Scientists have asked the public for help as they survey 1,000 sites across Northern Ireland for traces of a fungal disease that could devastate our tree population.
Over the past four years, 100,000 young ash trees have been destroyed in an attempt to eradicate the potentially devastating ash dieback disease.
The disease is widespread in Europe, where it is killing mature trees.
It has been found at around 100 sites in Northern Ireland, mostly in saplings planted within the past five years. It is spread on the wind by spores.
Scientists at the Agri-Food and Biosciences Institute (AFBI) in Belfast are trying to establish if that is happening here.
The authorities have developed a smartphone app called Tree Check, which the public can use the report potential problems, including a picture and a GPS position.
They will then be assessed and, if necessary, the site visited by experts.
Jim Crummie, plant health expert with the Forest Service, urged the public to download the app and to be the "eyes and ears" of the authorities in the fight to combat the disease.
Prof Alistair McCracken, a plant pathologist with AFBI, said: "This is a serious disease. In Denmark they say up to 90% of the ash trees have been infected.
"Now they haven't all died, but they have been affected and are looking quite sickly so potentially this could have a huge impact on the Northern Ireland environment."
The fungus appears between June and September on leaf litter. Each pinhead size fungus can produce large numbers of spores.
When they land on a fresh ash leaf they germinate, penetrate the surface and grow into the stem. The first symptom is wilting leaves. As it gets worse the trees will develop a diamond-shaped dark-coloured lesion on the bark.