Chalara fraxinia.

Classic lozenge shape marks on the trunk of a dying/dead ash tree.  Being among the dead/dying most days of the week prompts thoughts thereon.  One keeps encountering people who suggest that there are trees showing partial or total genetic resistance, although they quote no specific source in most cases.  While the absence of obvious disease or limited effect on some trees is apparent, I think this may always be due to environmental circumstances rather than genetic resistance. There are certainly some mature trees which show little obvious sign of the disease despite longstanding proximity to diseased trees.  For almost all mature trees in the diseased geographical areas, diseased tips can be seen, but new growth below the tips grows past the diseased tip, concealing it (where new growth seems slightly bushier, look for the dead tip in the middle - a similar effect to the vigorous epicormic growth from the trunk of more heavily diseased trees).  On some mature trees it is difficult to spot any disease with certainty and they seem to be those where autumn leaf litter does not remain underneath because leaves are swept up or blown away (leaf litter provides a part of the disease cycle).  I wonder (and suppose some botanist is working on this) whether the rate of spread once the fungus in a tree is in the layer below the bark may depend on the relative circumference of infected tips to the circumference of the host twig or branch circumference.  The lozenge shapes suggest the fungus moves vertically more than circumferentially and down the tree more than up the tree and a host branch/trunk only dies when the disease from one or more branches/tips has completely blocked the circumference of the host branch/trunk.  Cutting successive logs from branches/trunks exposes the proportion of the circumference that is discoloured by the fungus and confirms the inclination of the fungus to spread strongly along, rather than round, a twig/branch/trunk.  If the absence of leaf litter is a successful way of breaking the disease cycle then as the population of ash declines there may come a time when the remaining ash trees (in isolated and windy sites) are too far apart for the disease to be transmitted by wind blown spores and leaf litter.  Does the under-bark fungus continue to spread within the tree from one season to the next, or does it die away and the fungus have to reinfect from the leaf litter each year?   How long do the spores remain viable without finding a new host?  Must do a bit more looking up - I am soooo ignorant.