The UK plant health authorities are currently consulting about a serious new disease affecting ash trees (Fraxinus) of many species. It is a fungal disease, now called Chalara fraxinea, and appears to be spreading rapidly through Europe (though has not yet become established in the UK).
Though the disease has been most widely spread in north western Europe and Poland, it has recently been reported in southern Europe too (in France and Italy).
Symptoms to look out for are lesions and cankers in the bark, with discolouration of the underlying sapwood. When these lesions girdle parts of the tree, it dies back - leaves, shoots, twigs, branches, then the whole tree. There may also be necrotic patches on leaflets, leaf veins and stems.
Have any forum members come across this new killer yet?
You can see the UK risk assessment
here.