Herbicide resistance issues were the key topics on the agenda at the HGCA Agronomists’ conference
Herbicide resistance issues and the loss of triazole fungicide efficacy to septoria tritici were two of the key topics on the agenda at the HGCA Agronomists’ conference prior to the Christmas break. Dominic Kilburn attended.We may have lost some battles against herbicide resistant weeds, but we can still win the war. That was the clarion call by Rothamsted Research scientist Dr Stephen Moss (left) who pointed out that over 16,000 farms in 34 counties are now suffering from herbicide resistant black-grass.Speaking at the HGCA Agronomists’ Conference staged at the East of England Showground in mid-December, Dr Moss said that it was now an accepted fact that some degree of resistance occurs on virtually all farms where black-grass herbicides have been used regularly for the past 25 years.He questioned whether stacking ever-more pre-emergence herbicides was the complete solution to increasing resistance to post-emergence herbicides such as Atlantis (mesosulfuron + iodosulfuron) and pointed out that resistance was also an issue with pre-emergence actives. “I’m not saying don’t use them, but it’s important to recognise that resistance occurs to all the key pre-em actives including flufenacet; pendimethalin; prosulfocarb; flupyrsulfuron; DFF; tri-allate and chlorotoluron.”Fortunately, there is usually only partial resistance to these herbicides and it appears to increase slowly,” he added.Dr Moss highlighted three current HGCA research projects that were looking at the overall problem of combating weed resistance to herbicides in the UK, the first of which – a collaboration between Rothamsted and NIAB TAG – is focusing on the effects that delayed drilling has on black-grass populations.He pointed out that delayed autumn drilling has three benefits; it will usually result in less black-grass in the crop as more can be destroyed before drilling; better control from the herbicide programme as conditions for residual herbicides will usually be more suited (due to more moisture in the seedbed) and that black-grass emerging in later drilled crops is normally less competitive.In winter wheat field trials (2011, 2012 and 2013), delayed drilling showed that, in untreated plots, there was an average reduction in black-grass plant populations/m2 of 38 per cent when drilling was delayed from mid-September to October.In plots treated with pre-emergence Liberator (flufenacet + DFF) + Defy (prosulfocarb), followed by Auxiliary (clodinafop-propargyl + prosulfocarb) + half rate Crystal (flufenacet + pendimethalin), on average there was a 68 per cent reduction in black-grass infestation in the crops drilled in early October compared with those sown mid-September.The most consistent finding on all trials was that herbicide efficacy increased as drilling was delayed, with a 19 per cent mean benefit when drilling in October compared with mid-September.
“This won’t happen in every field in every year, but this effect was seen in all the Rothamsted and NIAB TAG trials in three contrasting years so generally speaking this seems a robust finding,” he pointed out.”In 1975, Crop Monitor data showed that only five per cent of winter cereals in England were sown during September and now that figure is over 50 per cent. This increase is unsustainable where herbicide resistant black-grass is concerned and things must change,” he stressed.Plant traits
The aim of the second project, a PhD by Izzy Andrew, involves collaboration between Rothamsted, University of Nottingham, Agrii and Syngenta, and aims to understand how plant traits contribute to competition in cereals and then develop a protocol for indexing competitiveness of new wheat varieties. “We need information on variety competitiveness when they are first introduced and not when they are about to drop off the Recommended List,” claimed Dr Moss.Variety traits currently under study include plant height, rate of tillering, total green area, flag leaf length and final tiller number. “To date, no single trait in trials has consistently related to suppression of black-grass seed return across container and in-field experiments. At the moment the project is about half way through and the challenge is to find some traits that are consistent in their suppression of black-grass,” he said.He added that HGCA bursary student Emily Stratford (Bath University) had also looked into variety allelopathy and its relation to weed suppression. Variety-produced allelopathy reduces root growth of black-grass and chickweed in laboratory studies, said Dr Moss, but there were no significant differences between the varieties. “Allelopathy is a term used widely but its effect on black-grass is not proven in the field and it’s something that has been over-sold in my opinion.”The third and final HGCA project Dr Moss referred to was being led by Lynn Tatnell at ADAS Boxworth and supported by DuPont, BASF and Dow, and its aim was to prevent the wide-scale increase in broad-leaved weed resistance to ALS herbicides.He said that ALS-resistant poppies featured on over 25 farms in 9 counties in England – predominantly from Yorkshire down the east coast through Lincolnshire, East Anglia and into the south east.Field-scale poppy assessments in Cambridgeshire (2013) on a field with known target site resistance delivered much lower poppy plant numbers in fields treated with Crystal at pre-emergence followed by ALS herbicide Jubilee SX (metsulfuron) or MCPA 50 (MCPA), compared with those treated only with Jubilee SX at post-emergence.As expected, the ALS herbicide used on its own in these trials gave virtually no control – on a par with untreated plots – whereas poppy numbers were much lower due to the pendimethalin in Crystal, added Dr Moss. This was despite the soil being highly organic, which might have been expected to reduce the activity of a residual herbicide like pendimethalin.”Resistance to broad-leaved weeds tends to be specific to ALS inhibitors like sulfonylureas but other herbicides remain fully effective; pendimethalin on poppy, fluroxypyr on chickweed and ioxynil or bromoxynil on mayweed.”These three weed species have developed resistance to ALS herbicides in a lot of countries, particularly mayweed in Germany, and at the moment it can’t be explained why resistance has developed so widely in these three weeds, while most others remain susceptible at present.
“It’s something we have to watch out for though and any further loss of existing herbicides due to regulatory action would greatly increase the threat that growers face,” he added.In summary, Dr Moss urged advisers and farmers to think more about the benefit of integrating non-chemical methods with herbicides on farm. “There’s no blue print on how to do it and every farm is different, but non-chemical methods such as rotational ploughing, delayed autumn drilling, spring cropping and higher seed rates, in combination with herbicides, can result in a huge reduction in black-grass populations.”I doubt whether ‘resistance’ to non-chemical weed control methods will develop in the next 20 years so learning how best to use them on each individual farm will pay dividends in the longer term. Compare this with the current black-grass herbicides available to growers – I doubt whether many of these will still be very effective, or even around, in 20 years’ time,” he concluded.Triazole trouble?
The days of half rate triazole applications for the protection of winter wheat against septoria have gone. That was the stark reminder from ADAS senior research scientist Jonathan Blake (left) who pointed out that in recent HGCA-funded wheat fungicide performance trials, in a protectant situation, half rates of the best azoles provided less than 50 per cent control. However, he said that all SDHI and azole mixes gave 91-99 per cent control.In trials that spanned seven sites in 2013, with contributing partners including ADAS; NIAB TAG; SRUC (in Scotland) and Teagasc (Ireland), septoria trials included quarter, half, full and double label fungicide rates.In single active comparisons in a protectant situation; triazoles Ignite (epoxiconazole) and Proline (prothioconazole) gave comparable performances and continued the trend of delivering very similar efficacy over the years at all label rates, Jonathan pointed out. However, the widening gap between triazole and SDHI efficacy at half label rate was highlighted when comparing their performance with SDHI actives Vertisan (penthiopyrad) and Imtrex (fluxapyroxad) – both performing much better than the triazoles as protectants against septoria.”Phoenix (folpet) gave 50 per cent control of septoria, and, as a multisite protectant fungicide, it will help. Bravo (clorothalonil) also still performs well in this situation,” he added.In the mixtures, he pointed out that Aviator Xpro (bixafen + prothioconazole), Adexar (epoxiconazole + fluxapyroxad) and Vertisan + Ignite all performed extremely well in a protectant situation at half label rate, with Adexar having the slight edge.In over two year trials (2012-2013), the gap between Imtrex and the triazoles was again very clear, while the mixtures again all performed very well, he highlighted.
“Solo SDHI chemistry is very active but triazoles and multisite partners are important to broaden activity and reduce resistance risk,” he stressed.Septoria eradicant data 2012-2013 demonstrated that triazoles on their own were still providing useful activity although the level of efficacy had continued to reduce over the years.”For the mixtures; Aviator and Adexar activity followed similar lines and while it is hard to say if Vertisan + Ignite is better, it’s certainly as good as the other two products,” he commented.In terms of the rusts, he explained that there was no new data for brown rust trials in 2013, mainly due to the very cold spring, however the disease remains a concern. “Over 50 per cent of RL varieties are rated ‘5’ or less for brown rust and we shouldn’t forget what happened in the 2006/07 season when it was a major problem. Winter conditions will determine the risk for 2014 but I understand that it was already being found in places during December.”Brown rust trials at Andover in 2012 showed that Proline is a little weaker than Ignite or Imtrex in a protectant situation but adding SDHI mixtures enables very good control of the disease.
Adding a word of caution, Jonathan pointed out that the French are finding that SDHIs are less effective on brown rust and, as a result, UK growers shouldn’t come to rely on them too heavily for protection.”Brown rust is still one to watch out for,” he added.
Turning to yellow rust, according to Jonathan epidemics were delayed last season by the cold conditions in March and early spring but it is still a major issue in Oakley and other susceptible varieties which include Solstice, Gallant, KWS Kielder and Viscount.Trials at Terrington in Norfolk (2013) showed that triazoles are still performing well on yellow rust with Imtrex and Vertisan adding to the activity. In the mixtures; Seguris (isopyrazam + epoxiconazole) was very active on yellow rust at full label rate although fell a little behind Aviator Xpro, Adexar and Vertisan + Ignite at quarter and half rate applications.In over two year trials at Terrington (2012-2013), triazoles again performed very well, as did Seguris which remains a key active for the control of yellow rust, said Jonathan.In HGCA’s 2013 barley fungicide performance trials, where target diseases included powdery mildew, rhynchosporium, net blotch, brown rust and ramularia, SDHI’s Siltra Xpro (bixafen + prothioconazole) and Adexar showed good broad-spectrum activity consistent with previous years. “SDHI mixes are all performing well and are quite closely matched,” he commented.”Proline is still a highly effective azole on barley diseases, while strobilurin Comet (pyraclostrobin) remains effective against net blotch and Phoenix has some activity on rhynchosporium,” he concluded.Aphid advice
With neonicotinoid restrictions now in place, Rothamsted Research’s Steve Foster (left) described the concerning level of resistance in the peach potato aphid. Over 90 per cent of the UK field population now contains super-kdr (pyrethroid) resistance and MACE (pirimicarb) resistance, he said.
Dr Foster stated that although the new spray option pymetrozine will provide a valuable control option, the limit to one spray in the autumn will increase dependence on getting application timings spot on. He said the industry should keep track of local aphid pressures by signing up to HGCA Aphid News. Stewardship of SDHI fungicides is key
- A maximum of two SDHI fungicide-containing sprays to be applied (per crop)
- Always use SDHI fungicides in a mixture with at least one fungicide from an alternative mode of action group which has efficacy against the target pathogen(s)
- Tank mixing two SDHI fungicides is not an anti-resistance strategy