We have a large, stately American Elm which has been the centerpiece of our garden for years. It does not have Dutch Elm disease (we've been treating it with injected fungicide, etc., for years). It has always had a serious case of elm black spot (anthracnose), however. In recent years the black spot has gotten progressively worse. It is so bad now that the tree drops 60-80% of its leaves between June 15 and July 1. Remarkably the tree seems to remain otherwise healthy -- leafing out fully each spring. Dropping diseased leaves all over our garden every spring and ruining the beauty of the tree is such a drag. It has been treated with sprayed fungicide vigilantly this year and last, but this seems to have no had no effect. Does anyone know a more aggressive treatment?
Here are some other facts. The tree now has a very dense crown (thanks to pruning several years ago and regular fertilization). It leafs out very densely in April, with layered decks of leaves. Both this year and last spring in Massachusetts has been rather wet and cool, but not off-scale. We've made sure to remove all leaf litter around the tree. We've heard that dense foliage inhibiting air circulation can encourage the disease. On the other hand it's hard to believe that thinning the crown could control such an infestation.
Any ideas?