Evidence from Europe, where the disease has been present longer, indicates that about 1% of all ash trees show a high degree of tolerance to ash dieback. These trees show minimal crown dieback (less than 10%), and it is these trees that we are keen to learn about. We are particularly interested in healthy trees that are located within woodlands, surrounded by other ash trees that are showing a high degree of dieback.
The easiest time to do this in during the summer months, when it is possible to make as assessment on crown health. Trees showing up to 25% crown dieback are also of interest. The following images are reproduced from the Forestry Commissions publication: ‘Diseases and disorders of forest trees: a guide to identifying causes of ill health in woods and plantations.