SPAWS spots Phoma coming
Oilseed rape growers must be on the look out for Phoma leaf spotting over coming days, with rapidly increasing numbers of aerial spores being picked up by SPAWS (Syngenta Phoma Advance Warning System). In mild conditions growers have seven to 10 days respite, before Phoma Leaf Spotting breaks out from spore infection.
Treatment with Plover (difenoconazole) will cure existing Phoma leaf spots, and prevent further infection for up to six weeks. ADAS pathologist, Dr Peter Gladders, who operates SPAWS for Syngenta, warns spray thresholds in eastern counties are likely to be reached in more crops in early to mid-October.
"With increasing spore production over the last few days, and more rain forecast, Phoma leaf spotting is expected to keep increasing," advises Dr Gladders. With early infection small plants are most at risk of disease spreading rapidly from leaf spots to infect stems, and causing cankers next spring.
Spraying thresholds should be reduced on small plants - including late sown and re-drilled crops - and varieties with low Phoma resistance. "With this level of spore production being picked up by SPAWS, it is imperative growers monitor crops carefully, particularly those close to last year's oilseed rape stubbles," adds Dr Gladders. Some Phoma leaf spotting has already been reported in high risk crops across Lincolnshire and the eastern counties.
In the west, where rainfall has been lower, the SPAWS traps at ADAS Rosemaund have yet to register any spore activity.
Dr Gladders advises growers to continue to follow the developing Phoma situation using the SPAWS web site, which is free to growers using Plover for control of Phoma and Light Leaf Spot this autumn. Growers can register fro SPAWS through the Syngenta web site: www.syngenta-crop.co.uk
Phoma resistance falling
Don't be lured into a false sense of security with the promise of Phoma resistant varieties, warns leading disease expert Dr Jon West of Rothamsted Research. Vatietal resistance can slow down spread of the disease and provide useful flexibility in fungicide application timing. but different races of Phoma will inevitably overcome most major resistance genes, he says.
Under typical eastern counties disease pressure, Dr West believes that most resistance genes may be effectively overcome within two to three seasons. "Early action could avoid infection reaching the stem and reduce the impact of resistance breakdown, which makes it all the more important to control," he advises.
Syngenta Oilseed Rape Technical Manager, Beth Hall, highlights varieties with lower resistance ratings will still continue to yield just as well, but the contribution from a Plover fungicide application will be even more important – ADAS has calculated fungicides typically give an additional 0.1t/ha yield response per resistance rating point, worth an extra £13.50/ha.
With continued unsettled weather Mrs Hall advises growers to study weather forecasts when assessing treatment thresholds. "Phoma leaf spotting can move from low threshold to pandemic levels in a week or less. If the forecast suggests spraying may be difficult, treat at earlier signs of infection to avoid the risk of a severe Phoma attack.
For small plants thresholds should be reduced down to 10% of plants infected, she recommends. "Prioritise treatments on crops showing the greatest levels of leaf spotting, plants with the smallest leaves and the most susceptible varieties," adds Mrs Hall.
