Plum Pox Virus
Sharka, or plum pox, is probably the most dreaded disease of stone fruit trees. It plagues orchards in Europe, has been in Chile for several years, and sadly has now been confirmed in the United States. In October 1999, infected trees were found in several orchards in a small area in Pennsylvania. The area is under quarantine and every effort is being made to eradicate the disease. For more information, pleae check the Pennsylvania Department of Agriculture website.
To see good pictures of the symptoms, go to West Virginia University's plum plox gallery.
Any further information pertaining to California will be posted here as it develops.