Disease threat lurks in the wheat fields

Widespread disease in winter wheat crops mean growers will need to be spot-on with their early control of yellow rust and septoria to halt a potential epidemic.

The mild winter and lack of frosts have encouraged rust and septoria diseases, with crop experts advising growers to get on with an early fungicide this month as soon as fields dry out.

With a large area of forward winter wheat crops in the ground and plenty of disease inoculum around after the absence of cold weather, conditions are ideal for an upsurge in disease, they warn.

See also: Yellow rust on course for worst-ever season

Most wheat crops have septoria on their lower leaves, while yellow rust outbreaks in the eastern side of England have highlighted the threat that this could be a high disease year.

“Most crops have a good level of infection after the mild autumn and winter and there is the potential to have a disease epidemic,” says fungicide expert Jonathan Blake at crop scientist group ADAS, based in Herefordshire.

First fungicide spray

Mr Blake suggests the first fungicide T0 spray, usually applied in March, will be important to keep disease in check and prevent problems escalating later.

“The T0 application will be more important this season, especially on rust-susceptible wheat varieties,” he says.

Mr Blake says that a triazole-chorothalonil T0 spray will give good control of rusts and septoria, and even suggests chlorothalonil alone for varieties with good resistance, or an HGCA score of 7 and above for rusts.

He says there is a concern over yellow rust in the east of England and brown rust in southern England, while septoria tends to cycle more slowly earlier in the season.

Most triazoles give control of rusts, but few give good curative action against septoria in the early season, and so chlorothalonil is usually more than enough to control septoria at this early stage for many crops.

Septoria control

“Septoria is pretty evident at the moment, but crops can grow away from the disease if it turns out to be dry,” he says.

Bill Clark, commercial technical director of crop consultant NIAB, has already warned that this year could be the worst ever for yellow rust, while he adds that septoria is a problem most years.

“Whatever you do for yellow rust will have some impact on septoria, and fungicide programmes are largely driven by septoria,” he says.

Mr Clark points out that the yield response to T0 sprays are generally very small, but a good T0 spray does buy growers some flexibility in the timing for future T1 and T2 sprays.

He again suggests a triazole-chlorothalonil product as the best approach at the early T0 stage to stay on top of disease pressures.

Fast-acting triazoles

Fast-acting triazoles such as tebuconazole and cyproconazole are advised to give good quick knockdown activity against rusts, although most triazoles give some level of rust control.

Of the triazoles, epoxiconazole and prothioconazole give good control of septoria, with metconazole having some level of control.

“Rust is the priority for most growers, with little septoria seen on the higher leaves, so a triazole-chlorathalonil approach would seem appropriate,” he says.

The biggest influence on septoria spread is rainfall in April and May, with the heavy rain in 2012 heralding a big disease year and a dry spring in 2013 giving low septoria levels.

Faye Ritchie, ADAS plant pathologist, based in Cambridgeshire, says she is seeing a lot of yellow rust on susceptible varieties, and more septoria than usual in eastern England.

“It is important to catch rust in susceptible varieties at the T0 timing, and to stop it developing as early as you can,” she says.

Disease inoculum

Dr Ritchie advises that a triazole-chlorothalonil mix will give action against both rusts and septoria, while chorothalonil will be useful as a protectant fungicide at the future T1 timing.

Agronomist and Association of Independent Crop Consultants member David Lines is seeing septoria in virtually every one of his wheat crops over in the Welsh border region with plenty of disease inoculum around after a mild and wet winter.

“If the weather stays like this there could be a septoria explosion, but a month of dry weather would stem its spread,” he says.

Mr Lines covers north Herefordshire, south Shropshire, Worcestershire and Powys, and adds that winter wheat variety Cougar has seen less disease than many other varieties.

Cougar is the only winter wheat variety on the HGCA Recommended List with a resistance score for Septoria tritici of 7, while all others are in a narrow range of between 4 and 6.

“It’s not critical, but it is useful to have this higher resistance when growing a range of varieties. It gives a bit of flexibility if spray days are tight,” he adds.

Resistance ratings

Apart from septoria, the variety has a good resistance rating of 8 for yellow rust, 9 for brown rust and 7 for mildew.

Celia Bequain, lead wheat breeder at RAGT, which bred Cougar, argues that the variety’s rating for septoria is pushing an 8.

“Results from the high disease pressures in 2012 suggested it had less septoria symptoms than any other variety,” she says.

Over in the east of England, Ian Gibson has seen septoria in winter wheat since the autumn and the disease is now very virulent in the early-sown crops he manages in Essex.

Septoria is seen on the lower leaves of the 330ha of winter wheat out of the 500ha of arable land he manages at Radbourne Farms, Aythorpe Roding, some five miles south of Great Dunmow.

Septoria protection

He has decided on a triazole-chlorothalonil approach for his first T0 spray, which he is aiming to apply in the last 10 days in March to give control of septoria and protection against rusts, and has chosen epoxiconazole as the triazole component because of its good activity against septoria.

“Septoria will be key at T0, it is on the lower leaves and not on the upper leaves and that is the way we would like it to stay,” Mr Gibson says.

David Ellerton, technical development director of distributor Hutchinsons, warns that this season could be worse than the high-disease year of 2012.

“This year we’re started from a much higher base of disease. So there is potential for the situation to be as bad, if not worse,” he says.

Two years ago, the biggest problems occurred when growers lost control of disease and ended up chasing the situation through the season.

High-risk disease season

“Whatever the variety, it’s going to be a high-risk season,” he says.

Although a high-disease season may be more challenging to manage, it can deliver the biggest yield benefits from fungicides programme.

He points out that the yield benefit of a fungicide programme was more than 3t/ha in HGCA variety trials in the 2011-12 high-disease season, worth more than £450/ha at current feed wheat prices.