There are two plant diseases that are being actively watched by the USDA and the cause of quarantines.

First, SOD (Sudden Oak Death) is caused by the plant pathogen Phytophthora ramorum.
Over the past ten years, oak mortality has occurred at unprecedented levels along the central California coast. The Forest Service is working closely with USDA, California, Oregon, Washington and other states and local governments to coordinate and fund a program to limit the spread of Sudden Oak Death on federal, state and private lands. Public outreach is being used to locate those already planted in backyards. The disease affects oaks and a wide variety of other plant species including Rhododendron, Oregon Myrtle, Camellia, and Viburnum. On these plants other than oak, SOD causes leaf spots and may cause some dieback. If plants are found to be infected, they must be destroyed since they harbor the pathogen. SOD has not yet been found in the wild in the east but forests in the far west have been infected. Certain forested areas in California and Oregon have been devastated by the disease.

For more information, go to:
http://www.fs.fed.us/foresthealth/management/fhm-invasives-sod.shtml

The second, Boxwood Blight, is caused by a fungal pathogen Cylindrocladium pseudonaviculatum (Syn.Calonectria pseudonaviculata).
Boxwood Blight renders the emerald green leaves brown and dry. Boxwood Blight has been reported throughout Europe and the United Kingdom since the 1990s. Boxwood Blight is a new disease for the United States. It arrived in Connecticut in 2011 and is now reported from Connecticut to the Carolinas. Once infected, a boxwood will quickly yellow and drop its leaves. There is no treatment or cure and the infected plants and surrounding leaf litter must be burned and soil sterilized to prevent reinfection. Boxwood Blight does not bother rhododendrons and azaleas, but many of us have boxwoods in our gardens. If you live in a area free of Boxwood Blight, only buy locally grown boxwoods. Some big box stores have been shipped infected plants.

For more information, go to http://www.boxwoodblight.org.