My friend Julie sent me a photo of lumps on manzanita leaves in her yard asking me what they were. I’ve seen many similar lumps and bumps on willows, poplars, oaks, apples and many other plants during my career as a horticulturist. My answer is they are galls. Galls can look knobby or warty, can be round or oblong. Sometimes they are quite attractive colors, from red, green and yellow to black. Some are like blisters; others like hairy felt-like growths. And, each type of plant may have its own gall-producing insect. Generally, it’s younger growth, rather than mature growth that is affected. Galls are actually a growing part of the plant and require the same nutrients the other parts of the plant need. Because of this, galls may take nutrients away from the normal parts of the plant and this could possibly slow plant growth. The next question is what causes galls. As in so many horticultural conundrums, the answer is “It depends.” Galls are basically abnormal growths and may occur not only on the upper or lower leaf surfaces, but also on other plant parts including roots and flowers, which can then be deformed in shape or size. They often are found on the stems of the leaves. Galls are most often caused by insects such as aphids, eriophyid mites, adelgids, midges, flies or wasps that are feeding or laying eggs on the plant. The insects’ saliva irritates plant cells, which in turn stimulates plant hormone development increasing cell size and/or cell numbers. If you cut a gall in half, you might see insects inside of it, where they have hatched and grown, protected from the outside world. On the other hand, galls can also be caused by plant disease organisms such as bacteria, fungi, viruses or even nematodes. The amount of gall production varies from year to year with weather and pest population size. While galls might interfere with normal plant functions, they are rarely a significant problem for healthy plants, except in appearance. Chemical control is not often effective, particularly without insect identification and clear knowledge of its life cycle, and is rarely necessary. When there are a few galls, they can be hand-picked or cut out and discarded. For information and great pictures of galls go to Cornell University at: http://idl.entomology.cornell.edu/wp-content/uploads/Galls-on-Plants.pdf. JoAnne Skelly is Associate Professor & Extension Educator Emerita at the University of Nevada Cooperative Extension.
