AN important feature of North Yorkshire's riversides is under threat from an "ecological disaster".

Anglers, farmers and walkers have been asked to help stop the spread of a lethal fungal disease which is affecting alder trees on streams and river banks.

The Environment Agency said a survey along the Rivers Ouse and Ure showed about 75 to 80 per cent of alders were now affected by the disease phytophthora and about a quarter were already dead.

The problem was first identified in southern England in 1993 and spread rapidly. Cases were spotted in Yorkshire by 1996, and it is now present on all Yorkshire's main river catchments, though many smaller tributaries remain unaffected.

Agency ecologist Martin Fuller said: "These results at first glance look frightening - the destruction of alder populations along our river banks would be an ecological disaster and would completely change the landscape of rivers.

"It could also lead to other problems such as bankside erosion.

But agency officers have found healthy alders alongside dead ones in some areas.

"This means that the disease has not spread throughout the whole population and indicates that some trees might have greater natural resistance to the disease or greater powers of recovery.

"In other regions it has been reported that coppicing, the cutting down of trees to ground level and letting them re-grow, promotes recovery in affected alders, but it is too early to tell whether this will prove successful in the long term," added Mr Fuller.

The fungus is not spread by the wind or other natural means, but it can be carried in mud so anglers, farmers and walkers are thought to play a significant part in its spread.

An agency spokeswoman said there was no "cure" for the disease so they were trying to contain it. Its code of best practice suggests anyone who enters rivers or walks along their edge should: clean off all footwear, equipment or machinery before moving upstream or to another water course; delay moving straight from one catchment to another as the fungal spores only live for a few days; avoid transferring any river water between watercourses or upstream; and anglers should ensure all tackle is thoroughly dried before they move to another water.

andrew.hitchon@ycp.co.uk