Potato growers advised to check weed control before blight spraying

Potato growers are being advised to check fields to determine how well their herbicides have worked before the blight spraying season kicks off.

Andrew Goodinson, potato agronomist at crop production specialist Hutchinsons, explains that assessing how well growers’ herbicide sprays have worked can indicate any errors in sprayer set-up.  

“Most importantly, section control must be set up accurately to cover the correct area and avoid missing any area of the crop,” he says. 

See also: What a new fungicide-resistant blight strain means for UK farmers

“If you can see an area of weeds, it is very likely you missed them with the sprayer, and it is likely you will miss them again when you go in with the blight spray,” says Mr Goodinson.

He emphasises that effective blight control is about building up the product in and on the crop, so to minimise the chance of missing a blight spray when wet weather sets in, he recommends starting the seven- or 10-day spray programmes on a Monday.

“This means if you miss a day or two, you are not then up against a weekend.”

Nozzle choice

In addition to ensuring the right fungicide choice at the right time, spray operators need to choose the right spray nozzle for the particular task.

“The IDTA04 Flat Fan and 3D Ninety nozzles offer the best coverage because they can be set to cover the different parts of the canopy, including the underside of the leaves.”

He reminds growers that some of the newer blight strains are able to incubate during colder periods, and that spores can travel a long way in the wind.

“We often underestimate the spread of blight inoculum from unsprayed crops, including those in allotments and gardens.

“If weather conditions are dry when crops are at maximum canopy, you may be able to stretch your spray intervals a little or use cheaper alternative products, but the potential effect of late blight on the investment you have made in your crops remains huge.

“Ensuring you have a robust blight programme is more essential than ever.”

Andrew Goodinson’s fungicide strategy

Mr Goodinson likes to start blight control programmes when the crop is at the rosette stage, following a “mix-and-match” strategy with actives according to conditions at the time the Hutton Criteria is triggered.

“We rotate out chemistry across the different fungicide groups to avoid putting a single active under pressure.” 

He says the choice of fungicides is not set in stone, but very much depends on the conditions in the field at the time. Growers also need to follow the Fungicide Resistance Action Group guidelines.

Although outbreaks of fluazinam-insensitive blight strain 37_A2 (formerly known as Dark Green 37) were less prevalent in 2022 than in recent years, he warns growers to only use it mid-programme in a mix at full rate (400ml/ha).

“We used to use fluazinam for the first spray, but we now tend to opt for cyazofamid, and for the second spray we often follow it with fluopicolide + propamocarb because it works well with the crop at this stage.”

Mr Goodinson finds mandipropamid remains a good-value fungicide and it works particularly well as a second spray, particularly when it is mixed with cymoxanil.

“Mandipropamid can be useful in unsettled weather because of the speed of its rainfastness,” he says, highlighting the need for caution because of the recent rise of new blight strain 43_A1, which is insensitive to mandipropamid.

“One of the concerns is that other actives within the same fungicide group [CAAs] may also be at risk, so they should not be used alone.”

Although 43_A1 has not so far been found in Britain, it has proliferated in Denmark and has been seen in Belgium and the Netherlands.

“The addition of an adjuvant with mandipropamid to reduce drift has shown better effectiveness, but it will not affect the strain’s insensitivity,” he says.

 At mid-season, he finds oxathiapiprolin very useful because it is effective, persistent and it moves well through the plant.

“Although oxathiapiprolin is one of the more expensive options, when the pressure is really high, we tend to use it in sequence with cyazofamid, fluopicolide + propamocarb or fluazinameither side,” he notes.

He adds that fluazinam still has a place in the strategy because of its effectiveness against sclerotinia and botrytis. However, he avoids using it on its own.