There is a lot in the news at present about the appearance of ash die-back disease and its threat to the health of our British woodlands. There are a number of interesting points raised.
The pathogen is often identified as Chalara faxinea, an ascomycete fungus. It then turns out that this is simply the vegetative form of a fungus known as Hymenoscyphus pseudoalbidus in its sexual and spore-forming phase. It is the spores produced by the latter that attack the leaves and stems of our ash trees; and so far this sexual and thus spore-forming phase of the pathogen H. pseudoalbidus, has not been identified in Britain. So, though confusing, there may be some point in having two names for the same genetic species.  We shall continue to be attacked from across the sea, but there is just a faint chance that we can escape full-scale infestation if the sexual phase fails in Britain.
As of 16th November  there are two types of infected site in Britain: [a] woodlands, [b] nurseries. Woodland sites where infection has been noted are confined to the east coast, and focussed on East Anglia (though spreading northwards to Northumberland and Fife). The strong impression is that spores are blowing in from Northern Europe. The nursery sites are dotted around the whole of Britain, wherever there is a nursery unfortunate enough, or pathetic enough, to purchase stock from the continent. The saddest part of this whole story is the realization that this staple tree of the British landscape, a tree that can be found seeding itself all over our gardens, hedgerows and field-margins is being bought in from abroad because we are too lazy to produce our own seedlings for sale.
In general, fungal diseases need warm moist conditions for the germination of spores. Once the mycelium is inside the plant tissue, the atmosphere is a negligible factor in growth. My experience is that 2012 has been an outstanding year for high moisture levels during the 6 months of 'summer'. It would be a relief if the whole question of Ash Die-back disappeared with a fine dry summer in 2013.
Two excellent websites provide information of considerably better quality than the common press and radio: