Another year for yellow rust?

Yellow rust looks set to pose a threat to yields and profits once again, with the disease being spotted in winter wheat crops before Christmas.

Although the recent cold snap will have slowed it down, if warmer weather arrives in March, yellow rust levels could ramp up again.

Last season the warm spring temperatures led to the disease cycling so quickly that two weeks after T1 treatments, yellow rust was breaking out even in varieties with seemingly robust ratings of 7/8, says Prime Agriculture agronomist James Southgate, who advises across Suffolk and Essex.

I think yellow rust was in some crops before T0.

The temperatures we had meant it was rampant and I was going back two weeks after a T1 and all of a sudden there was yellow rust breaking out on leaves that should have been protected.

When it came around to T2 the flag leaf had emerged and wasnt protected because that typical three-week spray window was just into the lower leaves and trying to wipe out the inoculum down there is really quite important.

Going quick and with low water volumes doesnt help with trying to get the disease out of the crop.

You need a relatively fine spray and you dont want big, coarse nozzles when it comes to fungicide control. Formulation is also likely to play a part, says Bayer technical manager Ben Giles, with a big difference seen in curative activity between oilier tebuconazole and a suspension concentrate (SC) during last years drought.

He says: We used Toledo which is much stronger and more concentrated, but is an SC formulation.

There was a big difference, particularly in speed of curative activity between the formulations.

If youve got rust because of high temperatures, even tebuconazole choice can be a factor in that. The balance between SDHI and azole contents in products varies greatly, so should be an important consideration when choosing which product to apply, he says.

Although variety resistance ratings are the best available estimate of the threat posed by yellow rust, they are only based on which races the plant was exposed to during trials.

With new strains of the disease emerging, if yellow rust is present it should be treated, says NIAB technical director Bill Clark.

We have to predict the races which will be prevalent next year, but its only a best estimate.

When you see yellow rust on 8/9 rated varieties you dont know what the race is, so you have to assume it might be a new race.

There is masses of yellow rust out there at the moment.

The bottom line is if you look for yellow rust and you find it, you have to treat it there and then.

I never thought Id be recommending T1.5 for yellow rust but its getting to the stage where you might even have to do that where its really rapidly cycling.

With a rise in varieties with good resistance ratings, growers have more opportunity than ever to cut back on fungicide applications.

By adapting fungicide programmes to suit the season, location and, most importantly, the variety, savings of £10-£25 per hectare could be made, says Iain Hamilton, senior field technical manager at Syngenta.

In 2018, AHDBs Recommended List variety trials saw yield responses to its blanket fungicide programme range from 0.24-5.8 tonnes/ha, highlighting the variation between each variety and trial site.

Syngenta has been carrying out a series of regional trials on 12 sites across the country comparing a full fungicide programme with programmes tailored to location and wheat variety.

In 2018, which had the highest disease pressure across the three years the trials have been running, crops treated with varying fungicide programmes adapted to variety disease profile and trial location yielded just 0.1t/ha less than plots treated with a standard fungicide programme of chlorothalonil + propiconazole + cyproconazole at T0, chlorothalonil + epoxiconazole + isopyrazam at T1 and either Ascra Xpro (bixafen + fluopyram + prothioconazole), Adexar (epoxiconazole + fluxapyroxad) or Elatus Era (benzovindiflupyr + prothioconazole) at T2.

Cheaper

Mr Hamilton says: Standard treatments were about £125/ ha but the adapted programmes were significantly cheaper.

We were spending £20/ha less to get the same result. In the same trials in 2019, 0.2t/ha difference was seen in the differing fungicide programmes, but variety adapted fungicide programmes were £14/ha cheaper.

In 2020, trials results were limited by challenging drilling conditions, but crunching down into the numbers for each disease, spend on septoria control tended to be a similar amount of money regardless of the fungicide programme due to it being a harder disease to combat, Mr Hamilton adds.

For rusts, Elatus Era + folpet showed good control of both yellow and brown rust, with a 0.4t/ha yield lift in brown rust trials compared to other programmes at T2 in a variety with a resistance rating of 3.

Response

Newer varieties with stronger disease packages showed more flexibility in the field, including Graham which compared to untreated plots in 2019 showed only a 0.9t/ha yield response to fungicides and £25/ha savings were made by adapting the programme to the varietys resistance profile.

Mr Hamilton says: With lower risk varieties, youve clearly got more flexibility.

We cant predict the weather, so build a programme based on what you know in terms of variety profile.

As growers move into an era without epoxiconazole, early eradication of yellow rust will be particularly important, principally at T0.

AHDBs Fungicide Performance trials over the last three seasons show that although Ascra Xpro, Elatus Era and Revystar XE (fluxapyroxad + mefentrifluconazole) performed well on yellow rust, a significant level of disease was left behind even at full label rate, according to Dr Jonathan Helliwell, business development manager at BASF.

He says: No product on the market now as a standalone treatment can control yellow rust.

Going into 2021 its really important that with the loss of epoxiconazole we are getting on top of that infection and early to keep yellow rust out.

T0 and T1 is becoming increasingly important regardless of what fungicide you decide to use.

Despite 2020 being a low pressure year for septoria on the whole, trials by Bayer and NIAB saw significant yield responses of more than onetonne per hectare to fungicide treatments, even in varieties with high disease resistance ratings, including KWS Extase, Graham and KWS Siskin.

qPCR testing ruled out the presence of disease in the crops, leading Mr Clark to put the yield response down to expected drought tolerance triggered by SDHIs.

He says: In 2010 we did drought tolerance trials with SDHIs and found in the middle of the day, untreated plants began to look very spikey and were obviously under stress.

When the leaves start rolling it is essentially the plant stopping photosynthesising.

Even though it recovers at the end of the day when its cooled down, the plant has lost a lot of its yield potential in that period.

Provided it is only transient drought, keeping the crop green and leaves open and not rolled is a good thing.

Uptake

When water uptake was explored, the study found water uptake beyond onemetre was almost double where an SDHI treatment had been applied, due to rooting further down the soil profile.

Mr Clark adds: We also measured photosynthesis under drought conditions with and without an SDHI and we saw a massive difference because the crop was not under stress so the leaves and stomata were still open.

We still dont fully understand it, but we know theres an effect on rooting and photosynthesis thats different to strobilurins which delay senescence. Significant rooting effects were also seen from flag leaf sprays in separate trials, Mr Clark says.

You tend to think all the roots are there by flag leaf but theres a big turnover of rooting and some are being formed or dying back so theres always production of new roots.

Despite low perceived disease pressure early on in 2020, results across AHDBs Fungicide Performance trials saw an average yield uplift of 0.6 tonnes per hectare compared to untreated plots from half-rate Revystar XE at 0.75 litres/ha.

Dr Helliwell says: In reality, septoria did enter most wheat crops much later than usual in 2020 and the longevity of Revystar gave a prolonged period of protection against disease.

There was still a yield benefit of using one-litre/ha to protect the flag leaf instead of 0.75-litre/ ha even in 2020, although this is more evident in the multi-year analysis from 2018-20.

If a decision had been made at T2 to cut back in terms of strength of chemistry or reduce the dose, you wouldnt have filled the potential of crops, regardless of what the disease pressure was telling you at T2.

Were not just protecting crop potential by using Revystar and Xemium, but were building yield in the absence of disease in certain scenarios.