Arable News

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Prevention is better than cure

The wet weather in 2012 gave newer SDHI fungicide chemistry a real test against high disease pressure during the growing season, delegates at the HGCA’s Agronomists’ conference were told

The wet weather in 2012 gave newer SDHI fungicide chemistry a real test against high disease pressure during the growing season, delegates at the HGCA’s Agronomists’ conference were told. Dominic Kilburn watched and listened in on the event via a live webcast.  Don’t rely on eradicant control of septoria in winter wheat crops, even at robust spray rates, and growers should make use of SDHI fungicide mixtures for best protection against the disease, which was found at almost unprecedented levels in UK crops during 2012.That was the key message from NIAB TAG director of crops and agronomy, Stuart Knight (left), speaking at the HGCA Agronomists’ conference staged in Peterborough just prior to Christmas.Referring to the performance of eight fungicide products in six HGCA-funded wheat trials in 2012, conducted by ADAS, SRUC and NIAB TAG in England and Scotland, and one trial conducted and funded by Teagasc in Ireland, he pointed out that, last season, even the very best performing actives only achieved up to 50 per cent control of septoria as single sprays in an eradicant situation.Conversely, up to 85 per cent control was seen with products applied in a protectant role, prior to the disease establishing on the target leaf layer.Septoria trials
“Proline (prothioconazole) and Ignite (epoxiconazole) were used in the septoria trials as standard azole treatments, and they gave very similar levels of eradicant performance, but even at full label rate only delivered about 25 per cent control.”It’s very hard in a season like 2012 to eradicate septoria once it got a foothold in the crop,” stressed Stuart.Although intended as a protectant material to be used in mixtures, in the same trials he noted that Phoenix (folpet) gave useful activity against septoria while straight Xemium product Imtrex (fluxapyroxad) – as in Adexar – gave a good level of eradication.The SDHI on its own gave better control than the azole standards, he suggested, but he cautioned that SDHIs must always be used in mixture with at least one effective and appropriate partner to help safeguard their mode of action.Considering the performance of SDHIs in mixtures, he said that Aviator (bixafen + prothioconazole) and Adexar (fluxapyroxad + epoxiconazole) out-performed Seguris (isopyrazam + epoxiconazole) in an eradicant situation.In terms of their protectant efficacy, Stuart said that 85 per cent control was achieved from the best products when applied at their full label rate.
“We can do a better job of protecting crops against septoria with the products available to us than we can in eradicating it.As in the eradicant trials, the azoles prothioconazole and epoxiconazole showed similar levels of protection to one another, with a reasonable performance from folpet again. Imtrex also showed good protection against septoria, better than the triazoles, and results for Bravo (chlorothalonil), which was only tested at half dose, show that it remains a very effective protectant,” he commented.For the SDHI mixes there was little difference between Aviator, Adexar and Seguris, which all gave similar levels of protection,” he added.
Even the very best performing actives only achieved up to 50 per cent control of septoria as single sprays in an eradicant situation in 2012 trials.Yield response
Stuart pointed out that when the final yield in the same trials was considered, again, there was little difference between prothioconazole and epoxiconazole, but neither performed as well as fluxapyroxad which gave a better yield response. Used alone, folpet gave a small yield response, but below that achieved with the azoles.”Aviator and Adexar SDHIs had similar yield responses but Seguris, with not such good eradication levels, not surprisingly had a lower yield response than the other two.”
Stuart then turned to mean data over the past three years across 16 different trials, confirming very little difference in overall performance between the two azoles, or, for the SDHIs, between Aviator and Adexar.Seguris was only slightly adrift of the latter for septoria protection, but less effective for eradication. “Half doses of Bravo have remained very good over the past three years,” he added.Up to 85 per cent septoria control was seen with products applied in a protectant role.Trends over time
Stuart reminded delegates that over a longer period of time, average levels of septoria protection with azoles were now considerably lower; a half dose of epoxiconazole achieved 85-90 per cent control when first evaluated in HGCA fungicide performance trials in 1995, with similar levels of control for a half dose of prothioconazole when first evaluated in 2001. Now, both products were showing around 60 per cent control, he said.”At full label rate it’s a similar story in terms of azole protection but the decline is less steep with about 70 per cent control for epoxiconazole and prothioconazole in 2012.”With Bravo, however, there has been no change with control levels remaining around 60-70 per cent between 1995 and 2012,” he added.Stuart stressed that the decline in eradicant control levels was steeper, even at full label rate, with the two key azoles delivering half the level of control now (less than 40 per cent) compared with when they were first evaluated.Summing up the septoria trials Stuart highlighted the clear reduction in field performance of azoles over time but said that growers should use SDHIs with azoles, and multisite protectant products where possible. “The three SDHI mixtures have shown similar protectant activity in the trials while Aviator Xpro, Adexar/ Imtrex were the leading eradicants.”Imtrex has provided excellent septoria control, but it is lower yielding than Adexar and is another reason why it should always be used in mixture.”
Rust trials
Turning to rusts in winter wheat, Stuart pointed out that 2012 was a very different season weather-wise compared with 2011 and there were significant early yellow rust epidemics, especially in susceptible variety Oakley. “We were expecting problems with brown rust too but, despite early observations in crops, the unfavourable conditions delayed epidemics.”Stuart noted that 55 per cent of varieties on the Recommended List have a rating of ‘5’ or less for brown rust resistance and so the risk remains for 2013, he stated.In the yellow rust trial in 2012, with three per cent yellow rust present on leaf-3 at the time of the fungicide application, he said that there was a clear separation between prothioconazole and epoxiconazole in terms of yellow rust control, the latter performing much better.Strobilurin Comet (pyraclostrobin), although only to be used in mixtures, was close to epoxiconazole’s performance and remains useful for the control of yellow rust in winter wheat. Fluxapyroxad also contributes to yellow rust control but, applied alone, it was not as effective as epoxiconazole, he added.”For the SDHIs; Adexar and Seguris gave very good yellow rust control while Aviator was similar to its prothiconazole component,” commented Stuart. “The best yellow rust control comes from epoxiconazole-based treatments or mixtures,” he added.Also included in trials was Brutus (epoxiconazole + metconazole) which, with epoxiconazole included, was seen to be doing “a good job” on yellow rust, he suggested.In terms of brown rust, Stuart said that prothioconazole was again adrift of epoxiconazole while Imtrex was effective. All three SDHIs performed well on brown rust, adding significantly to the brown rust activity of their azole partners.In the yellow rust trial there was a clear separation between prothioconazole and epoxiconazole in terms of yellow rust control. Barley performance
Fungicide performance trials in barley during 2012 focused on control of rhynchosporium, net blotch, brown rust and ramularia diseases, continued Stuart.He said that the 2012 trials showed that there was useful rhynchosporium activity from Phoenix with up to 40 per cent control. Of the azoles (Proline and Ignite), Proline offered the better performance. Of the SDHIs; Adexar performed similarly to Proline and slightly better than Bontima (cyprodinil + isopyrazam) but Siltra Xpro (bixafen + prothioconazole) was the most effective when compared at below label rate. When averaged over three years, the picture was similar.
“Both Phoenix and chlorothalonil contribute to rhynchosporium control, but have produced a fairly flat dose response,” said Stuart. “Comet has also given useful control,” he stated. “In terms of rhynchosporium protection over three years; Proline was again the strongest out of the two azoles and there was a good performance from the SDHIs, notably Siltra Xpro and Ceriax (pyraclostrobin + fluxapyroxad + epoxiconazole), he pointed out.For protection in barley against ramularia in 2012, according to Stuart, all the SDHI fungicides gave good control, particularly Siltra Xpro (containing prothioconazole), as did prothioconazole on its own in Proline.”SDHIs are also a very useful addition to the armoury where net blotch is concerned with all giving high levels of control.”For mildew, we know that on barley Proline is still giving good control of mildew, as is the eradicant Torch (spiroxamine), but out of the specific mildew fungicides trialled, including Cyflamid (cyflufenamid), Talius (proquinazid) and Flexity (metrafenone); Cyflamid has been the best in terms of eradicant activity,” he said. “But we are really looking at these three products as protectants,” added Stuart.Timely messages on mycotoxins
Prolonged wet weather during the growing season and a delayed harvest resulted in higher levels of mycotoxin-producing fusarium in 2012 wheat crops than in recent seasons, and Professor of plant pathology at Harper Adams University, Simon Edwards used the conference to deliver timely messages on the subject.Professor of plant pathology at Harper Adams University, Simon Edwards.He told delegates that of the first of Fera’s 130 samples from harvest 2012, 11 per cent had exceeded EU legal limits for fusarium mycotoxin deoxynivalenol (DON) in wheat destined for human consumption, while 15 per cent had exceeded the zearalenone (ZON) type of mycotoxin. Both are key threats in the UK and result from fusarium head blight.”These are high levels, although not as high as in 2008,” commented Simon, who pointed out that due to the high risk of wheat currently in store, nabim requires a risk assessment score and a DON test for each wheat load leaving the farm all to be recorded on growers’ grain passports. “Some mills may relax restrictions and not require a DON test based on local and regional risk,” he added.”Farmers are classed as food operators under EU law and mycotoxin legislation applies at the first point of sale, i.e. the farm gate,” continued Simon. “There are currently higher guide limits for wheats destined for animal feed but the Commission warns that if those are ignored then tougher legislation will be put in place,” he stressed.Simon pointed out that, produced in the field; there is little that can be done about removing mycotoxins from wheat once the crop is harvested. The worst case for mycotoxin development is when a warm and dry spring aids spore production followed by heavy rain in June where splashing increases the likelihood of spores infecting the ears. Infection occurs at flowering time, particularly in warm and humid conditions. High summer rainfall, as occurred in 2012, can increase the spread, he explained.”Fusarium needs a certain temperature in the spring (an average of 12C) to develop and so we are seeing it more in the South and East, and less in the North, but in 2008 we saw it nationally because of the delayed harvest that year,” he said, adding that head blight infecting species microdochium nivale, a species that doesn’t produce mycotoxins, dominated infections in 2012 probably due to the cool summer conditions.He reminded delegates that minimum tillage cultivations following maize provided the greatest risk of spreading fusarium when surface debris harboured inoculum. Research has also suggested that wheats that are drilled late (after sugar beet for example) are also of concern, he said. “If possible, avoid maize as a previous crop, particularly grain maize, and plough in crop debris. If using min-till, then chop and mix the debris as much as possible.”French data has shown us that, following maize, the more chopping and mixing that takes place the greater the reduction in mycotoxins in following wheat,” he added.In terms of fungicides, Simon said that metconazole, tebuconazole and prothioconazole remained the key products to tackle fusarium at T3 growth stages (59-65) – the key timing. “Infection is very much weather dependent but the optimum timing is just prior to infection. If there is a spell of dry weather prior to flowering then hold off with the spray, but if rain is forecast at growth stage 59 then get it on.”You need robust rates when disease pressure is high,” he stressed.
Simon pointed out that work at Harper Adams has looked at other spray timings and that there is evidence of some control at T1 and T2. “Each time you put a fusarium-active product on there will be a benefit and T1 and T2 applications will be useful, but T3 is the most important spray.” He concluded by saying that the risk of fusarium mycotoxins in wheat remains high for 2013 due to high levels of inoculum in planted seed, crop debris and soil, and, with many crops this season drilled late into cold and wet seedbeds, this could exacerbate the problem.Mycotoxin legislation
Maximum legal limits for unprocessed wheat intended for human consumption:
DON 1250 ppb
ZON 100 ppb
HT2+T2 50 ppb – proposed investigative limit
(Legislation applies at first point of sale. There are higher guideline limits for wheat intended for animal feed).

Good Agricultural Practice for controlling fusarium mycotoxins:

  • Use fusarium resistant varieties
  • Good rotation
     – avoid maize as previous crop, especially grain maize
  • Cultivation
     – plough in crop debris following a cereal (particularly maize)
     – if using min-till, remove debris and/or chop/mix as much as possible
  • Use a high rate of a recommended fusarium head blight fungicide at correct timing (GS 59-65)
  • Avoid lodging
  • Timely harvest

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