Plant galls come in a fascinating variety of strange forms, textures and colors. Some are irregular, bumpy, or warty; others are smooth and spherical. Some galls sport thick growths of fuzz, hair or spines. One of my favorites is the aptly named mossy rose gall - it occurs on rose and looks like moss. It never ceases to amaze me that a rose plant can produce a growth that looks like moss just because of a tiny wasp larva.
Galls result from an intricate interaction between two living organisms. The gall-maker (insect disease, mite) causes the plant to change the course of normal growth and modifies growing tissue into a special swelling that surrounds the gall-maker. In the case of mossy rose galls it is a small cynipid wasp.
The gall provides some protection from adverse weather, predators and parasites. Galls are rich in proteins and carbohydrates. Thus the gall-maker lives a sheltered, luxurious live surrounded by a ready source of food in which it grows, develops and may even reproduce. The new adult gall-makers emerge from the galls in the summer or the following spring.
Despite their unsightly appearance, most insect galls do not seriously affect the vigor of healthy plants. Mossy rose galls formed on the leaves of the rose are harmless. Occasionally, a heavy or prolonged gall infestation on the rose stems may weaken or kill portions of a plant above the gall. If mossy rose galls occur on the stem they can be pruned out.
Galls cannot be cured after they have formed. That is, spraying or treating does not make them go away. Preventive treatments applied before the galls form may be effective but are not usually practical.