As some ash species show very few symptoms after infection, they may act as undetected carriers.  There is evidence of low susceptibility to disease in some Asian ash trees (Drenkhan and Hanso, 2010). (Moderate confidence)

Common ash (Fraxinus excelsior) is the most severely affected species.  Young trees are particularly vulnerable to C. fraxinea and succumb to disease rapidly.  (Kowalski, 2006; Forest Research, 2012).  (High confidence)

There is no evidence that C. fraxinea can spread to tree species other than ash. (High confidence)

Infection is via spores from fruit bodies on leaf litter.  Spore production (in fruit bodies) occurs on infected fallen leaves and shoot material in the growing season after infection (High confidence)

C. fraxinea infection starts primarily on leaves, and is progressive over time, with dieback and stem lesions usually manifesting in the next growing season.  Leaf symptoms can be detected within two months of infection. (Moderate confidence)

C. fraxinea causes infection from June – October, mainly in July – August. (High confidence)

Spores are produced on Chalara fruit bodies formed on fallen leaves and shoots the year following infection. Natural spread is by wind-blown spores (ascospores). Wind-blown spores cause the disease to spread up to 20-30 km per year. Longer-distance spread occurs via infected plants or potentially via wood products. (High confidence on wind dispersal; Moderate confidence on untreated wood products)

There is low probability of dispersal on clothing and footwear or via animals and birds. (High confidence).

C. fraxinea is found in seeds (Cleary M., et al. 2012), and this is reflected in the legislation, which restricts the movement of plants and seeds. (High confidence)

There is a lower risk of C. fraxinea spreading over the winter because there is now a ban on ash plant and seed imports into the UK, restrictions on plant movements through Statutory Plant Health Notices, and spore production is not expected to resume until June 2013. (High confidence)

larger trees can survive infection for a considerable time and some might not die. (High confidence)

The impact of C. fraxinea infection depends on tree age, location, weather conditions and co-presence of honey fungus (Armillaria) or other secondary pathogenic / opportunistic organisms. Trees in forests are more susceptible because of the greater prevalence of honey fungus. Timber trees are generally felled before they are killed by honey fungus.

  • Trees under 10 years of age are likely to die from C. fraxinea in 2-10 years.
  • Trees under 40 years old will die in 3-5 years if also infected with honey fungus, and likely more rapidly if the tree is already debilitated.
  • For mature trees more than 40 years old, there is no direct evidence of tree deaths just from C. fraxinea to date, but there is little comprehensive survey data from Europe on which to base firm conclusions.
    (Moderate confidence)