Dr Heinrich Losing of Versuchs-und Beratungsring Baumschulen, who has been carrying out research into the disease in Germany, said that trees could be affected in nature as soon as they leave the nurseries, because the disease exists in forests and cities.
Losing told delegates at the International Plant Propagators Conference at East Malling Research yesterday: "The disease is spreading and we don't know how far it will go or how severe it will be.
"Chemical control is possible but it doesn't help us."
He added: "Species are very different if you look at susceptibility."
He said that work on the selection of tolerant clones was ongoing in Denmark and these could be grown in the UK.
Ash dieback first started to appear about nine years ago. Two years ago, the fungus Chalara fraxinea was identified as the pathogen resopnsible for the disease.
For the latest on the how the industry is dealing with the disease in the UK, read Disease Strategy.