At the Tree Barn Christmas tree farm in Oxfordshire, which supplied 10 Downing Street's tree this year, grower Andrew Ingram said: "Necrosis is not understood at all. It causes needle drop and although people are beginning to research it, we expect to lose 10 per cent of our crop a year.
"We just have to write it off unless we can find a cure. From one year to the next the trees seem to recover but then they get it again."
Welford Christmas Tree Farm owner Will Miles, who supplied Downing Street's door wreath, said: "We had someone look at our trees and he identified one or two with it. It is a worry."
British Christmas Tree Growers Association (BCTGA) secretary Roger Hay said: "It's affecting individual farms in counties including Suffolk and Kent. It is some kind of fungus that has only appeared in the past two years and is worse this year. The trouble nowadays is so many plants are coming from around the world through the EU that are not properly checked."
The BCTGA is hosting a meeting in March 2012 for its 350 members - 75 per cent of the UK's growers - to discuss a strategy. Defra, Forest Research and chemical companies will attend.