Despite September 2016 being warm growers are being advised to check OSR crops for phoma. The ‘Indian summer’ has seen much of the country bask in warm sunshine but there has still been sufficient rain events to fuel the disease according to ADAS plant pathologist Julie Smith.
Some areas reached the 20 day rain threshold by the middle of last month, she says. “Despite the perception that it’s been a dry autumn I think there is the possibility of an early epidemic in some parts of the country. September has been warm – mean daily temps of about 15°C at our Herefordshire site – but there’s been enough rain too,” she says.
She expects symptoms to be visible in some crops very soon. It only takes 120 degree days for leaf spot symptoms to appear after ascospore infection given the warm weather that period could be as short as seven calendar days. “At the Herefordshire site we have a plot of PT211 that was drilled on 31 Aug and is now showing phoma leaf spots on the cotyledons.”
Yield losses on a susceptible variety can typically be in the region of 0.5t/ha with more possible under greater disease pressure, or in small crops which are particularly vulnerable. “Small crops are most at risk because early leaf infection leads to early stem cankers and this is the most damaging to yield. If growers suspect the disease is present an early treatment is advisable,” she suggests.
Bayer’s Tim Nicholson suggests that growers can start their light leaf spot (LLS) programme at the same time by using an active effective against both diseases such as prothioconazole (Proline).
A two-spray strategy is often best given the threat of both phoma and LLS, he explains, suggesting a single spray strategy carries a high element of risk. “Recent research has shown LLS ascospores can threaten crops from the moment they come out of the ground and right through the establishment phase as farmers undertake winter wheat cultivations, disturbing OSR trash. And with phoma, if the first spray is required before the end of October a follow up spray will be required within 4-6 weeks.
“For a single spray to be able to protect against both diseases you need a variety that has an RL rating of at least 7 for Phoma and a good degree of LLS resistance too. A robust rate of active against both diseases may be required,” he warns.