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Q&A: Base OSR spray decisions on in-field evidence


With oilseed rape crops emerging, Farmers Guardian and Bayer CropScience convened the second in an OSR roundtable discussion series to assess autumn disease control priorities.

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There was a move from the east to the west of England for the second oilseed rape roundtable discussion, with Bridgnorth, Shropshire farmer Andrew Williamson providing the venue for the gathering of agronomy experts. Another of Bayer’s small group of InVigor Developers – farmers hosting ‘real world’ OSR trials for the firm looking at the effect of different input variables – Mr Williamson’s strip trials are comparing different fungicide strategies on strips of Harper, Incentive and DK Extrovert. He grows 72 hectares (180 acres) of oilseed rape on his 320ha (800 acres) cropped area.


Protecting crops from autumn disease attack was the meeting’s theme.


Q:Why has turnip yellows virus become prominent again and what can be done about the threat posed to OSR crops by aphids carrying it?


Mark Stevens: Early infection in September-October can reduce oilseed rape yields by up to a maximum of 30 per cent, while in on-farm situations where aphids are migrating in from the wild, up to 100 per cent infection can be possible in ‘hot spot’ areas when aphid numbers are high, with yield impacts of at least 10-15 per cent.


The virus relies totally on aphid transmission, with the peach-potato aphid, Myzus persicae, the most important vector.


Previous research has identified hotspot areas depending on the season, though, and if conditions are conducive to flying then aphids can invade crops as late as November-December. Monitoring is crucial to support the industry regarding the decision to spray. An aphid threshold does not exist, so keeping an eye on the weekly AHDB bulletins is essential. If you have sprayed once, a second application may be necessary if later invasions have taken place. But it is early infections which do the most damage.


It can be difficult to stop both primary and potentially secondary infection – neonicotinoid seed treatments were a relatively simple management tool to stop population build-up and virus spread, with the option to apply an October/November follow-up insecticide if necessary, doing a good job of protection from drilling through the crop’s first six weeks. With the foliar products remaining, application must only be made if peach-potato aphid presence is confirmed. Alongside Plenum (pymetrozine) and Teppeki (flonicamid), Bayer has applied for autumn use approval for Biscaya (thiacloprid). This option would provide rapid kill of aphids, lending itself to early application if a two or three spray programme is likely to be required.


At this time of year, the aphids themselves are not doing much noticeable damage to the crop, but the reddening and purpling which can be seen come February/March is the first sign of virus infection. It can be easily mistaken for stress caused by cold, waterlogging, compaction or nutrient deficiency.


Current mild September conditions will favour aphid development and build-up and there are the first signs of renewed activity.


If aphids are found, the problem is there is no recognised threshold for OSR – that is something requiring more research. My advice would be do not spray until you find wingless aphids on plants.


Antony Wade: With the protection of neonicotinoids, we have not had to be too concerned about the impact of TuYV in oilseed rape, but now they are gone I am watching aphid alerts more closely. Populations could well increase from the mild weather we have had during September. Growers will need to prioritise treatments according to the main threat to their crops.


Q: What is the outlook this autumn for phoma and light leaf spot?


Antony Wade: Unlike some of the UK, on this side of the country we had a fairly wet August, and although we are in a dry period now which may delay it, that means there is an increased phoma risk. Earlier-drilled – and therefore larger – plants can counter this, but here we tend to be drilling later in August. Early infection of phoma is more detrimental, as smaller plants with smaller leaves are more at risk of the disease travelling down leaf petioles and causing stem cankers. Factoring in phoma warnings and rainfall is important. Big, strong crops can cope with early infection much better and are at lower risk of damaging stem cankers forming.


Gareth Bubb: I have never seen as much light leaf spot as I did in plants and on pods last season, so there is a pool of disease inoculum from last year’s stubbles. Unless you see it before November, then phoma should be your priority. By November, if plants are big and healthy, then phoma should not be a worry any further. Light leaf spot needs to be the focus from then, aiming to keep it off the developing pods down the line.


Antony Wade: I would agree. At least some of the disappointing harvest 2014 OSR yields in this area could be attributed to light leaf spot (LLS) infection. Previous crop residues produce ascospores which travel on the wind and are then spread by rain splash within the crop, so if it stays dry the risk may fall. But LLS is far more difficult than phoma to spot early. If symptoms are identified, a mealy scorch-like watery mark with a pinkish white centre – an almost imperceptible white halo of spores – will be the prime evidence if leaves are incubated in a plastic bag in a warm environment for diagnosis, but by then the disease will have been there for a while. Typically, full lesions do not show until winter or early spring, but protectant treatment before Christmas is essential for control.


My strategy would be to use an early spray on phoma-susceptible varieties, but only at first evidence of the disease reaching threshold, with focus on small or backward crops, with an aphicide if necessary. That would be later followed with an LLS spray. Varieties stronger on phoma, and particularly those which also develop big, strong plants, will simply get an LLS-targeted spray in November.


Gareth Bubb: LLS needs to be treated like septoria, with preventative action, before it gets out of control. By the time lesions are

identifiable in early spring, it is too late. Flusilazole acts on both LLS and phoma, and was one of the standards for phoma control, but its recent revocation means it can be used only up to October 12 this year, and then it is gone from the market. While it is good on phoma, Plover (difenoconazole) does not have LLS activity. The reverse is true of Folicur (tebuconazole), though. Proline (prothioconazole) acts on both, but remember it has no growth regulatory effect, and always bear in mind the critical autumn application timings for each disease are different.


The problem this autumn is both diseases are prominent, and LLS is pretty widespread across the UK. Spraying from November is critical to control, but LLS has long been regarded as a more severe problem in the North because it is more difficult to travel on the land from October to March/April, making spraying more difficult between autumn and spring, while disease development continues even when lower temperatures mean crop growth has slowed.


Look at your varieties and identify those at risk. Some will need a focus on both diseases, where others such as Harper score 9 for phoma, mean growers may get away with one early to mid-November fungicide targeted at LLS, for which it is rated 5.


Anyone who chooses to make two autumn sprays would previously have been advised to apply two half doses, but a half dose in November will be insufficient to carry through LLS protection to the next likely opportunity early in the new year. It must be a three-quarter dose to last through to a January-February timing, when topping up will be advisable if travel is possible. Then, tracking back from that November LLS timing to earlier in the autumn, if 10-20 per cent phoma lesions have been recorded then treat with a half-dose of prothioconazole.


Q: Given the potential cost complexity of these strategies, and the additional issues raised by the neonicotinoid ban, what is the future for oilseed rape as the key UK break crop?


Andrew Williamson: I will admit to being a bit concerned at the number of passes we could be making. And at £250/tonne, with the inputs required and yields having plateaued over the past 10 years, it is becoming borderline profitable. But perhaps we should be paying greater attention to crop variables pre-Christmas to protect the building of potential. The last couple of years have been a good illustration of how crop appearance in April/May is a good indicator of final performance.


Gareth Bubb: Few other break crops can provide the early harvest entry OSR does, and cutting inputs is not the answer to boosting profitability. The key to limiting passes is to keep ahead and act protectively rather than curatively, but spraying just because you can get on is no good. Correct timing is critical. As soon as you have made a decision, for example, to go on with an early phoma spray, which will not do light leaf spot at that timing, then applying an LLS follow up four-six weeks later is essential, even if it is not visible. If it is not done, you will spend the same amount of money trying to control LLS curatively, and not succeeding. In terms of combining disease and weed control tasks, Proline can be tank-mixed with propyzamide, but not carbetamide.


What are the InVigor on-farm trials you are conducting here this year?


Antony Wade: We have some Harper, Incentive and DK Extrovert strips, each in the field centre, with the headlands down to Harper. All were established off the back of a subsoiler. Because Harper is strong on phoma, we are going to trial a one-spray/two-spray fungicide strategy and assess disease development and yield. Incentive is weak on phoma, Harper is stronger, and Extrovert is good on both diseases.


Round the table

  • Andrew Williamson, farmer and host
  • Antony Wade, independent agronomist
  • Gareth Bubb, regional manager, Bayer CropScience
  • Mark Stevens, BBRO lead scientist and TuYV/aphid control expert
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