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Time for a rethink on OSR disease control?

These are challenging times for oilseed rape growers, who face a much reduced price and a growing list of potential agronomy pitfalls.


Take a more protectant approach to light leaf spot control and keep a close eye out for verticillium wilt. This is the advice from Frontier national technical manager Stuart Hill, who also believes growers in the Midlands and the South should be paying more attention to light leaf spot resistance ratings.


Light leaf spot levels in this year’s harvest season were at their highest level since 1989, and were significantly higher than the long-term mean.


The disease affected crops in all regions and, according to CropMonitor, was recorded on 87% of crops and 44% of plants.


Light leaf spot is spread pretty much across the whole of the UK and growers outside the traditional high risk northern region need to take this into account, says Mr Hill.


“We saw a significant take-off of light leaf spot this spring. We had good leaf wetness and conditions were conducive to disease development,” he says.


The increase in light leaf spot is probably the result of our changing climate allowing disease recycling and re-infection, says Mr Hill. And tighter rotations are also contributing to increased disease pressure.


He says: “In Scotland, there has always been a criteria of not growing anything with less than a 6 rating for light leaf spot resistance.

“And now we are going to have to take this into account in other regions. We have got some 8 and 9 ratings for phoma, but for light leaf spot it is more challenging.”


Levels of phoma in 2013/14 were also higher than in the previous season, with 88% of crops and 33% of plants affected.

Mr Hill says: “We cannot discount phoma. Phoma levels are still high. The lowest level of infection was 65% in the North, according to CropMonitor.”


Verticillium wilt continues to be a cause for concern for some. This season’s early drilling will increase disease risk and growers which have used farm-saved seed should bear this in mind, says Mr Hill.


“The best time to monitor is before harvest, but there is also a soil test available from NIAB TAG. The potential for infection is there.”


The ban on neonicotinoid seed treatments takes effect on winter oilseed rape for the first time this autumn. While there are no like-for-like alternatives to Cruiser (thiamethoxam), Modesto (clothianidin) and Chinook (imidacloprid), a number of seed treatment options have been available, including Hypro Duet (prochloraz+thiram) and Mesurol (methiocarb).

Phoma protection

Hypro Duet will give crops some protection from early phoma. But seed treated with Mesurol to combat cabbage stem flea beetle will not have received any fungicide seed treatment. This is unlikely to affect autumn disease control programmes, says Mr Hill.


He says: “We don’t think we will need an extra phoma spray where Mesurol has been used, but it depends on the weather.”


For many growers, the early season culminated in an early harvest and so an early start to oilseed rape drilling was inevitable. But what will this mean for disease development?


Mr Hill says: “With phoma you see leaf infection first and lesions move down to the stem in smaller crops. So a big crop is a good thing in terms of phoma as you can do remedial work with a fungicide.


“But with light leaf spot, a big crop means there is more potential for infection as there is a bigger area to become infected.”


The high levels of light leaf spot seen last season raised questions over the continued effectiveness of triazoles against the disease and there have certainly been shifts in the efficacy of some fungicides against the disease, says Mr Hill.


“Certainly we had some reasonable doses put on under high infection pressure and it was difficult to control disease this spring,” he says.

The mutation conferring resistance is the same one which confers resistance to triazoles in septoria, he adds.


In the field this means growers are going to have to take a much more protectant approach to light leaf spot control, Mr Hill suggests.

With prothioconazole underpinning oilseed rape disease control programmes, the launch this summer of DuPont’s SDHI+strobilurin oilseed rape fungicide Refinzar is perhaps a timely one.


Mr Hill says: “When programmes are so dominated by triazoles, it is good to take some of the pressure off.”


Refinzar is a co-formulation of the SDHI penthiopyrad and the strobilurin picoxystrobin; the former is the key to the product’s efficacy on phoma, while the latter is effective on light leaf spot.


Frontier has been looking at using Refinzar as a preventative treatment, applying 0.5 litres/hectare as the first spray in the autumn in September/early October. The second spray would be Prosaro/Corinth (prothioconazole+tebuconazole).


Mr Hill says: “I don’t think I would necessarily be waiting for thresholds as we need to be more protectant. Refinzar will fit in early, in the protectant role. The second spray, heading into November, is where you start looking at the PGR effect.”


Philip Marr, Agrii

Philip Marr, Agrii

LIGHT leaf spot is the main reason for the disappointing performance of many oilseed rape crops across the country this season, according to Agrii oilseed rape specialist Philip Marr.


Mr Marr has found infections causing yield losses of 30-50% in many cases, through a combination of very small seeds and premature pod opening.


He says: “Verticillium is being blamed for disappointing yields by some, but this season has seen some of the lowest levels of verticillium infection in recent years.


“In complete contrast, light leaf spot has become more and more obvious on a national scale as the season has gone on. From widespread leaf spotting as the weather warmed up in the spring, it has progressed into the most severe stem blotching I have ever seen on susceptible varieties, coupled with serious pod lesions in many crops.


“Yield losses have been severe in varieties with poor disease resistance ratings. In addition to small seeds resulting from restricted translocation, as well as photosynthesis during grain fill, pod infections have led to high levels of shedding ahead of combining, even without the sort of storms many have suffered around this year’s harvest.


“One of the most resistant varieties in our Brotherton trials showed hardly any signs of infection and went on to produce more than 5.5 tonnes/hectare. Yet, alongside it, we brought in barely 3.5t/ha from one of the most susceptible which was riddled with stem and pod lesions.”


Under these circumstances, with high levels of light leaf spot incidence recorded across much of the UK and with a far faster apparent decline in light leaf spot sensitivity to triazoles than the shift in septoria sensitivity in wheat, Mr Marr urges growers to take a ‘northern’ approach to future crop management.


This includes insisting on a minimum light leaf spot resistance score of 6 in the varieties grown and autumn spraying to prevent infections becoming established.


“Light leaf spot only becomes evident when the weather warms up in the spring,” he says. “But by then, unless we have decent levels of resistance in our varieties, the triazole shift means we have precious few defences left these days.


“Increasingly, this means we need to be nipping infections in the bud when they take place in the autumn – adding extra light leaf spot activity to our phoma spraying wherever the disease has previously been evident. As an extra protection, we should also be putting more emphasis on growing varieties with high levels of pod shatter resistance.


“More than half the varieties on the current East and West Recommended List have a light leaf spot resistance score of less than 6. 


“These can clearly perform well where the disease is not a problem. However, on the evidence of recent years – and this season’s losses in particular – I consider most to be far too much of a risk for most UK growers; especially with every kg of yield so vital at current crop values and a good selection of varieties with higher levels of resistance so widely available.”


Dr Fiona Burnett, SRUC

Dr Fiona Burnett, SRUC

AFTER a year of difficult to control light leaf spot, growers are probably pondering what to do this autumn, but SRUC’s Dr Fiona Burnett says significant changes are not necessary.


She points to a more stable situation in Scotland where more light leaf spot resistant varieties are grown.


“Control remains challenging, but severity data in monitored crops is no worse now than it was five years ago, despite the variable azole sensitivity we have noted in HGCA work as far back as 2003,” she says.


So why the problem south of the border? According to Dr Burnett it is down to a combination of factors.


She says: “The weather has certainly been more favourable to the disease in recent seasons and with the intensity of rape grown, any trash-borne disease is bound to benefit. This is not helped by agronomic factors. A focus on phoma means fungicide timing and product choice can also be a factor.”


With both phoma and light leaf spot a significant threat in many English counties, and growers unwilling or unable to make a third autumn pass, agronomic practice needs tweaking, she adds. Control comes down to variety choice and the effective use of fungicides.


Dr Burnett says: “The one thing we have learnt in Scotland is the importance of varietal resistance. Oilseed rape remains the most profitable break crop, so rotation intensity or cultivation strategies are unlikely to change, but which varieties you put in the ground can.


“In Scotland, our general rule is to look for a minimum of a 6 RL light leaf spot score. English growers need to factor in RL scores when selecting varieties – it might be less straight-forward but any loss of performance is likely to covered with better control of the disease.


“In addition, certain fungicides which are effective against phoma offer little activity against light leaf spot.


“In cereals, we are relying on two primary azoles in mixtures, yet in oilseed rape, we are relying on two azoles as straights or as a combination. Growers are heeding the advice to use at a minimum of a three-quarter rate in wheat, even when combined with an SDHI to get control of septoria, and we need to consider raising doses in oilseed rape as well.


“When it comes to autumn fungicide applications, split phoma treatments are typically too early for light leaf spot, so the ability to delay applications to late autumn is useful. Late October to mid-November is standard and there is a compromise to be made if land is likely to become unpassable much into November.


“You need to stay in a protective situation, and ideally see the crop through until you can get back in after winter. A third pass is probably impractical, so the only real option growers have is to manipulate dose, and in my view you just cannot get away with a low rate any more, regardless of when the application is made.”


Dr Burnett hopes in time better forecasting will help, and highlights an HGCA-funded project which aims to do just that. “This research should help us identify the timing of epidemics as well as the risk. We can then pass this on to growers which will help them target application timing more effectively and remain in a protective situation.”

Patrick Stephenson, independent agronomist

THE loss of flusilazole from this autumn – existing stocks must be used up by October 12, 2014 – means further pressure on prothioconazole, but independent agronomist Patrick Stephenson is not too concerned.


He says: “We are possibly seeing the beginning of a loss of activity against light leaf spot, but we are still at the top of the slide so there is no need to change current strategies.”


For Mr Stephenson, those strategies remain building autumn disease control around natural resistance combined with a prothioconazole-based programme.


He says: “The simple fact is with the loss of flusilazole there is not a great deal of choice. Difenoconazole offers little light leaf spot activity, tebuconazole’s PGR activity makes it more appropriate for the spring in most seasons, and in oilseed rape, the new SDHI options offer no additional activity over robust rates of Proline. There is no point in adding another mode of action which offers nothing in product stewardship and only increases the cost of your programme.”


According to Mr Stephenson, the only question is one or two passes? And he prefers two.


He says: “The problem is persistence. Light leaf spot cycles quickly and when you don’t get a real winter you enter the spring with an infected crop. Spring fungicides are economic, but only protect crops from new infection, so the damage is done.


“A high rate of prothioconazole will give you a degree of persistence, but when do you go? In high risk situations my view is once is risky whereas two sprays give a better spread of protection against phoma and light leaf spot and will help see a crop through the winter. Whatever route you take though, dose and timing is critical.”

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