COLD AND frosty conditions will delay phoma development in oilseed rape and give growers time to treat susceptible crops, Syngenta has advised.

Prior to the start of the latest cold spell, very large numbers of spores were found in eastern counties during early to mid-November, according to the Syngenta Phoma Advance Warning System (SPAWS).

Most crops were found to be above the 10-20% of plants affected threshold and some crops had over 90% of plants showing signs of the disease, said the firm’s technical manager, Iain Hamilton.

“Cold weather will temporarily slow the development of phoma lesions, which gives growers time to get disease back in control.”

But thick, leafy crops could create a warmer microclimate under the canopy and allow the disease to develop, he said. “Wet leaf surfaces encourage phoma movement down to the stem, where cankers will grow later.”

Later sown small to moderately sized plants and more canker susceptible varieties are in greatest need of protection, he said.

North Yorkshire agronomist, Patrick Stephenson agreed. “Last year we saw some damage on those types of crops [small, backward]. At least, because the mild weather has encouraged those crops to fill out a bit, it is easier to justify spraying them.”

Mr Hamilton suggests using Plover (difenoconazole), which also provides activity against Light Leaf Spot through to mid-December.

For more on why the cold weather could remove the need for a second phoma spray this autumn, see Farmers Weekly (25 November).