A combination of very high disease pressure, weather-related spray timing issues and a reliance on azoles at insufficient doses has been responsible for the control problems that many growers have experienced with septoria in 2012. That’s according to Bill Clark, commercial technical director with NIAB TAG.
“The panic and disbelief has almost subsided now and people have accepted their fate,” he says. “But there are some key messages to get across and discuss with growers, as some valuable lessons have been learned.”
The first of these is that this season has been exceptional for septoria disease pressure, he says. “It’s been much higher than we’ve seen for many years. It’s almost as though we’d forgotten what can happen when the weather conditions work in the disease’s favour and against its control methods.”
The idea that many people forgot the basics about the disease leads Mr Clark to another important point. “The septoria population has been drifting to less sensitive types for several years, but this was forgotten when fungicide programmes were being planned and applied.
“Growers didn’t adjust their inputs to take account of this loss of activity, which is known to affect even the best septoria fungicides.”
As a result, the most popular straight triazoles – epoxiconazole and prothioconazole – have looked very ordinary in such a tough season. “And that’s the case regardless of whether they’ve been used at high doses or not. They were up against it right from the start.”
There’s also no doubt that the T0 and T1 sprays played a crucial role this season, continues Mr Clark. “Where they were missed, or applied late, the disease was able to spread rapidly up the plant. The use of a low rate of triazole plus Bravo just wasn’t enough to contain the problem, even though it had done the trick for the last few years.”
However, Mr Clark says that timely, robust applications at these two timings seem to have made a big difference.
The commercial availability of the SDHIs has certainly helped, he says. “The trials performance of the three SDHI fungicides – Adexar, Aviator and Seguris – looks exceptionally good where they were used at T1 and T2. The addition of an SDHI at T1, when compared to straight triazoles, has given much better septoria control.
“So that’s something to consider for future seasons, especially where disease pressure is high. But SDHI use must remain within the agreed guidelines.”
The rainfall pattern created further challenges, says Mr Clark. “The rain meant that we had almost continuous infection pressure throughout April and early May.”
This meant that leaf two got infected as it was emerging at the end of April. “We then had cool conditions, so it took nearly four weeks before the flag leaf emerged. As a result, leaf two was already showing symptoms.”
In many areas, this resulted in a T1-T2 gap which was too long. “If you were advised to spray a pre-T2 fungicide to protect leaf 2 and did so, it made quite a difference and the situation was less serious,” says Mr Clark.
Rates were generally too low, he continues. “We started the season with the threat of drought, which tends to result in low disease levels. By the time people realised that this had changed and there were high levels of disease, it was almost too late. Putting high rates on the flag leaf couldn’t retrieve the situation.”
While the SDHIs have served growers very well this year, with high doses looking very good, the use of low doses has looked very ordinary, reports Mr Clark. “Where a low rate SDHI was mixed with another azole, it hasn’t been a great result. That’s even more so where timings were poor and the gap between T1 and T2 was stretched.”
In summary, he highlights high disease pressure, poor spray timings and use of azoles at low rates as contributing factors. “There’s no doubt that most of the problem cases had a T1-T2 gap issue, and leaf two was showing symptoms before the flag leaf spray was applied.”
However, in just one season, the SDHIs have become essential for good septoria control, he stresses.
Fast track septoria?
Growers will have to wait another month to find out if there is a new strain of fast track septoria, reports Mr Clark.
“Testing is being carried out at NIAB, because there have been a number of cases of very poor control that can’t be explained by poor timings, low rates or inappropriate product choice,” he says.
Furthermore, patches of the disease in an otherwise green crop suggest that there are strains of septoria that can cycle very quickly, or have a shorter latent period, which aren’t controlled by normal timings, he adds.