Damn, you must be gutted!
I have a few on my land and every year I always fear the worst. I believe that trees are susceptible when the bark is of a sufficient thickness for the female scolytus beetle to lay their eggs in late summer. The beetles lay their eggs in galleries which they excavate between the bark and the wood carrying the spores of the fungus Ophiostoma novo-ulmi on their boddies.
The fungus then sporulates in these breeding galleries and the larvae which hatch from the eggs feed by extending them, (which can be seen as hundreds of squiggly lines when the dead bark falls off), the larvae pupate in the outer sapwood and and develop into young beetles which emerge the following spring and eat their way out of the bark leaving hundreds of small emergence holes.
The young then fly off carrying the spores of O. novo-ulmi on their boddies and begin feeding in the twig crotches on young bark wher the spores become lodged and begin to germinate.
The fungus grows in the xylem vessels (not the phloem) where it produces more of the yeast like spores and as the fungus moves through the xylem vessels it produces phtytotoxins that disrupt the trees metabolism. Phenol based substances are then formed in the vessels preventing the uptake of water and nutrients and the tree is now dying.
As already pointed out, Elm disease has been around for centuries in the form of Ophiostoma ulmi, which all but the weakest trees could tollerate, however in the last century a more aggressive strain known as O. novo-ulmi appeared and wiped out our native trees.
Susceptible species to the more aggressive O. Novo-ulmi are Ulmus procera (English elm), U. Hollandica ( Dutch elm) and U. carpinifolia var. sarniensis (Wheatley elm).
U. Glabra (Wych elm) and some spp of U. carpinifolia ( smooth leaved elm) are less susceptible and if infected, die over a longer period of time.
Interestingly, the gene that is affected by U. novo-ulmi has been identified and Anti-fungal DNA has been inserted into the Elm genome using coat
ed micro ball bearings! Unfortunately, present Law governing geneticaly modified crops prevents it from being implemented outside the lab and Personaly, I find this unfair as the female of the species is infertile so cannot be corrupted by pollen from GM trees.