Andrew Goodinson, potato agronomist at Hutchinsons, has provided advice on how to handle late blight control. Before the season starts, he says that growers should assess the efficacy of herbicide applications, which could indicate any errors in sprayer setup.
“Most importantly, section control must be set up accurately to cover the correct area and avoid missing any area of the crop,” he said. “If you can see an area of weeds, it is very likely you missed them with the sprayer, and it is likely that you will miss them again when you go in with the blight spray. Just tweaking sprayer set-up can offer payback when blight pressure is high.”
He warned that growers should take no chances, despite low pressure last year and the likelihood of low inoculum carryover. Blight control programmes should start at the rosette stage, with a mix-and-match strategy with actives according to the conditions.
“We rotate out chemistry across the different fungicide groups to avoid putting a single active under pressure,” Mr Goodinson explained. “The choice of fungicides is not set in stone, but very much depends on the conditions in the field at the time and we also need to follow the Fungicide Resistance Action Group (FRAG) guidelines.”
While outbreaks of fluazinam-insensitive blight strain 37_A2 (formerly known as Dark Green 37) were less prevalent in 2022, he warned that growers should only use it mid-programme in a mix at full rate (400ml/hectare).
“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.”
He added that mandipropamid remains a good-value fungicide, which works particularly well as a second spray when mixed with cymoxanil.
“Mandipropamid can be useful in unsettled weather because of the speed of its rainfastness,” he said. He also highlighted 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.”
While not found in the UK so far, 43_A1 has proliferated in Denmark, and 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,” Mr Goodinson said. At midseason, he finds oxathiapiprolin useful, especially in sequence with cyazofamid, fluopicolide + propamocarb or fluazinameither, while flauzinam still has a place for its effectiveness against sclerotinia and botrytis.
He said that effective blight control is about building up the product up in and on the crop, and recommends starting seven or ten day spray programmes on a Monday to avoid being up against the weekend if wet weather stops play.
The right nozzle choice is also vital. “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.”
“We often underestimate the spread of blight inoculum from unsprayed crops, including those in allotments and gardens,” he concluded. “If weather conditions are dry when crops are at the 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.
“Ensuring you have a robust blight programme is more essential than ever.”