Be on the look out for the early appearance of phoma leaf spot in oilseed rape crops, Rothamsted Research’s Jon West has warned growers.

“It is looking like an early start to the phoma leaf spot phase, with a high risk of infections on very early leaves and even cotyledons,” he said.

“A few places, such as parts of Lincolnshire, seem to have escaped the worst of the August and September rain so they may not have such a problem but generally the risk is high.

“The key thing with an early epidemic is that fungicides need to be applied promptly, once the first 10-20% plants are infected, because on young plants the fungus has only a short distance to grow to reach the stem.”

One fungicide would probably not prove to be enough this season where early infections took place, Dr West said. “A split application is better than one spray, with the second spray a month after the first. The aim should be to protect at least the first six leaves from infection, as these leaves usually produce the most severe cankers.”

Eastern Crop Watch agronomist Bill Barr reported phoma was present in his region. “It is just starting to make an appearance in forward crops, although it is not at threshold levels yet. Crops need checking carefully.”

The increase in minimum tillage combined with tighter rotations, high disease carryover from last season and low varietal resistance add up to it being a high risk year, BASF oilseed rape product manager Diane Heath said.

“Crops need monitoring carefully for symptoms, and leaf spotting treated within two weeks of symptoms appearing.”

HGCA trials last season showed BASF’s metconazole products, Caramba and Sunorg Pro matched other fungicides for phoma leaf spotting, she said.

In the first year of the project aimed at providing independent data on the effectiveness of products Caramba did prove to be slightly weaker in controlling stem cankers than Proline (prothioconazole), Prosaro (prothioconazole + flusilazole) and Plover (difenoconazole), although it didn’t make any difference to yields, the project report states.

Caramba could also be used to help achieve optimum plant structure in the autumn, Mrs Heath said, something not all other products could offer.

“Plants need to have a strong rooting system so that nutrient uptake is optimised and lodging risk reduced. Short main stems also mean less chance of phoma moving to the stem and offer better winter hardiness.

“Trials work in Germany shows Caramba increases root length by 17% and root mass by 28%, improving its root/shoot balance and improving its ability to survive the winter.”

For more on phoma control and growth regulation see the forthcoming issue of Crops (14 October).