Crop Doctor: Disease set to strike wheat if weather turns wet

Disease is lurking in winter wheat crops across the country and April showers could easily spark an epidemic of the crop’s most yield-damaging disease, septoria.

Despite a warm and dry spring which had led to generally low levels of this wet-loving disease, inoculum of septoria leaf blotch is being seen on the lower leaves of most crops.

The presence of this disease, which can cut yields by up to 50%, is putting growers on the alert to make sure they control septoria at the vital T1 fungicide timing in late April.

See also: How growers produce wheat to make Weetabix

To assess the wider picture, Bayer’s Crop Doctor helicopter conducted a whistle-stop tour with disease experts Fiona Burnett of Scotland’s Rural College and Jonathan Blake of crop consultancy Adas to give a nationwide outlook.

Three-stop tour 

The team visited a yellow rust “hot spot” site on the edge of The Wash, a wet-weather septoria-favourable site in the west and then to the wheat heartlands of Suffolk.

 

The big question for wheat growers is whether their upcoming T1 late April spray will incorporate an SDHI fungicide to improve disease control or not, with rainfall in April likely to sway many wheat growers in their choice of products.

Both experts looked to tackle this key SDHI question, with an eye for achieving good control and protecting this vital chemistry from succumbing to disease resistance.

If the SDHI-azole approach is adopted, the likes of an established product such as Aviator, Adexar, Keystone or the sole SDHI Vertisan – often applied with multisites like chlorothalonil – would likely be used.

Meanwhile newer products such as Ascra and Elatus, and also Adexar are likely to be used at the T2 flag leaf stage.

Jonathan Blake © David Jones/RBI

Jonathan Blake © David Jones/RBI

If the azole-chlorothalonil approach is taken, the two most popular options for the azole component would be prothioconazole or epoxiconazole.

The experts say late-drilled crops of varieties with good disease resistance may only get an azole-multisite fungicide mix, while growers of early-sown susceptible varieties could opt for more expensive, but more effective control by adding an SDHI.

Currently, the lower leaves of many wheat crops are seeing infection from airborne septoria ascospores, and this disease can easily spread to the important yield-building upper leaves by rain-splash in the spring.

April weather

Mr Blake, who is based at Adas Rosemaund in the wetter western climes of Herefordshire, says much depends on April weather, and if weather stays dry an azole-multisite fungicide or a reduced rate of an SDHI mix may be appropriate.

Actives

  • Aviator: SDHI bixafen + azole prothioconazole
  • Adexar: SDHI fluxapyroxad + azole epoxiconazole
  • Keystone; SDHI isopyrazam + azole epoxiconazole
  • Vertisan: SDHI penthiopyrad
  • Ascra: SDHIs bixafen and fluopyram plus azole prothioconazole
  • Elatus: SDHI benzovindiflupyr (solatenol) plus azole prothioconazole

“Weather conditions up to the April T1 spray window will dictate how much growers will need an SDHI approach,” he says.

Virtually every wheat crop in Herefordshire has septoria on the lower leaves, while he is only seeing another damaging disease, yellow rust, on the susceptible variety Reflection.

Up in Scotland, Prof Burnett says disease is easy to find in wheat crops, even though they generally look fine after good growing weather through the autumn.

She agreed that with late-drilled crops there is scope to go with an azole-multisite approach, but Scottish growers are often forced to drill early because of shorter autumn drilling windows.

Early drilling and a warm autumn has encouraged disease levels, and she is seeing septoria, yellow rust and mildew in crops from the Scottish Borders to Aberdeenshire.

“A lot of growers will use a SDHI at T1 and this is sensible and broad spectrum approach to disease control,” she says.

Yellow rust 

She warns yellow rust is more prevalent this season with the disease being seen from the borders to Angus, and in the last two years many growers were caught out due to cold springs and the gap between T1 and T2 was extended to as much as eight weeks, when three to four weeks is generally seen as the maximum.

Yellow rust found at the Long Sutton site near The Wash in Lincolnshire © David Jones/RBI

Yellow rust found at the Long Sutton site near The Wash in Lincolnshire © David Jones/RBI

“Last year many growers cut their azole rates and got stung by yellow rust,” she says.

Prof Burnett highlights that mixing different groups of chemicals such as SDHIs, azoles and multisites is more important than the choice of products.

Many growers will be putting on their first T0 sprays on in late March and early April, and many are likely to use an azole plus a multisite such as chlorothalonil, with the best azoles for yellow rust being epoxiconazole, cyproconazole, tebuconazole and closely followed by metconazole, with a multisite often added for protection againts septoria.

Long Sutton, Lincolnshire

Septoria is the main disease concern for David Hoyles, who aims to get very high yields of wheat on some of the most fertile soils in Britain close to The Wash.

The disease is currently present on the lower leaves, while yellow rust can be seen only on susceptible varieties such as Reflection on a farm trial run by Bayer.

Lincolnshire grower David Hoyles © David Jones/RBI

Lincolnshire grower David Hoyles © David Jones/RBI

Mr Hoyles is growing feed wheat varieties with good resistance to septoria such as Evolution, Motown and Graham, and is able to cover his 190ha of wheat in one day with his sprayer.

“Septoria is the dominant factor here. We look to grow lush thick crops for higher yields so we are looking for the cleanest varieties,” he says.

Although this corner of south-east Lincolnshire is seen as a “hot spot” for yellow rust, he has only seen a few spots in his Graham wheat, although there is plenty in susceptible varieties in the trial.

He is budgeting for 12t/ha wheat yields on his deep silt soils and although these slipped to average 11.7t/ha in 2016 his top crop made 15.6t/ha in the bumper harvest of 2015.

Ticking away

Prof Burnett says the disease pressure on the farm was not high, but septoria will be ticking away in the bottom of the crop, and a turn to warm wet weather could see the lesion of the disease start to appear.

Mr Blake says with a lot of new growth in the crop, septoria was currently confined to the second newest leaf, but the disease could easily emerge on higher leaves.

Jack Hill, Bayer’s commercial technical manager in neighbouring Norfolk, says septoria is “bubbling away at the base of the crop”, and the timing of a T1 spray will be crucial to protect against the disease splashing up the crop.

He adds that wheat crops have a lot of potential this year after establishing well in the autumn, and fungicide strategy needs to focus on drilling date and variety.

Three blocks

Mr Hoyles has three blocks of wheat divided by drilling date, with his early drilled wheat having received a T0 in mid-March while his middle-drilled crops put in the ground in last few days of September had a T0 on 25 and 26 March.

Fiona Burnett, Bayer's Jack Hill and Jonathan Blake © David Jones/RBI

Fiona Burnett, Bayer’s Jack Hill and Jonathan Blake © David Jones/RBI

Later-drilled wheat after sugar beet or potatoes on his 700ha Monmouth House Farm, near Long Sutton, should get a T0 spray in the first week in April.

His choice of T0 was the azole-chlorothalonil mix Cherokee, as although frosts have kept rust in check, warm and moist mornings near the coast are ideal conditions for the development of the disease.

Mr Hoyles says he will probably go down the SDHI route at T1 and use either of the SDHI-azole products Aviator or Adexar with a multisite added such as chlorothalonil, and a lot will depend of product pricing.

“We have lush crops and damp morning with seas frets and mist, so we are looking to keep as many tillers until June to maximise yields,” he says.

Woolpit, Suffolk

One Suffolk farm has seen yellow rust in its wheat crops for the last 28 years while the wet-loving septoria can be a big headache with heavy rain in April.

Disease pressure appears low on the farm, with some septoria, yellow rust and mildew seen in crops, but this threat could easily exacerbate with a change in the weather.

Disease levels are low but this is misleading as septoria will come back
Bill Angus, plant breeder

Farm manager Steven Offord says septoria is the main disease problem in this area of mid-Suffolk despite relatively lower rainfall levels.

He is growing 250ha of winter wheat split between the varieties Skyfall and Evolution, both with good resistance to septoria, drilled in late September/early October.

Disease threat

“The disease threat is here this year with a background of septoria, which with heavy rain in April could easily explode,” he says.

Plant breeder Bill Angus, who is conducting trials on the farm in conjunction with Bayer, warns that septoria is a nasty pernicious disease that can cut yields dramatically.

Suggolk farm manager Steven Offord © David Jones/RBI

Suffolk farm manager Steven Offord © David Jones/RBI

“Disease levels are low but this is misleading as septoria will come back and so will yellow rust,” he says.

He argues that there is a synergy between chemistry and genetics, and growers would be crazy to cut back on fungicides.

“This farm has never failed to get yellow rust in the last 28 years, and the tractors often come back yellow,” he says.

Mr Blake says septoria is seen on the second to last leaves to emerge, and there was clearly yellow rust on susceptible varieties such as Santiago.

Septoria explosion

“There is plenty of septoria to allow for an explosion to occur in the next few weeks if you get rain,” he says.

Prof Burnett also agrees that septoria is definitely established in the crop which could spread rapidly with the right conditions, while yellow rust is also lurking.

Bayer's Ella Crawford © David Jones/RBI

Bayer’s Ella Crawford © David Jones/RBI

Ella Crawford, Bayer’s technical commercial manager for Suffolk, says the disease threat is being held back by dry fine weather.

“There is a lot of disease on the lower leaves which could easily move up the crop with rain splashes,” she says.

Mr Offord, who manages Clopton Green farm near Rattlesden, 10 miles south east of Bury St Edmunds, says he is looking to apply his T0 in early April.

He is planning to use the azole-chlorothalonil mix Cherokee in the first week of April with the two azoles aimed to control rust and the multisite to protect against septoria.

Callow, Herefordshire

As usual Callow is the dirtiest of all the Crop Doctor sites for septoria
Jonathan Blake, Adas

Disease levels were higher in Herefordshire than the other two eastern sites, with septoria being seen on the tips of the last winter wheat leaves to emerge.

Mr Blake says disease levels are as bad as he has seen for some time in the county with yellow rust and mildew as well as septoria being seen in many wheat crops, especially at the Crop Doctor site at Callow, just three miles south of Hereford.

High humidly through February and March fuelled by plenty of dews has created conducive conditions for disease spread, leading to infection occurring just as new leaves start to emerge.

Dirty site 

“As usual Callow is the dirtiest of all the Crop Doctor sites for septoria. At Suffolk and Lincolnshire, lesions could be found on the second last leaf to emerge, here it’s on the tips of the last leaf,” he says.

High levels of mildew are being seen on susceptible varieties, while the damp mild conditions are perfect for yellow rust development, which has started to come into crops in the last week of March.

Herefordshire farmers Mike and Oliver Price © David Jones/RBI

Herefordshire farmers Mike and Oliver Price

The farm run by father and son team Mike and Oliver Price grows Costello, Graham, Siskin and Reflection varieties of wheat, while also incorporating a Bayer trial site on the farm.

They say that crops have come out of the winter well, whether early or late drilled, and spring rainfall has been quite low since Christmas which has suited most crops.

As bad as it gets

Gareth Bubb, Bayer’s commercial technical manager for Herefordshire, says the disease situation is “as bad as it gets” with septoria being found on most wheat varieties across the site.

He reiterates Mr Blake’s observations that the disease which thrives in damp humid conditions is on the tips of the last leaves to emerge in some varieties, creating a source of infection for the next leaves when they emerge.

“The risk from septoria is definitely here, and much will depend on rainfall from March through to the end of June,” he says.

Video: How to get wheat T1 fungicide timings spot on

Independent agronomist Luke Wheeler demonstrates how to dissect a wheat plant to tell if leaf 3 is fully emerged, at which point the crop is ready for a T1 fungicide spray.

 

Differences between varieties are surprisingly small at this stage with even the more septoria resistant varieties, such as Sundance and Graham, showing substantial infections.

Bayer's Gareth Bubb © David Jones/RBI

Bayer’s Gareth Bubb

Mildew is present at high levels on the early-sown susceptible variety Santiago on site, while yellow rust is just starting to come into Reflection due to the favourable damp and mild conditions.

Plans at Monkhall Farm are to apply an azole-multisite T0 spray in the first week of April, before a T1 fungicide is applied usually about 22-24 April.

Mr Blake say T1s are mainly protectant sprays, but he suggests that adding in more curative activity may be advantageous, assuming weather in the next three weeks is conducive to septoria.

The extra curative activity will come from the addition of an SDHI, and Mr Blake suggests an SDHI-azole-multisite may be the best approach, especially on susceptible varieties. Mr Bubb plans to go in with an Aviator-chlorothalonil mix.