What Causes my Ash tree and Sycamore Leaves to turn Brown?

This is a common question I hear each Spring in Sonoma County.  Usually the problem is caused by a foliage disease called antrhacnose.  Antrhracnose is one pathogen in the category leaf, shoot or twig blight (Pests of Landscape Trees and Shrubs, University of California Publication 3359).  The disease shows up during moist warm spring weather.  It causes mottled brown spots on leaves of ash, sycamore, elm and oaks.  In modesto ash trees often a tree