A deadly tree and plant disease first found in the UK in 2002 has spread to Wales, the Forestry Commission said today.
Phytophthora ramorum is a fungus-like organism which kills many of the trees and plants that it infects and is commonly known as sudden oak death because of its impact on oak trees in the US.
It was first identified on a viburnum plant at a garden centre in Britain in 2002 and has since infected shrubs including rhododendrons, viburnums and bilberries.
Fewer than 100 trees were identified as being infected, most of which were found close to rhododendrons.
But last year Japanese larch trees in South West England were found to be infected - the only place in the world where it has attacked large numbers of commercially grown conifer species.
The Forestry Commission said today the pathogen had spread to Japanese larch trees in South Wales, probably carried as spores in rain, mist and air currents across the Bristol Channel.
It has only been confirmed in one area of larch forest in Wales so far, but the Forestry Commission Wales expects to find wider infection as it undertakes ground inspections in areas where aerial surveys have raised suspicions.
Roddie Burgess, head of the Forestry Commission's plant health service, said: "Given the damage it has caused elsewhere, we were very concerned when Phytophthora ramorum turned up in Britain in 2002, and we and our partner organisations have moved quickly to deal with it and prevent it from spreading wherever it has appeared.
"We managed to fell most of the infected trees in the South West before this year's new needles formed and therefore before new spores could be produced.
"This appearance and spread into larch trees in Wales add to our concern."
But he said: "Based on our scientists' knowledge of local weather patterns and how it spreads, we remain hopeful that by taking quick action now in Wales as well we might still prevent the infection from the large trees from spreading further north and east outside South Wales and South West England."
The Forestry Commission said it was working with partners to monitor woodlands, fell infected trees and destroy other diseased plants as quickly as possible.
Signs have also been put up in the infected areas asking the visiting public, as well as forestry workers, to takes steps including washing boots, equipment and bike and vehicle wheels to help prevent spreading the disease.