Bayer's Crop Doctor North: Cold weather masking disease levels

It’s often geography that ultimately dictates Scottish cereal disease threats and although that is a factor this season, it is more to do with drilling dates.

Contrasting drilling dates and conditions mean a variable picture, not just by region but by every farm. After the autumn of 2019, many made the most of fine early autumn conditions, in some cases wheat going into the ground during the first week of September.

But a change in weather interrupted drilling plans, with crops also sown in late October and during November. As a result, there is now variable crop development, but also shifting disease pressure.

Although varietal ratings have improved in the varieties favoured by Scottish growers, many still lack high septoria ratings. With many wheats drilled early, it’s a key concern for SRUC’s Professor Fiona Burnett,

And that's especially so given the loss of chlorothalonil (CTL). A key T0 component and buttress for T1 sprays, it helped keep septoria at bay and better varietal and alternative multisites don’t fully compensate its loss.

She felt that azole + multisite still had a place at T1, but only where a resilient variety has been sown late, and the weather hasn’t stoked up disease pressure.

“It’s not just the septoria risk that increases with earlier sowing but eyespot too, especially as crops have sat wet through the winter. Including an SDHI will boost septoria and eyespot control, especially if combined with prothioconazole with its protectant activity,” she said.

With yellow rust around the coast, eyespot in early crops, and mildew in prone areas like the Black Isle, she looked upon prothioconazole as a logical T1 option. Where yellow rust is the key target, Elatus is a good fit, but it isn’t so strong on against septoria, so Ascra is a better option.

Yellow rust on the lower leaves pictured earlier this month at the Long Sutton test site

A spiroxamine co-form with prothioconazole might add to knock-down where yellow rust is the issue and SDHI + prothioconazole is a good ‘all round’ choice to combine septoria and eyespot concerns.

Although yellow rust levels are not at the height of this time last season it is bubbling up in some coastal regions. Professor Burnett is still nervous about variety resilience to the disease and noted it has been seen in Skyscraper.

“The question is when will adult plant resistance kick in, especially in late sown, less developed plants. If the disease is visible on the lower leaves, it is a wise choice to factor it in to T1 sprays. But it is a very site-specific disease,” she said.

Where yellow rust is established, she added that a low dose of tebuconazole, or pyraclostrobin added to T1 sprays should boost eradicant activity. When replacing CTL, she would include alternative multisite like folpet in any T1 mixture,

She appreciated concerns on additional control and yield response but considers the need to protect azoles and SDHI as vital. Where disease pressure is light, Professor Burnett would reduce the rate of the ‘at risk’ azole and SDHI components.

For her, the question with folpet is more about dose. She is worried that if the maximum total dose of 3.0 l/ha is applied at T1 and T2 it leaves nothing back if septoria is a significant concern at the T3. “Our extended season means we can see a significant foliar threat late on.

"Holding some folpet back is a wise move, but beware of doses below 1.0 l/ha as we see significant drop off below this rate,” she argued, adding that fungicide rates can be altered where disease pressure is lighter. She preferred azole and SDHI rates were reduced.

She accepted that increased azole and SDHI doses would also deliver effective disease control, but that ups the selection pressure to resistance. Complex mixes are helpful and the removal of an additional mode of action, especially a multisite, weakens anti-resistance strategies. She would prefer this rather than upping rates and omitting folpet.

Dose rate will be the response of Sandy Dale, of Scoughall Farm, East Lothian, with the varieties Insitor and Elation. He is one of three Scottish growers taking part in Bayer’s National Disease Snapshot initiative.

Using its new Rapid Disease Detection Service, fast qPCR analysis means that Sandy gets results on septoria and yellow rust infection levels back 48 hours after sending them to the lab. When testing started it January, both Elation and Insitor returned positive tests for septoria.

Elation’s susceptibility to the disease was evident with pg/DNA levels of 3.705 – significantly above that of Insitor.

But since that peak, DNA levels have tailed off. With testing weekly from March on the newest leaf to emerge no result above 0.932 pg/DNA was recorded for Elation, and for Insitor new leaves were completely free of the disease.

Similar has occurred with yellow rust. Although visible in the base of crops the latest results detected no Septoria or yellow rust in newly emerged leaf 4 for both varieties.

But with T1s delayed, it could all change if warmer, wetter weather arrives. Sandy went early with a T0 spray for yellow rust as he wanted to apply digestate to ‘kick start’ crops, but needed to be wary of cut-off date.

With the key T1 and T2 timings approaching, rapid disease detection is giving him and his agronomist an insight into what is going on in plants, helping to shape final fungicide strategy.

Test results will ultimately decide whether they go with just an azole + multisite mix or include a SDHI. But what is almost a given is that rate will be adjusted to reflect variety rating variation.

“At 6.8, Insitor has good septoria resilience but it is weak for yellow rust. If it stays dry and generally cool the yellow rust that is in the lower canopy is unlikely to spread, and I’ll have some flexibility with rate," he said. “With Elation being drilled in September and a septoria rating of just 4.1 I’m unlikely to have the same flexibility."

Bayer’s Craig Simpson agreed with that in this situation, but there is unlikely to be the same flexibility with Elation compared to later-drilled varieties that are more resilient. Where septoria is a threat, Craig said the addition of fluopyram to a fungicide programme gives added assurance.

“The incomplete cross-resistance of bixafen and fluopyram gives broader control of septoria strains. Where we see a shift in sensitivity with one, we don’t see it with the other,” he pointed out.

The combination of prothioconazole alongside the SDHIs would help to deliver yellow rust activity at the T1 timing. Although recent frosts have kept yellow rust to a minimum, the coastal East Lothian location and the fast-cycling nature of the disease means it remains a threat prior to the onset of adult plant resistance.

The good news is that although the cool conditions have checked crop progress, it hasn’t dented it too badly. September-sown wheats are looking good and with proven performers, like Skyscraper, he is hoping for a respectable season.

To go with the digestate, he recently turned to inhibited urea fertiliser. This could pay dividends this season given the dry spring conditions.

Reduced volatisation means more N is going to get into the plant and he is also ‘drip feeding’ sulphur to ensure this and nitrogen are available throughout the season.

Rhyncho risk for winter barley

In winter barley, rhynchosporium has also been checked by cold, windy condition but in early drilled situations levels remain high. At Boghall, in the Lothians, the disease is present in a number of varieties.

Rhynchosporium setting in on winter barley just this week at the Boghall trial site in the Lothians

Given that conditions haven’t been totally conducive, Professor Burnett was surprised to see how dirty plots were. However, she noted that there were few varieties rated above six at the site.

Her view was that there’s sufficient infection for the disease to take off if weather turns wet and humid. But there is a good arsenal of products, including cyprodinil and spiroxamine – which growers used at T0 and early clean ups – and these allow for alternative modes of actions to be introduced at the main spray timings.

The question with spring barley is the need for a T1. Late sown spring crops drilled into cold, drier beds are likely to come through quickly once they get going.

“The T1 is often a bit of an unknown, and that is certainly the case this season. With crop ‘romping away’ and clean, most growers will want to hold off until the T2. But the unknown is disease levels in the plant,” she said.

Which is why she welcomed 'Rapid Disease Detection' system in conjunction with Bayer. SRUC will be undertaking frequent qPCR testing with results returned within 48 hours.

“It will give us an idea of what is going on in the plant. When T1 decisions are being made crops could appear free of net blotch, or rhynchosporium but the disease might be present but still to express itself,” she concluded.

PANEL:

Keeping resistance low in barley

The multisite question in barley is also a little unclear with inconsistent efficacy and yield gains in the barley crop.

Professor Burnett still looked on systemic chemistry as a good option when it comes to diseases like net blotch and ramularia.

Despite concerns with fungicide sensitivity, it is a decline but one that can be managed in terms of efficacy by avoiding reliance on individual actives.

Again, the message is to use azoles in combinations with SDHI and strobs, and make use of cyprodinil and spiroxamine when possible, especially as disease in barley is often a combination of threats.

With CTL gone, it does place more emphasis on cultural measures. Net blotch defence needs to start with certified seed or testing for home saved seed.

“If you start off with a problem you’re risking greater yield loss but also increasing pressure on the chemistry,” she warned.

Reducing stress in spring barley is also going to help prevent troublesome ramularia. The loss of CTL makes the disease harder to manage so again the use of azoles in combinations with SDHIs and strobs is the best approach.

Despite resistance in ramularia to strobs, they can still be an asset in managing other diseases and their greening properties could be valuable with senescence triggered stress.

She also noted that dire predictions with prothioconazole haven’t materialised and that it is still a good protectant against ramularia. “It isn’t as strong as mefentrifluconazole, but it has a later application window on malting crops,” she pointed out.