Oilseed rape growers should be on red alert for light leaf spot across the UK this autumn and regular, well-timed applications of active fungicides will be key to keeping a lid on the disease.

Light leaf spot is becoming a major headache and not just in its traditional heartland of Scotland and northern England, where prevailing cool and damp conditions help the disease to establish in crops through the autumn.

See also: Growers could see resistance ratings for OSR verticillium wilt by 2020

Pressure has also been ramping up further south in recent seasons, with significant infection seen in crops right down to the South-East coast and across into the West Country.

The disease is estimated to cause yield losses up to 1.5t/ha and cost the industry £30-£160m each year, despite an annual fungicide spend in excess of £10m to control it.

According to CropMonitor data, phoma was the most economically damaging disease threat to oilseed rape nationally until 2008, when light leaf spot took over that mantle.

Intensive area

There are a number of contributory factors to explain this, according to Scotland’s Rural College (SRUC) crop disease expert Fiona Burnett. These include a larger, more intensive area of oilseed rape and a shift to minimum tillage.

“It’s a predominantly trash-borne disease and there is plenty of oilseed rape out there now. We are also having fewer cold winters that can help nip disease in the bud as well, so control is becoming a real battle,” she explains.

Prof Burnett explains that light leaf spot is also an extremely adaptive pathogen that can readily overcome both varietal resistance and fungicides, with perhaps only yellow rust in wheat as dynamic.

“We have seen varieties with scores of 8 and 9 getting whittled away quite quickly, with Cracker an example of that here in Scotland. It had clubroot resistance and a 9 for light leaf spot, but fell a point every year since it was introduced.”

Grower behaviour has also played a part in light leaf spot epidemics becoming more widespread, she adds, with phoma shaping variety choice and fungicide programmes further south and allowing the disease to build.

Bad year

Keith Norman, technical director at farming company Velcourt, says 2015-16 has been as bad as he can remember and he has seen cases of the disease completely covering the top layer of pods, despite receiving four fungicide sprays.

Along with other pressures such as cabbage stem flea beetle, verticillium wilt, slugs and pigeons, he believes light leaf spot may have had a big influence in lower rapeseed yields this harvest.

Although too early to draw concrete conclusions, high levels of disease last season and the absence of a hot and dry summer would hint at another high risk year for the new rapeseed crop.

“I haven’t seen anything that will set us back and there will be some carryover with so many spores around, which will bridge the gap to the next crop,” explains Mr Norman.

Cultural controls

With plenty of oilseed rape trash lying around and the potential for new crops to be emerging alongside last year’s, little can be done avoid the very high light leaf spot spore movement with cultural methods.

“Many are locked into an early drilling slot, so later drilling isn’t an option and burying trash might help in a small way, but not much. The weather is the only limiting factor, not the amount of inoculum,” explains Prof Burnett.

By the time Crops goes to press, it will be too late for many to adjust variety choice and switch to a more resistant type, but Adas researcher Faye Ritchie says it is the best place to start to reduce this high disease risk.

Following that, appropriate timing of fungicide applications is the next most critical component of a strategy to reduce the impact of light leaf spot, she says.

“There are strains that are less sensitive to azoles now due to our reliance on the fungicide group, but their prevalence is relatively unknown and I think timing, and not resistance, is the biggest issue with control at present.”

Poor control

Bayer technical manager Gareth Bubb agrees and believes the role of azole resistance in increased light leaf spot epidemics may be over-played and instead poor timing or missed sprays are the greatest cause of poor control.

Traditional programmes in oilseed rape have typically included an earlier spray with activity on phoma, such as difenconazole, and a late autumn spray for light leaf spot in November.

However, Mr Bubb points to Kynetec farmer panel data that shows about 40% of the UK oilseed rape area received no autumn fungicide in 2015-16. Of the 60% that was treated, only 20% had two sprays – so about 80% of the treated area had just one application.

This can perhaps be attributed to growers either attempting to cover both diseases with one spray to cut costs or being unable to get back on the land for a second spray.

“It is asking a lot of any product to last from October all the way to stem extension in March. You might get away with it in a low disease year, but not anymore with the light leaf spot pressure we have now.

“I know there are issues with being able to travel at that time of year, but you have to think about the risk, plan ahead and try and get some protection on when you can,” he says.

Top-up

Dr Ritchie says getting a spray on with good light leaf spot activity in November should be a priority, followed by another spray as soon as possible early in the spring to top up activity.

The aim in light leaf spot control – much like septoria in wheat – is to keep the disease at the bottom of the crop and off the pods later in the year, so maintaining protection until stem extension is critical.

“You need to be back in there when you see as little sign of the disease as possible and you will get much better control, so consider spraying in February if you can travel.

“It is a polycyclic disease and will keep developing, so you may also need to top up light leaf spot control at the stem extension timing and again at flowering,” explains Dr Ritchie.

Fungicide choice

According to latest AHDB fungicide performance data, prothioconazole, tebuconazole or combinations of the two are the most effective light leaf spot fungicides, along with azole combination Orius P, which includes both prochloraz and tebuconazole.

There are also azole alternatives new to the market last year, such as SDHI-strobilurin co-formulations Refinzar and Pictor, offering comparable control to azole products.

Product choice and rate will boil down to varietal resistance, plant growth regulator (PGR) requirements and economics for CCC Agronomy crop consultant Richard Cartwright, who advises across Kent, East and West Sussex.

He starts his light leaf spot programme with a cost-effective product such as Orius P, added to propyzamide applications in November or early December, but has also used the more expensive Refinzar to good effect in the past.

Mr Cartwright then recommends a follow up of Proline in the early spring to bridge the gap until crops reach stem extension.

“Previously, we wouldn’t have to come back until then, but the last few years we have needed to because disease had started to reinfect the crop before new growth kicked off in the spring,” he explains.

Growth regulation

Stem extension is a key time for growth regulation and although specific PGR products are effective, they come at a considerable premium, so Mr Cartwright sees tebuconazole as the most economic option, with good PGR activity and control of light leaf spot.

“Depending crop development, you can vary rate to match PGR requirement. If the earlier fungicide has been missed or disease is established, you could add prothioconaozle to increase activity,” he explains.

Looking ahead to sclerontina sprays, he believes the first spray should have activity on light leaf spot and typically uses a combination of tebuconazole, prothioconazole and MBC fungicide thiophanate-methyl.

“I do try and change fungicide groups if I can, but I think essentially resistant varieties will be key to taking the pressure off chemistry and they can also give some us some rate flexibility too,” adds Mr Cartwright.

Fungicide resistance and light leaf spot

Azole-resistant light leaf spot isolates are present in UK populations and managing this resistance should be considered when formulating control strategies.

Two key genetic mutations were uncovered by a recently published PhD project carried out at Rothamsted Research and both result in less sensitivity to the fungicide group.

These mutants are similar to those found in septoria, with genetic changes within the fungi’s DNA altering its CYP51 protein. Resistance is much further developed in septoria, however, with more than 30 of these mutations know to scientists.

Another mechanism was also uncovered in the study that results in some isolates over-producing CYP51, making them harder to kill with azole fungicides.

Nichola Hawkins, research scientist at Rothamsted, explains that the frequency and distribution of these less sensitive light leaf spot isolates is uncertain at the moment.

She adds that AHDB fungicide performance trials are currently the best reference for any shift in sensitivity, so hopes a project can be set up to shed more light on the problem.

“Recent field samples have shown similar results to the previous study, with the same less-sensitive mutants present, but we would like to carry out further laboratory testing of samples from a range of locations,” explains Dr Hawkins.

Fungicide Resistance Action Group (FRAG-UK) chairman Fiona Burnett agrees the situation is uncertain, but SRUC testing has shown an extremely wide spread in sensitivity of light leaf spot isolates.

Because of this spread there is potential for variations in sensitivity to fungicides in the field depending on geographical location, but not enough work is being done to get a full picture.

“Also, field control will come down to a number of factors and poor results are not necessarily down to resistance.

“If you are applying a light leaf spot fungicide at the tail end of October or early November and not coming back until March, it’s no wonder you are struggling. Fungicides only last three to four weeks and that length of activity is a hurdle that needs to be overcome.”

As with other diseases where resistance is a concern, mixing and matching actives through the oilseed rape fungicide programme should be considered as much as possible, she adds.

“We now have Pictor and Refinzar that are matching azole activity on light leaf spot and we need that diversity in the chemistry to complicate things for the pathogen.

“With sclerotinia sprays there are non-azole options too, so there is potential to keep alternation going and try to think of the season as a whole – don’t just treat everything in isolation.”

Light leaf spot monitoring – what’s available?

The Rothamsted Research light leaf spot risk forecast – based on disease levels on pods and stems at harvest, summer temperatures and likely winter rainfall – is released every October, and is currently the only decision support tool for rapeseed growers.

It is a good system for those in and out of high risk areas year on year, as it can indicate the need for a fungicide, but for those in perennial high risk areas such as Scotland, it has its limitations and this has been recognised by researchers.

SRUC plant pathologist Neil Havis has been involved with an AHDB-funded project attempting to devise an accurate forecast of disease onset, to advise growers on the timing of their most effective fungicides.

He says the light leaf spot forecast will work in a similar way to the incredibly accurate phoma forecast – also on the Rothamsted Research website – which estimates a date when the threshold of 10% of plants infected will be reached.

Dr Havis adds the data gathering for the project threw up surprises.  The consensus before the project was that epidemics were related to autumn spore movement, but in fact a significant number spores were detected in the air through July and August.

“We believe this could initiate another disease cycle and subsequently develop more spores to infect later in the season.

“The modellers have all the information now and will combine that with all our prior knowledge to produce the forecast and we hope to have completed it within the next six months.”

Light leaf spot pressure leads to shift in variety choice

In the South this season Elgar and Allize have been solid performers on farm, despite lower average yields nationally and its perhaps no coincidence, with both possessing a resistance score of 7 for light leaf spot.

Howard Nason of independent buying group Crop Advisors says varieties such as Elgar, Barbados (7), Campus (6), Picto (6), Amalie (6) and candidate Flamingo (8 *breeder data) are all proving popular this season across its southern customer base.

He adds that timing of fungicides is critical for light leaf spot control, as the disease infects the crop over a long period of time and to protect crop right through that period is hard.

“To rely solely on fungicides is becoming riskier, so varieties with inherent resistance can buy you some flexibility at a time of year when it can be difficult to travel to apply sprays,” he adds.

Agrii’s national seed manager Barry Barker has also seen the same shift and tips Elgar and Nikita (7) of the conventionals and hybrids Exalte (8 *limited data) and Allize (7) to also prove popular.

“We will sell many more varieties with good light leaf spot scores this year – it used to be just in Scotland, but now it’s all over the UK,” he adds.