The Plant Clinic recently received a very interesting case of galls on viburnum. We have also seen this on forsythia in past years. The galls are caused by the Phomopsis fungus but look very much like a systemic infection of crown gall or possibly a stem gall from an insect. If you have taken the Master Gardener classes, you know that if you cut into the gall and do not see any chambers, trails, or evidence of insect borings, disease is a good possibility. Phomopsis-induced galls are about 1 to 2 inches in diameter, rather round, and have a bumpy, roughened texture. They look like a cluster of nodules pressed tightly together. The gall is a mass of undifferentiated plant tissue and fruiting bodies (pycnidia) of Phomopsis. We did not see the fungus on the dry tissue, but the pycnidia were visible as black, pinhead-sized dots on tissue that was incubated in moist chambers overnight.
Phomopsis galls can occur on many tree and shrub species, including forsythia, viburnum, highbush blueberry, American elm, hickory, maple, oak, and privet. Gall size varies with the host species and time. If the galls girdle the twig, dieback results. Otherwise, very little is known about the disease cycle. The only control measures we can suggest are to prune out affected or dead branches and promote tree vitality through good horticultural practices.