Crop Doctor: Rain in May could bring wheat disease woes

A very dry spring has led to subdued disease levels in winter wheat, but a turn towards wetter weather at the end of April could be just the spark septoria leaf blotch disease needs to spread through crops.

Oli Hill and David Jones boarded the Crop Doctor helicopter to assess disease levels across the length and breadth of England.

Crop doctor Jonathan Blake and Fiona Burnett

Adas’ Jonathan Blake and SRUC’s Fiona Burnett with the Crop Doctor helicopter

The helicopter travelled to south Lincolnshire, North Yorkshire, Herefordshire and south Oxfordshire to look for septoria and yellow and brown rust, as well as eyespot and mildew in wheat.

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

In Yorkshire, we took a break from focusing purely on winter wheat and turned to barley to see high levels of disease with mildew, rhynchosporium, net blotch and brown rust all found.

Wet weather warning

Wetter weather towards the end of April is set to encourage an upsurge in wheat diseases, which should alert growers to the risk of septoria leaf blotch spreading rapidly in May.

The dry spring has restricted wheat’s most damaging disease to the lower leaves, but wind and rain in the last week of April could splash the disease spores onto the vital yield-creating upper leaves.

As infected older leaves die back, crops can look deceptively clean of the disease, but newer leaves could already be infected, so putting an onus on making sure T2 May fungicides are potent enough to control disease.

To check on the risk to crops, the Bayer Crop Doctor helicopter travelled to four sites across England, with disease experts Fiona Burnett of Scotland’s Rural College and Jonathan Blake of crop consultants Adas, to give the latest advice for growers.

Prof Burnett warns against complacency with crops generally looking relatively clean, as she points out that disease levels can quickly escalate with a change in the weather.

“As lower infected leaves die off, crops look better than they really are and then suddenly disease comes back with a vengeance,” she says.

Stay vigilant 

Mr Blake adds that the dry April weather has held disease levels back, but a change to wetter weather means growers need to be vigilant leading up to the key T2 spray timing in mid to late May.

“On more disease-susceptible wheat varieties, rainfall will allow infection to become a real problem,” he says.

The experts warn rainfall does not need to be heavy to spread septoria, but wind and heavy dews may be enough to transfer infection from the lower leaves to the upper ones by contact.

With many growers applying T1 fungicides in late April and early May to their wheat crop, the weather in May will be critical on deciding on fungicide products and doses for T2 sprays.

Actives

  • Cherokee: azoles propiconazole + cyproconazole + multisite chlorothalonil
  • Aviator: SDHI bixafen + azole prothioconazole
  • Adexar: SDHI fluxapyroxad + azole epoxiconazole
  • Keystone: SDHI isopyrazam + azole epoxiconazole
  • Sparticus: SDHI bixafen + azoles prothioconazole + tebuconazole
  • Vertisan: SDHI penthiopyrad
  • Ascra: SDHIs bixafen + fluopyram + azole prothioconazole
  • Elatus: SDHI benzovindiflupyr (solatenol) + azole prothioconazole

Scottish situation

Prof Burnett reports that septoria is being found in most Scottish crops, but says it is unusual to see so much yellow rust so early in crops close to the coast from East Lothian up to Forfar in Angus.

It has certainly been a season to justify a T0 fungicide in Scotland, using a systemic azole to give control of  yellow rust in addition to chlorothalonil to protect against septoria.

A three-way SDHI-azole-chlorothalonil mix at T1 is often justified to control a complexity of diseases including septoria, yellow rust, eyespot and mildew.

Mr Blake, based at Adas Rosemaund in Herefordshire, says the dry spring in the usually wet western county has contained septoria, although early-drilled crops of susceptible varieties are showing more disease.

“When infected lower leaves are in close proximity to emerging leaves this is a sign of problems ahead,” he says.

T1 timings

Many growers will have used a three-way T1 mix of two systemic fungicides – SDHIs and azoles – which moved through the plant tissue, plus the protectant product chlorothalonil, and have often reduced the overall dose rate due to a perceived reduced disease risk.

For the T2 flag leaf spray, which across southern England often occurs in mid to late May, two new SDHI-azole products are newly available this season, namely Ascra and Elatus, alongside other established products such as Adexar and Aviator.

To assess the current situation, we took to the skies to check on the disease risk at four locations.

We first visited a yellow rust “hot spot” close to The Wash, which can also see plenty of septoria due to sea frets, followed by a North Yorkshire site with mildew showing up in wheat and barley.

In the wetter west, we visited a septoria favourable site, and a southern location where septoria is a key problem disease.

David Hoyles

David Hoyles

East

Long Sutton, Lincolnshire

Septoria waiting for wet to strike

David Hoyles chooses his wheat varieties from those showing the highest resistance to septoria, but even so, in a generally low disease year it could be found readily in his crops.

Very low rainfall on his farm in the south-eastern corner of Lincolnshire, three miles from The Wash, has kept septoria leaf blotch confined to the lower leaves, but if rain sweeps in that could be about to change.

Although his farm is in a “hot spot” for yellow rust due to its often warm and humid conditions, his main disease headache is keeping septoria out of his potentially very high-yielding crops.

 

“We’ve had 6mm of rain in the last seven weeks and there is little rain forecast,” he said in the last week of April.

Even though he is growing top septoria-resistant varieties Graham, Motown and Evolution, he is taking no risks with the yield-robbing disease with a T1 SDHI-azole-chlorothalonil programme.

Prof Burnett says septoria is easy to find on the lower leaves in his crops, but that the dry weather has kept the disease off the two newest leaves – leaves three and four.

Leaf two in Lincolnshire

Mr Blake says septoria was evident on leaf five and had the potential to infect newer leaves if the weather turns wet.

“The disease pressure is less after three dry weeks, but there could still be latent infection in the crop. May is more crucial than April in terms of rainfall and we are coming into a very important period,” he says.

Yellow and brown rust

Across a Bayer variety trial on the farm the two experts were able to pick up yellow rust in Reflection and brown rust in Crusoe, as well as eyespot and septoria.

Mr Hoyles aims for very high yields on his rich, fertile silty soils and last year was the first time in five years he did not beat his budgeting target yield of 12t/ha, but in the good harvest of 2015 his top yield came in at 15.6t/ha.

He grows 190ha of wheat on his 700ha Monmouth House Farm, Lutton, 12 miles east of Spalding, and, aiming for high yields, his fungicide spend is about £120/ha.

This year the crop had a T0 in late March of azole-chlorothalonil mix Cherokee, and then a T1 in late April/early May of SDHI-azole product Sparticus plus chorothalonil.

He cut the T1 dose rate due to a reduce disease risk saving himself about £5-6/ha, and adds he will be looking at the same three-way SDHI-azole-chlorothalonil approach at T2.

Mr Hoyles will be trying an increased use of Ascra this season after success with it last year on a limited area, but says the final decision will depend on disease risk and product pricing.

North

Cawood, North Yorkshire

Four big barley diseases found

Further north in Yorkshire, there was plenty of septoria in wheat crops and mildew in winter barley on this fertile silty clay soil site within a stone’s throw of the River Ouse.

As the weather warms up and with showers starting in late April, the risk of septoria spreading is increasing and warmth could exacerbate mildew on both barley and wheat.

Prof Burnett spotted mildew, rhynchosporium, brown rust and net blotch in most winter barley varieties, while the risk from ramularia would likely come later in the season.

 

“Four of the big barley diseases are present in crops and the risk from them will increase as the weather turns warmer and wetter,” she says.

To cope with the risk from ramularia she suggests an SDHI-azole-chlorothalonil approach at T2 in barley with the latter multisite product particularly important as the disease is resistant to strobilurins and resistance to SDHIs and azole has been seen in Germany.

The German authorities are now advising the use of chlorothalonil in all fungicide applications in barley.

Cheap insurance

Mr Blake agrees and says the addition of chlorothalonil is a relatively cheap insurance against the potentially very destructive ramularia hitting yields severely.

The Crop Doctor site at the Stockbridge Technology Centre, near Cawood, 10 miles south of York, is on a fertile silty clay soil with potential to produce really high yields, but only if protected from disease.

Independent agronomist Ben Boothman says as well as diseases in barley crops, there is plenty of septoria in the base of wheat crops and yellow rust in susceptible varieties in North Yorkshire.

Adam Tidswell

“Disease is in the base of crops and if we do get a warm spell in the next 2-3 weeks then disease levels will take off,” he says.

Adam Tidswell, Bayer’s commercial technical manager for Yorkshire, agrees, saying that with rain showers and warmer weather forecast, disease levels could increase.

“There is septoria in the bottom of many crops and rain will help the disease spread up the plants,” he says.

The standard trial site treatment was azole-chlorothalonil at T0 and Bayer’s SDHI-azole Aviator+chlorothalonil at T1, which was applied in the last few days of April.

If the showery weather continues, Mr Tidswell says that early-drilled crops of susceptible varieties will need a robust T2 fungicide spray to keep disease in check.

West

Callow, Herefordshire

Septoria worst in the west

Septoria disease levels were higher in the wetter west than any of the other sites suggesting that it may be a struggle to control the leaf blotch disease during May.

Large amounts of septoria inoculum are being detected on the lower leaves of wheat crops in the wet climate of Herefordshire despite a relatively dry month of April.

Mr Blake points out that septoria infection was higher up in wheat crops in the county than all the other sites, and that only a little moisture was needed to infect the newly emerging leaves.

 

“Septoria is on leaf four and as this leaf is often next to the emerging leaves then there are clear dangers of infection spreading,” he says.

Prof Burnett says septoria levels are higher than in Scotland and only a little moisture is needed to spread the disease as the lower infected leaves are brushing up against newly emerged leaves which will allow the disease to spread.

Despite a generally dry April, high humidity and heavy dews in the two preceding months have helped the disease develop on the lower leaves of crops in the area.

The trial site at Monkhall Court, Callow, is three miles south of Hereford city, where Costello, Graham, Siskin and Reflecition wheats are grown, while the farm also incorporates a Bayer trial site.

Gareth Bubb

Gareth Bubb

Gareth Bubb, Bayer commercial technical manager for Herefordshire, says septoria is higher up crops this season than last year.

Struggle with septoria

“Whether the disease kicks off will depend on rainfall. If we get normal rainfall from now on we will struggle to keep septoria out of crops,” he says.

The risk is higher this year because of the amount of overwintering disease inoculum and is especially high on early sown crops of susceptible varieties.

If growers went on too early with their T1 spray, the experts say this could potentially leave a T1-T2 gap wider than the optimum three weeks and allow disease into the crops.

The trial was drilled on 22 September and received a standard azole-chlorothalonil at T0 and SDHI-azole-chlorothalonil in the last week of April.

Independent agronomist Malcolm Williams says that he has seen a very bad season for  yellow rust in Herefordshire, and has had to take more effort to get the disease under control.

“This is the worst year we have known for yellow rust and we have had to tailor fungicides strategies to cope, using more azoles and strobilurins,” he says.

He has generally used a SDHI-azole-chlorothalonil approach at T1, and will back that up with an SDHI-azole at T2 and perhaps look at the new SDHI-azole products on the more susceptible varieties.

South

Hinton Waldrist, South Oxfordshire

Rust turns ground yellow

Yellow rust was so severe at the Oxfordshire site that the fallen pustules of the disease had turned the ground yellow in the worst case the crop experts had seen.

yellow rust Oxfordshire

Yellow rust turns the ground yellow in Oxfordshire

Although the variety was the susceptible Reflection in an untreated fungicide trial, the disease had virtually killed off the top leaves, leaving scattered yellow pustules covering the ground.

Prof Burnett says it was the worst case she had seen emphasising the growing problem with the disease for growers. However, the main preoccupation at the site backing on to the River Thames was the threat from septoria.

 

The disease was clearly seen in the bottom of wheat crops – although not as prevalent as in Herefordshire. All it needed was the right combination of warm and wet weather for the yield-damaging disease to climb up the plant.

“It’s all about the weather. The inoculum is there so if the weather is conducive, the disease will spread. Wind and dews will give the right micro-climate for the disease to climb up especially in thick crops,” she says.

Mr Blake adds that septoria was present on leaf five of the variety Graham, which has one of the top resistance scores for the disease, emphasising the pressure at the south Oxfordshire site.

Real problem

“With the right weather in May then this could be a real problem and would warrant a robust T2 fungicide spray,” he says.

Prof Burnett stresses her approach at T2 would include the multisite chlorothalonil to give an extra element to a SDHI and azole mixture in a moving situation regarding disease resistance to fungicides.

The site at Duxford Farm, Hinton Waldrist, is 12 miles upstream from Oxford on the fertile flood plain of the River Thames.

Jon Helliwell

Jon Helliwell

Jon Helliwell, Bayer’s commercial technical manager in Oxfordshire, says that the level of septoria is higher this season than last year despite the generally dry winter and spring.

“Septoria came into the crop early in the autumn, which was unusual for this area,” he says

He is seeing the disease hit varieties with good resistance such as Siskin, where he saw septoria on leaf six, which could infect newer leaves by direct contact.

“We are seeing the disease creep up the crop without major rain so it must be from direct contact between leaves and from heavy dews,” he says.