Backward oilseed rape at risk from late phoma

Oilseed rape establishment has been variable – to say the least – in many parts of the country and backward crops will be especially vulnerable from late phoma attacks, Peter Gladders from ADAS Boxworth has warned.


“If you haven’t sprayed already, you should be looking very hard at doing so. Late leaf spotting on very small plants is potentially very serious and growers should definitely be looking at two sprays this season.”


Many growers had already sprayed for early leaf spotting in September or October – generally the most damaging to yield – but small plants could be devastated if phoma got in later, he said.


“The economic optimum is normally two sprays, but it’s vital to take phoma out before Christmas. Spraying into February or March is wasting money, as canker is already established.”


Trials at ADAS’s Boxworth site in Cambridgeshire between 1997 and 2007 found an average yield response of 0.3t/ha from two autumn fungicides – with little to choose between products (see panel), Dr Gladders noted. “At current prices, that’s worth almost £100/ha for an investment of around £20.”


Very cold weather would slow fungal growth in the crop, although with the trend towards milder autumns and winters, crops were likely to be at increasing risk in future years, he added. “Under the worst case scenario, the disease onset phase could be 10-15 days earlier by 2050. But perhaps more significantly, milder winters will give earlier canker onset – possibly up to 80 days earlier. Canker will also be pushed further north.”


 






What product to use?


 


Still deciding what product to use for phoma control? The HGCA Appropriate Fungicide dose project for phoma and light leaf spot found relatively small differences between the main products. Instead, spray timing was likely to be more important than product or dose rate.


Two-spray programmes at half dose performed well, although results found that higher doses could be worthwhile on small plants where infection was above the 20% threshold.


Go to http://www.hgca.com then under Publications, search for “New fungicides for oilseed rape”.

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