Sudden Oak Death (NOT restricted to Oak trees):
http://www.ncipmc.org/alerts/suddenoakdeath/index.cfm

Oak Wilt:
http://www.na.fs.fed.us/spfo/pubs/howtos/ht_oakwilt/toc.htm

Asian Longhorned Beetle:
http://www.na.fs.fed.us/fhp/alb/

Emerald Ash Borer:
http://na.fs.fed.us/fhp/eab/

Birch Leafminer:
http://www.ento.psu.edu/extension/factsheets/birchLeafminer.htm

Bronze Birch Borer:
http://www.na.fs.fed.us/spfo/pubs/fidls/bbb/bbb.htm

Hemlock Woolly Adelgid:
http://www.caes.state.ct.us/FactSheetFiles/Entomology/fsen012s.htm

Dogwood Anthracnose:
http://www.na.fs.fed.us/spfo/pubs/howtos/ht_dogwd/ht_dog.htm

Winter Moth:
http://www.umass.edu/umext/ipm/featured_pest/archive/winter_moth.pdf

Plum Pox Virus:
http://www.aphis.usda.gov/ppq/plumpox/faqs.html