Don’t base disease decisions on previous seasons

Young wheat seedlings growing in a soil.

The good news is that most crops have come through the winter well, especially early drilled ones. The bad news is that reports of yellow rust and septoria have circulated since the turn of the year. We’ve been here before but a recent run of dry spring weather has helped check disease pressure. But don’t bank on a lack of April showers again, is the warning from AICC agronomists Patrick Stephenson and David Lines.

April average rainfall at NIAB, Cambridge, is 42.5mm, but for the last six seasons it has been just 29.0mm, in what have been six turbulent seasons weather wise.

Mr Stephenson feels a substantial reservoir of disease could fuel foliar threats and advises growers not to base fungicide decisions on recent seasons. He believes decisions need to be based on risk. That demands a rounded appraisal of variety rating, when it was drilled, crop biomass and weather patterns and forecast – all essential to protect emerging leaf layers.

His starting position for doing that is, well, at the start. Nip disease development ‘in the bud’.

That start could be at the T1, where a septoria resilient variety has been sown late; if not, then a T0 is a prudent investment. For susceptible yellow rust varieties, when prudence becomes a necessity, he considers a contact material like tebuconazole the best option.

T0 options for septoria

T0 options where septoria is a concern are multisite folpet or possibly defence elicitor Lamanarin (Iodus). He considers nothing is as reliable as CTL but both are alternatives, especially as wheat prices are still upwards of £200/t.

When it comes to the two key timings, he feels growers have a good arsenal to choose from. Ascra (prothioconazole + bixafen + fluopyram), Elatus (prothioconazole + benzovindiflupyr), Revystar and Univoq (prothioconazole + fenpicoxamid) are all likely to feature again, depending on disease spectrum and risk.

In his view, Univoq and Revystar are the strongest septoria options. Elatus is not the strongest septoria option but is for yellow rust, with Ascra being a good ‘all-rounder’. With its high prothioconazole loading it is particularly useful where the stem-based complex is a consideration.

So, he sees the former two as more natural T2 fits and the latter two as more suited to the T1. But not exclusively, if leaves 3 and 2 are in a protective position ahead of flag leaf sprays then Ascra and Elatus still have a place.

Keeping leaf 3 clean is why the T1 is vital in Mr Stephenson’s view. He understands the need to target inputs accordingly but is uneasy about rates being cut below minimum recommendations, even if we see another spring drought. “Lower input varieties allow a degree of flexibility but for the last two seasons some T1 rates have been extremely low. You are then relying on the weather to remain dry but if it turns, a few sharp rain events or a period of humidity can quickly change matters. It is likely rates have been a factor in the development of late disease pressure over the last two seasons,” he adds.

Varietal resistance

Herefordshire-based David Lines probably sees as much septoria as anyone. Walking mid-September drilled Extase and Graham in the middle of February, he didn’t have to look hard to find the disease. It hadn’t made its way to new growth but it was there in the base of plants.

He is perhaps fortunate that the septoria risk in his area means many growers have heeded the warning when it comes to variety resilience and he doesn’t manage anything rated less than a six. But he warns that even the most resilient varieties aren’t resistant. “There is no such thing as an immune variety,” Mr Lines stresses.

He points out rating really depends on when it was drilled. “Drill in mid-September and you can knock as much as a whole point off a variety’s septoria rating. Dawsum and Graham have good ratings of 6.4 and 6.7 respectively, but these are going to behave more like a variety rated at 5.5,” he cautions.

He agrees that we have the fungicide arsenal to meet a range of disease scenarios and like Patrick that can include a T0.

He also sees a role for Iodus, and for those with more susceptible varieties in the ground, then he would advise Thiopron or folpet as useful T0 additions, and elsewhere for the most susceptible varieties.

Ascra suits many T1 scenarios in his view. Rates will be tailored to variety rating, drilling date and environmental factors. He feels an azole + multisite does have a place but only for the most resilient varieties if later drilled and the weather has been kind. Revystar and Univoq will be held back for T2 applications, especially for more susceptible varieties.

There is also the issue of yellow rust spreading west. The disease was picked up in Extase at Fawley Court, Herefordshire, last season as part of Bayer’s CropCheck initiative, a variety considered resilient to all isolates apart from 19/215.

It probably is no surprise given the amount of Extase in the ground, Mr Lines saw the disease last year and mildew too. It could threaten lush crops again; however, he will only opt for a specific mildewicide in severe situations.