Heightened blight pressure raises stakes for tuber protection

August 8, 2019 by  
Filed under Crops

Growers are being urged to carefully consider how they protect potato crops through to the end of the season following a surge in blight reports.

AHDB Potatoes reported a sharp rise in blight samples submitted for testing in late June once weather conditions favoured sporulation. The most frequently detected clone of the disease was 36-A2 – a new strain of late blight that arrived in the UK from Europe in 2017.

Edward Hagues, Bayer campaign manager for root crops, says the rise in reported blight cases was to be expected following a period of warm and wet weather in early summer which created the humid conditions needed for disease to sporulate.

“The spike in blight cases was preceded by a flurry of Hutton Criteria warnings, so it is not surprising that growers are seeing crops under intense pressure. What is important however, is how growers plan to ensure crops are effectively protected through to the end of the season.”

Crop management

The extent to which growers succeed in keeping crops clean of disease will largely depend on how many products with recognised anti-sporulant activity they have retained for late-season use, says Mr Hagues.

“The focus must be on managing products to ensure good foliar and tuber blight protection. With 13 modes of action available for late blight control this might not seem too difficult, but only two are now considered to have good activity.”

The issue of tuber protection has taken on greater significance since the spread of fluazinam-resistant blight in 2016. Fluazinam was a popular anti-sporulant fungicide but has since been effectively limited to sclerotinia control in the early season.

Limited options

“For those who don’t want to run the risk of [blight] insensitivity, that leaves just two modes of action: Quinone inside Inhibitors (QiI), which belong to FRAC code 21, and the pyridinylmethyl-benzamide group containing fluopicolide, belonging to FRAC code 43,” says Mr Hagues.

Grower decisions now therefore determine their options later in the season. Resistance management guidelines from FRAG-UK state that QiI fungicides, such as those containing cyazofamid or amisulbrom, should not form more than 50% of the intended programme.

Similarly, Carboxylic Acid Amides group fungicides, such as those containing mandipropamid, dimethomorph or benthiavalicarb, should not exceed six applications when used in a mixture or four when used alone and no more than 33% of the programme.

Care needed

This is reasonably straightforward to overcome – so long as growers use products other than QiIs in the rapid canopy phase and alternate any remaining QiI applications with Infinito (fluopicolide + propamocarb), which can be applied up to four times per crop.

Product choice

Corteva Agriscience says conditions mean product choice and application timing are more important than ever this season. Growers should consider the length of protection they need and whether their chosen blight spray has any curative properties.

Corteva field technical manager Craig Chisholm says: “The potential for a high blight pressure year was already present due to the increase in volunteer potatoes across the rotation, coupled with the mild winter.”

Choosing an active ingredient with curative properties will ensure growers claw back some of days of lost protection, says Mr Chisholm. Flexibility on timing future sprays is also part of the conundrum, he adds.

“By including a molecule such as oxathiapiprolin (Zorvec) in a programme, growers will be better able to cope with inclement weather as the 10-day spray interval provides flexibility for growers.”