Sclerotinia attacks show importance of application timing

Many oilseed rape crops are now showing the true extent of Sclerotinia disease pressure this year, and it has really highlighted the importance of spray application timing, according to Peter Gladders of ADAS.

The long flowering of many crops, coinciding with an extended period of repeated sclerotinia spore release, resulted in potential infection from early April to the end of May. As a result even a few days difference in fungicide timing could have made a significant impact in the level of infection, he reports.

ADAS trials have shown very good effects from fungicide applications at the end of April and early May, but crops treated in early April are showing significantly higher infection. "With the level of disease pressure this season, growers will almost inevitably see some disease infection," he adds. "The question is, how bad would it have been without treatment? Fortunately, most appear to have used a one or two spray programme to minimise infection."

Nowhere is this more evident than in variable crops, as a result of pigeon damage or weed competition. In trials there are patches of crop at over 70% infection, when two yards away infection level may be 5% or less. "It is a very complex situation, where the weather, the susceptibility of the crop, the level of spore infection and the fungicide timing all play an important role," advises Dr Gladders.

The extent of the season's Sclerotinia attack is continuing to rise, as a result of late infection at the end of May and secondary spread of infection in lodged crops. But not all premature ripening and die-back is a result of Sclerotinia, he points out. "Growers should also be alert for signs of verticillium wilt which is more evident again this year. The symptoms of brown stripes spreading up the stem, turning to grey streaks as the plant dies, are evident on more than 50% of plants in infected crops," he warns.

Ian Holmes of Syngenta reports growers have seen a significant difference in Sclerotinia infection between different varieties treated with the same fungicide programmes. "This may be attributable to the length of flowering, but more likely what stage of flowering the crop was at when applications were made rather than any specific varietal influence. In future, growers may need to consider tailoring application timing more precisely to the flowering of individual fields depending upon the level of disease pressure.

"Some growers making a single Amistar application have achieved very good results where the timing was right, but clearly growers using a two-spray strategy have had greater success in protecting the crop through the extended disease pressure this year," he adds. After two consecutive bad years for Sclerotinia, Mr Holmes believes an Amistar two-spray programme may become the standard treatment for growers, especially where oilseed rape is grown frequently in the rotation.

Peter Gladders warns that the high level of Sclerotinia experienced in oilseed rape, combined with a continued weather pattern where repeated rain events have triggered further spore release, poses a significant threat for other susceptible crops this season. "Carrot, potato and pea growers will have to be especially vigilant and take necessary measures to protect crops from infection," he advises.