Last year, 876 hectares of Japanese larch trees were found to be infected by an outbreak of ramorum disease in Wales.
During the winter, Forestry Commission Wales put surveys of woodlands on hold as, once the larch trees have dropped their needles, it is difficult to spot symptoms in infected trees.
Since the trees regained their needles in late spring, Forestry Commission Wales surveyors have again been out in force checking woodlands to find out how far ramorum disease has spread.
Initial findings from these surveys indicate that this fatal tree disease has infected a further 227 hectares of larch trees in Wales.
Owen Thurgate, Phytophthora Project Manager at Forestry Commission Wales, said, "The worst case scenario would have been to have found the same number, or even more, trees infected by ramorum disease this year as last year.
"The advice of Forest Research scientists to prevent the disease's spread is to fell infected trees to kill the living plant material on which the Phytophthora ramorum pathogen depends.
"Whilst it is therefore worrying that we will have to fell a large number of infected trees again this year, it would seem that our decision to swiftly fell infected trees last year has played a key role so far in managing this major outbreak."
Most of the newly diagnosed trees are in woodlands adjacent to the areas found to be infected with ramorum disease last year in the Afan Valley, near Port Talbot.
Ramorum disease has also been diagnosed in a small number of larch trees at Bwlch Nant yr Arian, near Aberystwyth, where 60 infected trees were felled in 2010, and, for the first time, near the Alwen reservoir in Hiraethog Forest, north Wales.
In autumn, infected Japanese larch trees produce huge quantities of the spores that cause ramorum disease. The disease can therefore quickly affect and often kill large numbers of these trees within a year of symptoms first being detectable.
Weather also plays a role in the spread of ramorum disease as the spores seem to travel best during damp conditions.
"Last year, we found that the dry autumn helped to reduce the spread of the spores that cause ramorum disease," said Owen.
"We therefore expect that the disease's progress will depend on the weather this autumn, too, so the drier the conditions later this year, the better."
Forestry Commission Wales will continue to undertake surveys looking for the symptoms of ramorum disease until October.
In the meantime, the felling of trees that have been diagnosed with ramorum disease so far this year has begun.
Ramorum disease does not harm the timber, and biosecurity measures have been put in place to allow logs from infected trees to be taken to sawmills without spreading the pathogen to other trees or plants.
"We are determined to minimise the impacts of ramorum disease on woodlands and the forest industry, and the support of woodland owners in looking out for early signs of infection will continue to play a key role in our disease management strategy," said Owen.
Ramorum disease is not harmful to humans or animals, and all public woodlands remain open to visitors, except in areas where felling operations are taking place, which are temporarily closed for safety reasons.
Owen said, "We are appealing to everyone who works in or visits the affected forests to help us contain this outbreak by observing some biosecurity precautions so that the pathogen is not inadvertently spread on boots, bicycle or vehicle wheels, tools or machinery.
"Signs explaining these simple precautions will be placed at the entrances to forests where ramorum disease is present."
Woodland owners or managers in Wales who suspect infection in their trees should report it to Forestry Commission Wales's Grants & Regulations office at Clawdd Newydd, Ruthin, Denbighshire, LL15 2NL Tel: 0300 068 0300, e-mail: email@example.com.
Further information about Phytophthora ramorum is on the Forestry Commission's website at www.forestry.gov.uk/pramorum.