Galls are swellings or overgrowths of tissue, usually plant tissue. Causes include insects, mites, and pathogens such as bacteria, fungi, and nematodes. There are even some noninfectious causes of galls, such as growths around injuries or callous tissue. Try not to assume that the presence of a gall on a plant automatically indicates an infectious disease.
Some examples of bacterial galls include crown gall (Agrobacterium tumefaciens), bacterial knot of ash in Europe, fasciation (leafy gall), and a gall on honeylocust that is a bacterial suspect. Black knot of plum (Dibotryon morbosum), Phomopsis gall, Nectria gall, and Sphaeropsis knot are examples of fungal galls. There are hundreds of insect galls on oak alone. In Illinois, galls caused by insects far outnumber those caused by fungi or bacteria. The Plant Clinic has received samples this year with galls caused by mites (ash flower galls), insects (wood sower gall), fungi (Phomopsis gall), and a noninfectious callous gall. How do you know which type of gall is present? Cut into the gall and look for chambers, trails, or evidence of insect boring. Check reference materials to determine gall-forming possibilities on your host plant. Then observe the galls with a strong hand lens or dis-secting microscope, looking for fungal fruiting bodies. It may be necessary to incubate tissue overnight in a moisture chamber to cause fruiting bodies to form.
Phomopsis gall has appeared on a few hosts in Illinois the last few years. It may 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. 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. 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 usually do not see the fungus on dry tissue, but the pycnidia are visible as black, pinhead-sized dots imbedded in tissue that is incubated in moist chambers overnight. Dieback results when the galls girdle the twigs. Otherwise, very little is known about the dis-ease cycle. The only control measures we can suggest are to prune out affected or dead branches and promote tree vitality through good horticultural practices.
Crown gall is probably the most common infectious gall in Illinois. That disease is discussed in Report on Plant Disease (RPD), no. 1006, “Crown Gall.” It is available in Extension offices or on the Internet at http://www. ag.uiuc.edu/%7Evista/horticul.htm. Fasciation, or leafy gall, is discussed in RPD, no. 619, available at the same site.