A firm sense of proportion needs to be kept in oilseed rape disease management thinking
A firm sense of proportion needs to be kept in oilseed rape disease management thinking, believe leading agronomists and advisers, focusing preventative attention on the pathogens that cause the greatest damage to crops in the UK.HGCA Crop Monitor calculations suggest oilseed rape disease is responsible for average UK losses of just under 200 million per year at current rapeseed values. What’s more, at constant crop values they indicate annual losses have been growing steadily in recent years – from less than 150 million in 2005 to over 220 million in 2010.
While the balance between the losses caused by the different OSR diseases varies from year to year, phoma stem canker and light leaf spot are, between them, estimated to be responsible for 95 per cent of the average annual disease costs, with sclerotinia accounting for almost all the remainder (see Figure 1).Figure 1: Average Annual OSR Disease Costs 2005-2010 – Source: Defra-funded winter OSR disease and pest survey data delivered through HGCA Crop Monitor with rapeseed at 380/t.Against this background, Oxfordshire-based Agrii agronomist Iain Richards finds it concerning that verticillium has come to dominate the OSR disease headlines so comprehensively in recent years.
“Yes, we can often see signs of verticillium in OSR stubbles when we look for them,” he says. “It can cause considerable damage in some very close rotations but we’ve never seen any real impact from the disease, probably because we steer away from growing rape more frequently than one year in every three or four.
“While it makes sense to be conscious of the possibility of verticillium build-up in soils, we must not let this distract us from the threats that really matter – phoma and light leaf spot, and to a lesser extent sclerotinia and alternaria. For instance, no one in southern England should ever grow a variety with a low stem canker resistance rating just because it may have shown less signs of verticillium in the very few trials done to date.”It’s all a matter of getting our priorities right. Especially so since much of the ‘verticillium’ actually seen in many crops is, I suspect, actually mis-diagnosed phoma. And phoma looks set to become even more of a threat as average temperatures increase with climate change.”Major continental losses to stem canker in 2010/11, despite few signs of leaf infection, leave Dekalb North West Europe technical manager, Kuldip Mudhar acutely conscious of the need for particular vigilance on the phoma front.
He explains that a combination of high levels of autumn inoculum and small, backward plants that year meant a large number of susceptible crops suffered serious losses from widespread cankering at late-flowering. In Danish farm trials, for instance, the average yield of varieties with a phoma stem canker resistance of 5.0 or less was a good 15 per cent below that of varieties with a rating of 8.0 or more.”This underlines just how much stem canker damage a small amount of phoma can cause susceptible varieties,” he stresses. “The fact that over half the varieties on last year’s Recommended List had phoma stem canker resistance ratings of less than 5.0 is very worrying in this respect. All the more so given the extent to which this season’s spray programmes have been restricted by the weather and ground conditions.”Although early cankering and stem collapse are responsible for the greatest losses, Mr Mudhar explains that much of the damage from phoma results from fungal growth compromising water and nutrient uptake. This means that even later-developing and less obvious cankering is likely to have a significant impact on performance.
“Yield losses from phoma that result in early crop senescence, poor pod fill or both, frequently go undiagnosed,” he observes. “Either that or they are attributed to other diseases or climatic factors.”Danish growers certainly had a major wake-up call in 2011/12. Indeed, it led the proportion of plantings in highly resistant varieties to grow from less than 30 per cent in 2011/12 to almost 70 per cent this season.”Unlike other major phoma stem canker resistance genes, RLM7 is proving remarkably durable across Europe despite widespread use. Combined with high levels of polygenic resistance – so-called double resistance – to slow fungal spread and restrict stem canker-forming ability, this provides the best available insurance against limited winter spraying opportunities and the greatest possible flexibility in fungicide timing and choice.Light leaf spot threat
Alongside phoma stem canker, ADAS pathologist, Dr Peter Gladders (left) insists that light leaf spot must be the key focus of modern OSR disease prevention programmes. And, increasingly, not just in the north either.”Light leaf spot is a real threat across the country these days,” he points out. “Preliminary Crop Monitor forecasts this season put the percentage of crops with more than 25 per cent of plants affected at the highest levels ever in most UK regions. True, the forecast remains higher for northern England and Scotland. But over 55 per cent of crops are likely to see significant infection levels across the Midlands, Wales and the south and west. Even in the south east and East Anglia, around 30 per cent of crops are predicted to be significantly affected.”In recent years annual UK losses from the disease are estimated to have been higher than those from phoma stem canker, making varietal resistance at least as important in countering the threat for many.