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Rising to the light leaf spot challenge

Since 2008 light leaf spot (LLS) has replaced phoma as the principal OSR disease threat and losses can be devastating. In 2012 and 2014 CropMonitor data suggests LLS cost UK growers upwards of £140m. Whether it is septoria or LLS that is most difficult to control is open to debate but what isn’t is the similarities between both. Both are polycyclic and cycle in a wide range of temperatures, neither is killed off by a cold snap and once established there isn’t really any substantial curative option. ADAS plant pathologist Julie Smith (right) likens the disease to septoria and says the latent phase can be variable depending on the strain and the resistance rating of the variety. “The disease is driven primarily by temperature and rainfall. From infection to symptoms you’re looking at approximately 270 day degrees in a susceptible variety which means that the latent phase can be as short as four weeks if mean

daily temperatures are 10ºC. If the weather cools it can be double this.” She says growers have no choice but to stay ahead of it. Once LLS gets established there’s no real going back. “Being polycyclic it will continue to cycle under the right conditions. And that is what we’ve had of late – cooler summers and warmer wetter winters. With this more conducive climate plus the frequency of OSR in rotations and the move to non-inversion tillage, this has only benefitted this trash borne disease,” she warns.

Under attack Part of that prevalence might also be down to the fact that ascospores are being released from as early as the start of August. “These ascospores can travel significant distances in the right conditions and are being blown around right through the drilling and establishment phases, meaning that most crops are probably bound to be at risk,” notes Bayer’s Tim Nicholson (right).

Mr Nicholson shares the view that LLS is OSR’s ‘septoria’ and feels growers should treat it accordingly. “When you consider that ascospores could be flying around from August onwards I think you have to consider the disease will be there. If you wait until you can see it then it could be too late, especially if the disease has got to the growing point. As the plant grows so the disease will with it. We have to take a prophylactic attitude with septoria – LLS is no different.” And it is rainsplash that is at the core of another problem in dealing with LLS. Following on from the ascospore blitz and the start of the asexual phase, winter weather can often disrupt any attempt to spray the crop until the spring. The series of recent mild winters has seen the disease continue to cycle and mild wet weather compounds matters by making travel conditions tricky. As a result many agronomists have noted extensive LLS as crops come out of winter. Philip Wright of Wright Resolutions believes that LLS will remain a problem with current cultivation strategies leaving so much trash around. But even with the propensity for min-till there are measures growers can take to maximise travel opportunities up to stem extension. He recommends growers consider Controlled Traffic Farming (CTF) providing them with the opportunity to travel sooner. “Generally CTF produces more resilient

tramlines,” he notes. To assist travel post winter Mr Wright feels it is worth taking the trouble to optimise drainage. To do this he suggests loosening the tramline from the side rather than the centre. “Loosening soils at the centre can encourage water to sit beneath the tramline. Subsequent trafficking will then only lead to deeper ruts and compaction.” He stresses the need to reduce axle and tyre pressure loads and welcomes the introduction of flexible tyre carcase technology (VF) but he urges growers to ‘think smart’ to beat the disease. “LLS can devastate a crop so probably better to get into the crop even if it means some inconvenience. Half filling the tank might allow you to travel – the increased workload might be the better option than letting the disease take hold,” he suggests.

Mind the gap That gap to stem extension is why Bayer’s Tim Nicholson urges growers to only employ the most robust agronomy. For him that is a robust autumn dose of an effective product such as Proline275 (prothioconazole) and preferably two applications. “If you’re going into the crop with a phoma treatment then why not use an active that is effective against both diseases. We’ve seen in the past wet November weather hamper travel opportunities or ground temperatures delay combined applications with propyzamide. “This ‘pre-emptive’ strike buys some flexibility with the second timing. It’s important as there’s no curative option and if the disease continues to cycle you could have plants riddled with LLS come stem extension.” ■ 10R28B

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