10. Plant Galls
A common diagnostic misconception is that galls on plants are due only to insects, or that all galls are bacterial crown gall disease. The fact is that galls may be caused by many different factors. Webster's defines galls as: "A tumor on plant tissue caused by irritation due to fungi, insects, or bacteria." That is a good start, though not complete.
Insects, indeed, do cause many galls, for example, reportedly causing more than 800 characteristic galls by different insects (mostly species of wasps and flies) on oak alone. Most of these oak galls are of little concern to the health of these affected oaks, but some, such as the horned oak gall and gouty oak gall, are more serious because they occur on stems rather than leaves. Insect galls commonly occur as a plant response to egg-laying by the insects. Obligingly, the plant produces the gall that then becomes the home for the developing insect egg and larvae.
The best-known plant gall is bacterial crown gall, caused by the bacterium Agrobacterium tumefaciens, which reportedly occurs on more than 600 species in over 90 plant families (most prominently for horticulturists on rose, euonymus, and the stone fruits). This bacterium typically enters plant tissue through root wounds, then transmits a little chunk of bacterial DNA to the host plant root cell. This bacterial DNA then causes the plant to produce too many cells and cells that are too large, and voila - the tumor develops, potentially disrupting the orderly vascular flow in the area where the gall develops.
But these well-known galls are only the beginning. Many fungi also produce galled tissue, such as the fairly unusual roundish, half-inch diameter, wart-like growths of Phomopsis fungal gall on forsythia, and the very common blackish galled areas on the stems of plums and cherries caused by the black knot fungus (Dibotryon morbosa).
And common galls that even Webster ignored are those caused by eriophyid mites, such as the maple bladder gall mite, the maple spindle gall mite, and the mite that causes ash flower galls. There are even "galls" that appear to have no relationship to infectious disease organisms or insect or mite incitement. Hemispheric burls in wood and other stem-swellings may be due, in the words of Sinclair, Lyon, and Johnson's Diseases of Trees and Shrubs, to simply the plant response to "environmental insults" such as freeze injury. So, the next 20 times you see plant galls, try to learn from peers, books, samples to the PPDC, and other sources, which gall it might be.
Ohio State University Extension Research
Ornamental Plants Annual Reports and Research Reviews 2000
Special Circular 177-01