Deadly tree disease ash dieback has reached the county with three cases confirmed, the South Yorkshire Forest partnership says.
The condition has been confirmed at sites in Rotherham and Barnsley, and the cases have been highlighted on a map published by the Forestry Commission.
The exact locations have not been made public.
Robin Ridley of the South Yorkshire Forest Partnership said: “We want local woodland owners to be aware of the signs of the disease so they can help record its spread.
“So far we have not seen any evidence of the disease in the wild, but three cases on newly planted sites have been confirmed so we need to keep up the vigil.
“We are making available as much information as we can find, so that local woodland owners can help understand and track the infection if it occurs.
“We are in the process of contacting all the local woodland owners to update them.”
Scientists have warned that the outbreak has the potential to devastate the UK’s population of around 80 million ash trees.
The first confirmed case in the UK was recorded in March at a nursery in Buckinghamshire.
The disease, which is carried on spores, is spreading slowly across the country.
Robin added: “The loss of these trees will have a profound effect on the environment - not just our woodlands but also our streets and gardens too.”
The partnership has information about the condition on its website, and suspected sightings can be reported to http://ashtag.org/ or by calling 08459 335577.