Trials of candidate resistant specimens are ongoing, with some 155,000 trees currently being grown in Southeast England, he said. "We still need diversity because otherwise a mutation of the pathogen could still wipe them out. We know there are more virulent strains in Japan."
He added: "Early flushers seem to be more resistant, so if you have the choice locally, take seed from these. Trees in dense stands are more susceptible as it's more humid, and where they may be more stressed. Infected trees may die from secondary effects like Armillaria (honey fungus) or drought, or it can work the other way round."
The fungus responsible, Hymenoscyphus fraxineus, spread by spores released from dead material, though they don't spread more than 50m downwind, he explained. "But infected material can produce spores for up to five years. So remove leaf litter where possible, open up stands, but only fell trees where necessary as they might still be tolerant."
And he said of the emerald ash borer (Agrilus planipennis), which has devastated North America's ash trees: "If we don't do anything, it will arrive. It moves readily in wood and woodchips and we are importing more of these from North America than ever before. It could suddenly appear anywhere, and if it does we have only a short window in which to eradicate it. The worst case scenario is that we lose all the ash that survive dieback."