Light leaf spot levels on oilseed rape crops are at their highest since the epidemic year of 1995, adding to the problems being created by ravenous pigeons in many areas.

A DEFRA-funded survey, carried out by Central Science Laboratory in December, found 9% of oilseed rape crops, and 1.8% of plants were infected with the disease last autumn, markedly higher than the long-term mean of 3.3% crops infected and 0.4% plants affected.

Crops in the north were most at risk, with 28.6% of crops infected. But 15% of crops were also showing symptoms in the south east, the first time crops in the region have been affected at this growth stage since 1997.

The cold winter could increase the threat from the disease, too. Light leaf spot remains active at very low temperatures, so cold weather can exacerbate problems, SAC‘s Simon Oxley points out.

Crops left unprotected so far will need treating as soon as possible where the disease threatens, he says.

“Where crops were treated in October or November, a treatment might be needed before the typical top-up stem extension timing, if lower doses or less active products have been used. Proline and Prosaro have proved the strongest products in dose response trials. If you used half to three-quarter dose you should be OK [until stem extension]. If you’ve used a lower dose or an alternative fungicide, then you are at risk of running out of steam.”

In that case growers need to monitor crops, and new growth in particular, carefully for symptoms, he says. “If you see new light leaf spot on developing or upper leaves, then you would need to retreat sooner than stem extension.”

For crops that will be protected until stem extension he advises using Proline (prothioconazole) or Punch C (flusilazole + carbendazim) on those that don’t require any growth regulatory effect in addition to the light leaf spot control, or Prosaro (tebuconazole + prothioconazole) on crops that do.

Pigeon threat

Pigeons remain the biggest threat to crops in the south, Andrew Cotton, an independent agronomist based in Oxfordshire, says. “I haven’t seen any light leaf spot, but it is more a case of spot the leaf on my [clients’] crops. Any crops that were green before the snow have been defoliated since it melted. The pigeons are ravenous.”

Brian Ross, a Frontier agronomist in East Anglia, has similar concerns. “Backward crops are being eaten alive. Decisions are going to have to be made on crops in the next fortnight.”

Those that do survive will be very susceptible to disease, he notes. “If you see any disease you’ve got to jump on it.”.