Low rainfall fails to dampen septoria threat to wheat crops

Jonathan Blake and Dr Fiona Burnett with the Crop Doctor helicopter

Ahead of the flag leaf fungicide spray, the Farmers Weekly and Bayer CropScience helicopter tour scoured four wheat trial sites around England for the latest foliar disease status.

As many growers considered doing a rain dance after very low rainfall figures during the past month, it could be assumed wheat disease levels were equally as depressed.

But as the Crop Doctor helicopter took to the skies once more, experts on board found “suprising” levels of septoria in wheat trials around England and advise a tough approach to controlling the threat at the flag leaf fungicide spray timing.

Septoria tritici is the wheat growers’ nemesis and is present at varying levels every year, but causes most damage in wet and warm conditions through April and May.

The recent dry April has included cold night temperatures and frosts in some parts, so as Adas and SRUC disease experts Jonathan Blake and Fiona Burnett inspected wheat plots, they expected septoria to be confined to the base of the crop canopy.

Actives

Adexar epoxiconazole + fluxapyroxad

Aviator bixafen + prothioconazole

Bravo chlorothalonil

Brutus epoxiconazole + metconazole

Prosaro prothioconazole + tebuconazole

Vertisan penthiopyrad

However, there were brown septoria lesions containing the characteristic black pycnidia – or spore-producing bodies – evident on leaf four in the worst varieties and leaf five on the less susceptible types.

With the erect nature of leaves within the canopy, our experts agree that the close proximity of infected leaves to new growth is a real concern, and given the right conditions could damage yields.

“I thought there would be much less visible disease, but it has definitely held its level since we last visited the four sites,” says Dr Burnett.

“It has just been pegging along and it’s on leaf four or five and infecting the emerging leaves as they rub together. It could have been a lot worse if we’d had wet weather, but septoria is there on all varieties.”

Yield builders

Mr Blake notes that in the most septoria-susceptible wheats seen on the tour, leaf two and the flag leaf are likely to be exposed to infection as they emerge.

Along with the flag leaf, the top two leaves contribute two-thirds of the light interception during the grain-filling period and are crucial to the final yield of the crop.

“For that reason, the flag leaf T2 spray remains the most important application, as it protects the two leaves contributing most to yield.

“At this point, leaf two is only partially protected by fungicide applied to protect leaf three, so there will be latent infection there,” explains Mr Blake.

He advises that the flag leaf spray should include the best curative products available to help eradicate latent infection in leaf two and protect the flag leaf against septoria.

These would include SDHI/azole co-formulations such as Aviator or Adexar, which are also the most persistent and will help to protect the flag leaf for as long as possible.

“I would never advise cutting back on this timing, even where growers have ‘front loaded’ and used an SDHI at T1.

“It is fine to do that as long as it is not at the expense of the T2, and despite the dry weather, the septoria threat is still there. Just 5mm of rain is sufficient to create the ideal humidity and infection conditions,” says Mr Blake.

Splitting hairs

Dr Burnett agrees that cutting back the most important fungicide spray would be foolhardy and advises a minimum of 80% dose of the leading SDHI/azole septoria-controlling products. 

“If the dry weather continues you could tweak rates down, but you really are splitting hairs and would make only marginal savings.”

She adds that the SDHI fungicides will also give a physiological benefit at T2, making the top two leaves “work harder” by enhancing green leaf area and photosynthetic ability to capture more light and build yield.

“If you are only going to use just one SDHI in the programme, T2 is still the place to apply it,” says Dr Burnett.

Rise of rust

Yellow rust can be as devastating as septoria in wheat crops given the chance to take hold, and the disease could be found in susceptible varieties at all four sites.

New, more aggressive “Warrior” rust races now represent in excess of 80% of the UK population and these races cycle faster, produce more spores and are tolerant to higher temperatures.

With this is mind, Mr Blake warns growers not to let rust-prone varieties go more than three weeks without receiving a rust active fungicide to keep the disease at bay.

“If the crop needs an extra application between the leaf three and flag leaf emergence, then that should happen.

“An azole such as epoxiconazole would be preferable as it is active on yellow rust and more persistent than a strobilurin, so will give you a longer period of protection,” he says.

North

Stockbridge Technology Centre, Cawood, Yorkshire

Ear disease pressure heats up

The run of warm days and dry weather has heightened the risk of ear disease microdochium in wheat crops, according to Fera expert Phil Jennings.

Microdochium is seed and soil borne and the early symptom of stem-based browning (pictured) could be found easily at the Cawood trial site – accelerated by the dry conditions.

Stem Microdochium

Although it is not as much of a concern as fusarium head blight, which can cause 30% yield loss and increase mycotoxin risk, microdochium has been shown to hit yields by about 13% in trials.

“A dry few weeks has allowed it to develop and infect the base of the crop. It was easily controlled by the strobilurins until 2008, but resistance to that chemistry is now widespread,” says Mr Jennings.

The azole fungicide prothioconazole is the most effective active ingredient to fight the disease, with an early application at T1 shown to give the best reduction in inoculum, which later spreads to the ear.

Mr Jennings says that using a prothioconazole-containing product at T2 will also give some reduction, but the key timing for any ear disease control will always be at the early flowering T3 timing.

“If you are only targeting microdochium at T3, prothioconazole will give you the best chance of control, but where fusarium is the main concern a product such as Brutus is good,” he says.

The likelihood is that growers will be looking to control both, particularly if there is wet weather during flowering, and in that scenario, Mr Jennings says rates should be kept up.

“Whether you are using prothioconazole on its own or with partner actives, the rate must be kept up to deliver 150g/ha of prothioconazole. Below 100g/ha and you are getting too low,” he adds.

West

Callow, Herefordshire

Septoria threat still lingers

At the septoria hotspot of Callow, independent agronomist Malcom Williams (pictured) has not seen disease develop as expected due to the dry weather, but heavy dews and wind have helped the septoria threat linger.

However, Mr Williams does not feel there is the potential for a disease disaster, unless a couple of weeks of wet weather arrive.

Malcolm Williams

“The lack of rain has meant spray timings have been spot on so far, so it has taken the pressure off the rest of the programme, but an SDHI product will still be the main component at T2,” says Mr Williams.

He says the correct timing is crucial at T2, and growers should wait until full flag leaf emergence on the main tiller, but the biggest concern at this stage is that being disrupted by bad weather.

If the forecast looks like it is going to cause problems, Mr Williams advises it would be better to go slightly earlier than later to protect the crop.

“There is also talk on farm about cutting costs in the current climate, but once you have planted crops you are committed – don’t pull out of programmes.

“Last year when the pressure was high, we applied 1.25 litres/ha of Aviator at T2. This year we might be able to use one litre/ha for a small saving,” explains Mr Williams.

East

Long Sutton, Lincolnshire

Cool conditions keep disease at bay

Despite the perception of the year to date being dry, grower David Hoyles says that his rainfall for January to April has only been about 17% down on the five-year average.

But it is the dry, cool and windy conditions in the most recent weeks that have meant disease in his wheats has not materialised as expected.

After a robust T1 of Vertisan + Prosaro + Bravo, he is planning to follow that up with an application of Aviator on the flag leaf, which will emerge imminently on his most forward crops.

“In these thicker crops you can soon lose them if disease takes hold and there is plenty of septoria around.

“I’m looking to carry as much plant biomass as possible into grain fill, so I’m cautious with fungicide applications and adopt a prevention-is-better-than-cure policy,” he explains.

Mr Hoyles adds that with disease not romping away, there could be some room to tweak rates down, particularly on his more resistant variety, Evolution.

“Disease is fairly low, but for the sake of just £3-£4/ha on crops that have big potential, it would be a false economy to drop back on fungicide spend too much.”

Despite Mr Hoyles not seeing yellow rust in his commercial crops, Bayer’s commercial technical manager, Darren Adkins, says it is active in susceptible varieties at the Long Sutton trial site.

Yellow rust

With the disease able to slash yields by 50% if allowed to infect the flag leaf and ear, he says an SDHI such as Aviator will help protect against the disease.

“If you have active yellow rust in the crop at T2 though, the addition of a strobilurin such as pyraclastrobin is advised to give a fast knockdown of the disease,” he adds.

South

Hinton Waldrist, Oxfordshire

Cutting back ‘not justified’

After “front loading” his wheat fungicide programme with an SDHI/azole product at T1, independent crop consultant Sam Clarke (pictured below) will still use another on the flag leaf despite the dry spell holding disease.

He says crops locally are showing good potential, and with a 10t wheat crop currently worth about £1,200/ha, he doesn’t believe cutting back is justified.

“The mornings have been very dewy, so the disease is still bubbling away in the bottom of the crop.

Sam Clarke

“With showers forecast it is only going to go one way, so we’ll apply a robust fungicide at T2, with milling wheats getting an Aviator and the feed varieties a fluxapyroxad-based spray [for example, Adexar],” adds Mr Clarke.

He favours Aviator on the milling wheats as there is evidence the product helps with protein assimilation, crucial for hitting the 13% millers require for breadmaking.

“We are chasing premiums of £45-£50/ha, so making the quality specification is crucial to making money out of these crops,” adds Mr Clarke.

Bayer commercial technical manager Tim Nicholson reminds growers that the T2 timing will never be a fully protectant situation, with latent infection likely within leaf two.

He advises leaving out multisite active chlorothalonil when using Aviator at this time to maximise the curative activity of the product.

“The good news is that at the moment we seem to be on top of the job, so we should go into T2 in a good position to keep it that way,” says Mr Nicholson.