The appearance of cercospora – in crops in North West Norfolk – may come as a surprise to some, as the disease normally prefers hot conditions, which have been relatively few and far between so far this summer.
But, recent wet, humid weather has encouraged its development – cercospora will develop when there are five or six days when the temperature is above 17C and humidity above 98%.
British Sugar’s Colin Walters advised growers to be vigilant, but was not unduly concerned by the appearance of either disease. “Now is the time to be going in with a decent triazole and get everything treated within the next two to three weeks.”
Treatments for controlling powdery mildew usually last 4-8 weeks (depending on product and disease severity), so growers considering spraying before symptoms appear should take into account disease history and variety susceptibility, the British Beet Research Organisation added.
Applications now would lose two weeks of that effect if disease did not appear until late July, it said.
Where crops were backward (i.e. not covering or only just covered across the row) growers were advised not to apply a triazole or strobilurin fungicide yet. If powdery mildew was to come into these early, consider a treatment of Fortress (quinoxyfen) or sulphur, followed by a triazole in mid-August, it suggested.