every year the hawthorne trees on my street put out a fabulous flower display in may followed by a withering and complete leaf drop in june...does anybody know the cause?
It is difficult to say for sure, without a bit more information, but it sounds like the hawthorns may be affected by fireblight, a bacterial disease that kills young twigs and leaves. The bacteria survive the winter in cankers and ooze out during spring rain, and are then carried from tree to tree by bees and insects. Twigs will tend to have a distinctive "crook" shape when they die, if that is the case.
The likely cause of your hawthorn defoliation is scab disease (similar to apple scab). Sometimes, powdery mildew also infects so that in most years in the Vancouver area hawthorns are completely defoliated by about July. There isn't much that can be done, except perhaps to increase air circulation, remove fallen leaves so that the fungus does not get rain-splashed back into the canopy and use a fixed copper spray on the leaves in spring. Often, people just give up and replace their hawthorns with more resistant species (including other hawthorns).