Arable News

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Crystal clear on stem canker warning

While colder winter conditions and better plant resistance have helped cut recent phoma stem canker risks, oilseed rape growers are being advised

While colder winter conditions and better plant resistance have helped cut recent phoma stem canker risks, oilseed rape growers are being advised, for the future, to maintain their guard against a disease that can cost the UK over 150m a year.According to Rothamsted Research pathologist, Jon West (left), all the signs are that with climate change leading to warmer conditions pre-harvest, stem canker will become more important in future years.”While we’ve seen a run of cold winters helping to knock stem canker back a bit, this could just be a temporary blip; the disease remains a major issue in the south and could become more important in the long run given climate change predictions,” he says.
“With climate change likely to create greater carry-over and potential infection in the following season, we need plant breeders to develop varieties with better resistance for a pathogen that until the last couple of years was the number one disease for decades.”

Dr West says that the disease occurs when ascospores of leptosphaeria maculans from crop residues land on leaves and get into the veins. It is then transferred from the petiole to the stem, where damaging stem cankers can cause early senescence and yield loss.
“The degree of severity depends on how bad the phoma stem canker has been the year before.  Spread is worse in a wet, mild summer with spores released earlier in the autumn leading to an earlier cycling of the disease in the following year’s crops.

“In a mild autumn with a well-established big crop, the first leaves drop off the plant and the early infections rarely make it to the stem. In these crops it is important to protect leaves 6-8 as these will survive through the autumn and winter allowing the fungus into the stem in untreated plots. So, if you’ve got a nice vigorous crop one spray may be all that is needed,” he suggests.

“However, in backward, small crops, we’d advocate spraying before you see the phoma leaf spotting and in these crops two sprays may be needed to protect the leaf and prevent spread to the stem.” Either way, timing decisions each season can be helped by using a disease forecasting tool available on the Rothamsted Research web site.At the same time, though, growers also need to take into account other diseases, particularly light leaf spot, and ensure good protection against these, warns Dr West. For example, an additional late winter spray would have helped where light leaf spot was forecast.While the development of canker and the severity of the disease vary – those growing a susceptible variety and not using fungicides can see yields cut by 50 per cent. “So you do need good background resistance and a good fungicide programme.”Dr West says that varieties with the best resistance are those that have a wide range of genetic defence mechanisms within the plant. In essence, in varieties with major gene resistance, the plant recognises that the disease is there, it kills off a few cells – visible as a black speck – but the resistance then ‘walls-off’ and stops the fungus.In varieties that have multi-gene or polygenic resistance the fungus is recognised by the plant leading to some phoma spotting, but the plant then produces chemicals that slow the fungal growth.   These chemicals operate in the tissue including the leaf, petiole and stem, reducing the disease impact.Dr West points out that varieties with polygenic resistance are much more durable – offering belt and braces protection – as a range of resistance factors come in to play. “In contrast, resistance breakdown in varieties with one major gene can be very quick as there’s just one resistance mechanism at play,” he says. Better durability
KWS UK oilseed rape product manager, Tom Dummett (left) confirms that the newly recommended conventional oilseed rape Quartz has multi-gene resistance, potentially offering the better durability Dr West describes.”We know it has the major RLM 4 and RLM 9 genes plus other quantitative resistance mechanisms and this is reflected in Quartz’s highest possible 9-rating for resistance to the disease on the 2013/14 UK Recommended List,” says Tom.

KWS product development manager, John Miles suggests that as a result of its strong phoma stem canker resistance, Quartz will be particularly beneficial to those with larger areas of oilseed rape or who are growing it in closer rotations.

He confirmed that as a result of its polygenic resistance mechanism, growers may still see some phoma spotting in the autumn in Quartz, but this is unlikely to lead to stem canker. “Senescence occurs before this infection works its way down to the stem,” he explains.

Mr Miles did stress that autumn fungicide treatments still have a part to play on Quartz; it is all dependent on the size of the crop but protecting those leaves – normally leaves 6-8 – that will go through the winter, is the key.  Rothamsted Research’s prediction tool has been spot on at our trial site for a number of years, providing accurate information on likely phoma spot occurrence.

Early infection has a good chance of growing down the petiole if the season suits it so the timing of the first spray is more critical on susceptible varieties. Over three years of trials we have seen significantly less leaf spotting and cankers in Quartz when the first spray timing is missed.

Later spray timings can then be used to back this up and protect the over wintering leaves or be more specifically targeted at later diseases such as light leaf spot. 

“In trials with ADAS Terrington last year, Quartz was 0.25t/ha higher yielding than a stem canker susceptible variety in untreated plots. But, given a full three spray programme with Proline (prothioconazole), yields increased by an additional 0.3t/ha in what was a low infection season.

“The key thing with Quartz is the flexibility it brings on farm,” he says.  “The timing of autumn infections can vary every year, the spray windows aren’t fixed so a variety with strong stem canker resistance can be left until last.

“In addition, we now no longer have the strong north/south disease divide with oilseed rape diseases. So, with light leaf spot an increasing issue further south, those with Quartz can be more relaxed about phoma control, focusing their attention on the usually later November timed LLS spray.

“Quartz growers will also find they can reduce canker infections going forward, by leaving less disease on the rape stubble and trash they are left with after harvest,” he says.

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