This page has links to websites and databases important for research on plant pathology, including the plant diseases and the insect and other vectors that spread the plant diseases.
Center for Invasive Species Research, University of California - Riverside.
Lists all plant diseases and insects that cause or spread the diseases, as well as other insect pests. For each pest or disease, a map of California is provided showing the areas of infestation, and there is a detailed write-up about the disease or pest and a section devotec to the economic impact of the disease or pest.
http://cisr.ucr.edu/invasive_species.html
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74163.html
This website shows diseases in the entire country and maps showing distribution of the disease
http://www.nrs.fs.fed.us/tools/afpe/maps/
More nationwide Maps:
http://dontmovefirewood.org/gallery-of-pests/common-names.html
http://www.invasivespeciesinfo.gov/microbes/main.shtml
Widely prevalent bacteria in the United states:
http://www.prevalentbacteria.org/subject.cfm?id=647
Hungr Pests, very good maps:
http://www.hungrypests.com/the-spread/ByPest.php?pestabbr=sod
Invasive Plant Atlas of the United States:
http://www.invasiveplantatlas.org/trees.html
APHIS (US Animal and Plant Health Inspection Service)