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Are any ash species disease resistant?
Many ash tree owners and suppliers are talking about disease resistant ash species. Dr Kirisits of the University of Natural Resources and Applied Life Sciences in Vienna (a botanist who has been studying the disease full time since 2009, and who is a world expert on the subject) advises caution regarding claims that some ash species and varieties may be immune as more time and more observations are needed. Until test plantings and subsequent monitoring has taken place, no ash should be regarded as die back resistant. Overall, Fraxinus excelsior (Common Ash) and Fraxinus angustifolia (Narrow-Leafed Ash) appear to be highly susceptible, along with most of their varieties. The reproductive stage of C. fraxinea (Hymenoscyphus pseudoalbidus) has also been recorded in Europe on Fraxinus pennsylvanica, F. nigra, F. americana (Green Ash, Black Ash and American Ash, all from North America) and F. mandshurica (Manchurian Ash, from Asia). More exotic examples such as Fraxinus ornus (Southern European Flowering Ash) do not show the same susceptibility but would be of very limited value in the British climate. In fact, climatic conditions have a significant impact on the growth and spread of the disease. Countries with ash trees in damper climates (such as the UK) will have a tougher time containing the disease than drier, warmer regions on the continent. It also follows that in warmer urban areas, especially where leaves are routinely removed from streets, the risk is somewhat lower than rural farmland. Dr Kiritsits thinks this is important – good hygiene reduces the impact of the disease. As such, the advice and action around washing boots, animals, cars, tools and anything else that may have been in contact with the fungus should be taken very seriously.
In conclusion
There are three important points to take from this article:
- Please remember that it is natural for many trees to lose their leaves every autumn. Ash is one of these and loses its leaves earlier than most others. So don’t confuse natural autumn leaf loss with ash die back disease
- use the pictures on the previous page as a guide to look for cankers, leaf loss and crown die back
- Be reassured that leaf loss in trees other than ash will not be caused by the ash die back fungus
- if you’re still concerned, consult with your local nursery or tree surgeon for advice
- Practice good hygiene
- our advice is to rake up and burn all ash leaf and twig litter in winter.
Ash die back seems to be carried on imported stock, the wind, and via moisture. There are infected woods in the UK that were almost certainly infected “naturally”. There are about 80 million ash trees in the UK and any widespread treatment is simply not feasible. We must therefore resign ourselves to losing some large proportion of the ash tree population over the coming years. Not all, however, as some are naturally resistant to the disease. In 2011 there were 21 countries in the European Union that had ash die back, but all have ash trees surviving with no apparent ill effects in forests that have been decimated by the disease. So there is hope – in time we will inevitably propagate from resistant trees and breed even more resistant varieties.
My mountain ash (rowan) died of a honey fungus infection this year. Ash are resistant to honey fungus, rowan are resistant to ash dieback. It doesn’t really matter to the tree which fungus kills it.
I hate to disagree, but I think it does matter.
The two are fundamentally different. Honey fungus attacks through a “root system” (called rhizomorphs). It therefore travels relatively slowly but kills incredibly fast. It is not species specific (by the way, ash are not resistant to honey fungus – very few tree species are). It is preventable in the sense that it cannot cross barriers so planting susceptible trees inside an impermeable barrier in the top 6″ of soil should keep them safe. Honey fungus also tends to attack weaker and more stressed subjects, so the best prevention is to improve the soil using loads of organic matter such as well rotted manure, compost and so on. Healthy plants have their own defences.
Ash die back is different in a number of significant respects. It is spread by the wind and so can move many miles a year. It is species specific – it only attacks members of the Fraxinus family. It is indiscriminate – it attacks them all. And, critically, it does not seem that many Fraxinus have a defense system that can repel it. It is also not preventable and not treatable.