Variable oilseed rape crops, in part caused by the damage from cabbage stem flea beetle attacks, could leave the crop more vulnerable to sclerotinia infection.
The variability in growth stages could extend flowering, meaning a longer risk period for growers to protect these crops for. In addition, the warmer weather experienced so far this spring, potentially could also increase the amount of inoculum present as sclerotia germinate once soil temperatures reach 10degC, says Bayer.
The last severe outbreaks of sclerotinia were in 2007 and 2008, but localised problems occur most years so growers should take steps now to ensure they are accurately assessing risk, and taking protective action where necessary, says Bayer.
It adds that the three main factors to watch out for that aid sclerotinia development are:
To ensure efficient action against any development, Ella Crawford, commercial technical manager at Bayer, advises spraying from early to mid-flowering before any significant petal fall. “The weather in this critical flowering period will set the risk for sclerotinia development,” she says.
“A minimum temperature of 7degC and relative humidity of 80 per cent or above for more than 23 hours is required for infection to occur.”
Applications, though, have to be preventative, as fungicides have no curative activity against sclerotinia, Faye Ritchie, principal plant pathologist at ADAS, says. “Petals act as a nutrient source and also transfer spores down into the canopy, which is why fungicides need to be applied prior to petal fall and before any spores are present in the canopy, to be effective.”
Sclerotinia fungicides applied at the full recommended label rate will give approximately three weeks’ protection, Dr Ritchie says. “To get the most from a one spray approach to control this disease, it is best to apply fungicide as close to mid-flowering as possible - this will protect the crop as more petals fall into the canopy.”
But Bayer advises growers to consider a two-spray approach at high risk sites, where there is a history of severe slerotinia on farm, and/or the duration of flowering is extended, which it says could be the case in crops affected by cabbage stem flea beetle damage. It adds that the first spray can be applied prior to mid-flowering, with a follow up application no more than three weeks after that first application for continued protection.
Ms Crawford says: “A 75 per cent dose of Proline (prothioconazole) will provide effective protection for the first spray and can be followed up with another 50 per cent dose three weeks later where needed.”