At this time of year, I receive pictures from Spring-Green employees of weird growths on leaves that are causing concern from their customers. Often times, the leaves have fallen to the ground and covered with weirdly-shaped structures growing out of the leaf surface. These are galls. Galls are abnormal growths caused by various organisms, such as insects, mites, nematodes, fungi, bacteria and viruses.
Galls can take on an assortment of shapes and colors, which is why customers become concerned. Fortunately, these galls rarely threaten the health of the plant. The reason why leaves often fall when there are numerous galls present is due to the sheer weight that is added to the leaf. The problem is purely cosmetic and control is normally not required or recommended.
An assortment of insects and insect relatives are the source for many of these galls. These organisms may often secrete a substance in the leaf, which reacts by increasing its normal plant growth hormones. This results in an increase in the size or number of cells, which is what causes the gall.
Most of the gall production occurs in the spring when leaves are first beginning to open. The organisms that form the gall often live within the gall itself as it develops around them.
One type of organism is called eriophyid mites. Different species of these mites can form spindle galls, bladder galls or velvet galls. Psyllids will form nipple galls or blister galls. Nipple galls form on the underside of the leaf surface. Aphids will cause a gall to develop on the stem or petiole of cottonwood or poplar trees. Adelgids are the source of galls on many conifers, including the Cooley Spruce Adelgid gall.
There are even tiny species of wasps that cause galls to develop on the leaves of some trees. One of the more interesting is the Jumping Oak Gall. These galls are formed by a sting-less wasp and effects White Oaks. The female lays a single egg in the developing leaf bud. When the egg hatches, the larva lives in and feeds on the gall tissue that forms around it. In the early summer, the galls will fall to the ground and the larva will jump in an effort to escape the gall, similar to the jumping of a Mexican Jumping Bean.
Galls are just part of nature and their formation usually does not affect the overall health of the tree or shrub. They may cause some leaves to fall, but they are going to fall anyway, so don’t let them bother you.