Mounting concerns about SDHI resistance, combined with an unpredictable disease outlook are putting wheat fungicide strategies under closer scrutiny this spring.
Leading agronomy firm Hutchinsons, pointed out that SDHIs have become a cornerstone of many T1 and T2 programmes in recent years, but signs of declining performance against the principle threat of septoria tritici last season have reinforced the need for spray regimes that avoid exacerbating resistance problems. While the harsh spring cold may have reduced early disease pressure, cutting back early sprays risks allowing disease to establish and put later chemistry under extra pressure.
Cold weather only slows rust and septoria development, and both diseases can soon reignite with the right conditions and it remains hard to predict what pressure crops will be under once they reach growth stage 32 (T1) and flag leaf emergence (T2), pointed out Hutchinsons’ technical development director, Dr David Ellerton.
“At the moment pressure from rusts and mildew is lower than last season, but there’s a long way to go,” he argued.
The decline in curative control from triazoles and signs of similar issues starting to affect SDHIs puts greater emphasis on early sprays to protect crops throughout the growing season, without ‘firefighting’ disease, he said.
In Hutchinsons sponsored trials, the average yield response from fungicides had exceeded 2.5t/ha in the past two years with the most responsive varieties showing almost a 5t/ha in the past two years with the most responsive varieties showing almost a 5t/ha benefit in 2016/7 despite a dry spring leading to lower initial disease pressure.
When to use an SDHI?
SDHIs still offer the strongest curative action against septoria, so should remain the foundation of the all-important flag leaf spray, Dr Ellerton said.
The situation is less clear-cut at T1, where product choice should be tailored to variety resistance and other risk factors.
If you’re growing a high-risk variety sown early and spring weather is conductive to disease development you’re more likely to benefit from using an SDHI based programme at both T1 and T2. However, growers should consider only using one at T2 where early disease pressure is lower.
The key to optimising cost per tonne of production is to tailor fungicide programmes to risk on a field-by-field basis.
To manage resistance pressure, SDHI chemistry should always be mixed with a triazole and multi-site protectant at any timing. Chlorothalonil is the most consistent multi-site on septoria, although folpet and mancozeb are good alternatives.
A mildewicide, such as cyflufenamid or fenpropimorph, is also worth including a mildew found at T1.
Consider an early strobilurin spray as another option early on, which can reduce reliance on SDHI or triazole chemistry, said Dr Ellerton.
Although they lack the curative efficacy of SDHIs against septoria, azoxystrobin, fluoxastrobin or pyraclostrobin are strong against rusts and can boost root development and nutrient scavenging when applied earlier in the season (T0 or T1).
Where SDHIs have been used at T1 and T2, he said there may be scope to include a strob at T3 to help prolong green leaf area.