Arable News

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Learning the lessons of light leaf spot

Light leaf spot in oilseed rape appears to be getting a greater foothold in the southern half of England

Light leaf spot in oilseed rape appears to be getting a greater foothold in the southern half of England as each season goes by. Dominic Kilburn seeks advice as to why this might be happening.
OSR needs protecting against light leaf spot rather than trying to cure it after infection, say advisers.
There’s no doubt that oilseed rape disease light leaf spot (LLS) is gradually migrating south and, while there’s no concrete evidence as to why this is happening, it could be that growers in the south of the country simply haven’t focused on combating it.
That’s according to Indigro agronomist Damian McAuley, based in Northamptonshire, who reckons there have been increased levels of light leaf spot coming into crops in the region in recent years, and particularly so in the past two. “My feeling is that, and this is generally speaking for the south of England, we haven’t traditionally suffered with high infestations in crops and so we haven’t always concentrated on it as a priority for control,” commented Damian.
“I don’t think the spread of LLS is down to the weather,” he continued. “There’s nothing definite as to why it is moving further south but it has been relatively mild in recent years and LLS is a disease that likes the cold.”
He suggests that varietal tolerance is probably one of the key reasons for its continued spread – with variety choice in the south (from the East/West List) predominantly based on a good tolerance for phoma, as well as good yielding characteristics and vigour.
Varieties grown in the north however, and selected from the North List, have a far better tolerance to the disease. “Most of the varieties we grow in the south have a mid-range tolerance to LLS as the options for high tolerance are limited,” he suggests.
In addition, Damian says that LLS is not a main consideration of the usual OSR autumn fungicide programme, and therefore many crops may not be receiving the early protection that they might need at the time. “Growers have started to spray for it in the spring in the past few years as it has become more obvious at that timing, but I would suggest more attention needs to be paid to LLS in the autumn treatments. It’s a disease that needs to be protected against rather than trying to cure it after infection.”
He says that the increase in the amount of min-till and direct drilling on farm has meant more trash and stubble remaining on the surface between crops, providing an environment to harbour disease inoculum, adding that tighter rotations will also have played a part.
“Prothioconazole is very strong on LLS, as well as providing good phoma control, but tebuconazole is a cheaper option now that we have lost flusilazole,” he points out.
“There’s also the new product from DuPont, Refinzar (penthiopyrad + picoxystrobin) where the picoxystrobin element of the product has worked well against LLS,” adds Damian.
“Phoma of course remains our priority in the autumn early on but, if we are to learn lessons from those in the north, we need to include something that will protect against late autumn infection of LLS before we see the symptoms in the spring.”

Variety focus
Sam Clarke (left) advises in north Oxfordshire for farming and crop consultancy business Clarke Farming Partnership Ltd. Like Damian, he says he has seen a big rise in the incidence of LLS in the past two years and so variety choice for him each season will take into account RL ratings against the disease.
“We grow HOLL rape variety V316OL, which has a LLS rating of ‘7’, as well as hybrids DK Extrovert and Incentive, which have a rating of 7 and 6 respectively.
“However we also grow conventional variety Charger, which has a lower rating of 4, so at times it is a compromise of yield versus disease resistance,” he adds.
Sam reckons that unlike diseases such as septoria, where over-wintering infections on the lower leaves in wheat don’t present much of a problem, LLS can cycle at much lower temperatures in the winter with spores self-perpetuating.
“It’s really important to keep in front of LLS and not to let it in the door,” he continues. “Sometimes we hear of LLS and phoma affecting crops in neighbouring counties while there are no signs of them with us, so it’s all about monitoring your own crops and using the forecasting tools available to ensure that you are ready to act if necessary.”
He notes that some azole products and co-formulations control both phoma and LLS but it’s down to the individual season and disease pressure as to what programme of products is used. “All my fungicide programmes are written as to what is in front of me at the time. You need a list of options ready to deal with the two diseases – prothioconazole is one option for LLS and probably difenoconazole or tebuconazole for phoma leaf spot, with SDHI + strobilurin mixes also effective on both diseases. Budget for one LLS spray in the autumn and make a decision on a second if it’s required.”

Phoma still key
Phoma is still the greatest threat out of two of the key OSR diseases by a considerable margin, adds Nottinghamshire-based CJS Agronomy adviser, Christina Scarborough.
“Incidences of LLS are relatively few by comparison although we know that some inoculum is there as it sometimes shows itself as late as May, as it did last season.
“I think it was particularly cold last May which might have brought the disease on,” she suggests.
Christina points out that, by that stage of the season, there is little to be done about LLS incidence in the crop, however, if it’s deemed worthwhile, there is still the opportunity to add another fungicide in the sclerotinia tank mix if the crop is being sprayed anyway.
She says that as several available fungicides used to control phoma are co-formulations, rather than straights, often they already have a protective effect on LLS and therefore many growers in the region are spraying for phoma in the autumn and inadvertently protecting crops against LLS.
“It’s a disease that hasn’t got a foothold in this area but we are certainly on our guard. We can see it coming into the crop late in the season and so we are on the look out for it more now in the autumn too.”

Trial damage
Certainly the damage that LLS can cause was very evident at Bayer CropScience trial site, Thorney, Cambridgeshire earlier this year. With 18 varieties in the trial it represented commercial choices for the 2014/15 season, and, with LLS the most prevalent disease, the variation in LLS susceptibility.
Site manager Darren Adkins says it is easy to see why LLS cost the industry #140m in 2014. “When we undertook Green Area Index (GAI) assessments on 26th March it revealed some of the more susceptible varieties with GAI of just over 0.5, compared with some of the more resistant varieties being in excess of 1.5,” he notes.
“As you would expect, the varieties with the higher GAI scores were those with better resistance to the disease, but these differences were quite marked, especially when backed with an effective fungicide programme.
“Even varieties with RL ratings of ‘6’ or more responded well to a fungicide programme aimed at controlling phoma and LLS. Ours was two applications of Proline275 (prothioconazole) where a 0.32-litres/ha dose was followed by a second at 0.46-litres/ha. For varieties such as Harper, Trinity, Incentive and Popular it doubled GAI area – and that was in a season where the weather did provide some winter check to LLS.”


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