It is a crop of brilliant yellow that seems to be covering more of North Yorkshire's fields than usual this year, and one which has not always been welcomed by everyone.

But experts today swept aside the notion that oilseed rape can cause problems for hay fever and asthma sufferers - and explained why more people could be gropwing the crop in the area.

One factor could be farmers switching to rape from growing sugar beet, following the decision to shut the sugar beet factory in York later this year.

Rachael Gillbanks, spokeswoman for the National Farmers' Union North East, said: "Anecdotal evidence suggests that farmers in the area are growing more oil seed rape this year.

According to the British Medical Journal, there is no clear evidence that the crop has an adverse affect on human health, and it was suggested that the British have taken a dislike to it because of its flashy colour and intense smell."

She said other countries growing more rape than Britain were seemingly unconcerned about its prevalence. "It is also important to remember that rape flowers at the same time as many tree species, including the hawthorn, " she said.

Dr David Fair, of The Jorvik Medical Practice, in York, said he did not believe the crop brought on the symptoms of hay fever or asthma.

"My understanding is that oilseed rape doesn't release anything that brings on hay fever or asthma - any notion that it does is a complete myth, " he said.

"There has, however, as a result of the warm weather, been an increase in the levels of pollen from trees in the last week or so, which can bring on hay fever. It can also irritate asthma in those who suffer from it."

Dr Fair said there had been an increase in the number of patients with hay fever complaints in the last fortnight, but that he believed this to be a result of the tree pollen, and not the rape.

Mervyn Lewis, at York's Askham Bryan College, said said there were a number of factors encouraging the increase in the growth of rape in the area.

"One is that some farmers who previously grew sugar beet for the now closed York British Sugar Factory will now instead be growing rape. Another is the availability to farmers of energy contracts, under which growers are cultivating rape to be used as bio-diesel.

"The average price of rape has also been much stronger in the last 12 months - in 2006 it cost about £130 per tonne, but by 2007 this had risen to around £160."