Aug. 10--I found this odd growth on some of my roses. What is this? Are my roses in trouble? What steps should I take to fix this problem?
-- Tom Harris, Evanston
The picture you sent shows a condition called mossy rose gall. It occurs on roses and causes mossy-looking lumps that can reach the size of golf balls. The gall consists of plant tissue that has built up in response to the irritation of larvae that hatched from eggs laid by a particular type of wasp.
Like most insect galls, mossy rose gall will not seriously affect the vigor of your roses if the plants are healthy. There is little reason to worry, but if you are concerned, simply prune out the stems bearing the galls.
The galls cannot be cured after they have formed, so spraying will not make any difference.
Galls come in a wide variety of forms, textures and colors and can be found on many different kinds of plants. They are abnormal growths caused by various organisms including insects, mites, nematodes, fungi, bacteria and viruses.
Many plants have an intricate association with a particular kind of gall-maker. The gall-maker causes the plant to change its normal course of growth and modify growing tissue into a swelling that surrounds the gall-maker.
In the case of mossy rose gall, the gall-maker is a cynipid gall wasp that lays eggs on a rose stem. About a week later, the larvae hatch and begin to feed on the leaf bud tissue. This causes the plant to begin growing layers of tissue around the larvae to form the gall.
Once the gall forms, the larvae feed on the plant tissue inside it. The gall also provides protection from adverse weather, predator insects and parasites. The larvae will eventually become pupae and overwinter within the gall while they transform into adults. In spring, the adult wasps will emerge from the gall.
Eventually, the abandoned galls will turn grayish-brown. If they are not pruned off, they can remain on the branches for one to two years before they degrade and disappear.
A very heavy or prolonged gall infestation on your roses may weaken or kill portions of the plant above the gall, but this is rare.
Prune out the galls if you find them unsightly. Provide additional water during the rest of summer as needed and fertilize next spring to improve the health and vigor of your roses. A light layer of mulch also is good for them.
Johnson is senior director of horticulture for the Chicago Botanic Garden in Glencoe; ctc-realestate@tribune.com.
