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Successful potato blight control depends on a range of factors – and each requires growers to pay attention to detail. Top tips for controlling late season potato blight

Successful potato blight control depends on a range of factors – and each requires growers to pay attention to detail.

Before the blight season starts, it is important to assess how well your herbicide spray has worked. This can help identify any errors in sprayer set-up. Most importantly, section control must be accurately calibrated to avoid missing any area of the crop.

If you can see an area of weeds, it is very likely you missed them with the sprayer. This means you will probably miss them again when you go in with the blight spray. Just tweaking sprayer set-up can offer pay-back when blight pressure is high.

Despite low blight pressure in 2022 and the likelihood of low inoculum carry-over, growers should take no chances this year. It is also important to follow the Fungicide Resistance Action Group (FRAG) guidelines.

Rosette stage

Blight control programmes should start when the crop is at the rosette stage. Growers should adhere to a ‘mix and match’ strategy – matching actives according to conditions at the time the Hutton Criteria is triggered.

We rotate our chemistry across the different fungicide groups to avoid putting a single active under pressure. Fungicide choice is not set in stone, but very much depends on the conditions in the field at the time.

Although outbreaks of fluazinam-insensitive blight strain 37_A2 – formerly known as Dark Green 37 – were less prevalent in 2022 than in recent years, growers should 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.

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

Unsettled weather

Mandipropamid can be useful in unsettled weather because of the speed of its rainfastness, 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, and therefore 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 found 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.

At mid-season, oxathiapiprolin can be very useful because it is effective, persistent and it moves well round the plant – although it is one of the more expensive options – when the pressure is really high.

We tend to use oxathiapiprolin in sequence with cyazofamid, fluopicolide + propamocarb or fluazinameither side.

Fluazinam still has a place because of its effectiveness against sclerotinia and botrytis. We avoid using oxathiapiprolin on its own.

Spray programmes

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, it is good to start a seven or ten-day spray programme on a Monday.

This means that if you miss a day or two, you are not then up against a weekend. As well as ensuring the right fungicide choice at the right time, spray operators also need to choose the right spray nozzle for the particular task.

The IDTA04 Flat Fan and the 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.

Some of the newer blight strains are able to incubate during colder periods – and their 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 impact of late blight on the investment you have made in your crops remains huge.

Andrew Goodinson is a potato agronomist at crop production specialists Hutchinsons.