I first spotted the curious little structure on a willow oak growing at the edge of my yard, where the lawn merged into marshland. Except for its color, the brownish grey object resembled a battered ping-pong ball wrapped around a slender branch of the tree. Was it alive or dead? An outgrowth of the oak, an insect’s creation or evidence of disease?
The more I examined the tree, the more of the mysterious balls I found. I hesitated to touch them, fearing there might be something unpleasant inside that could sting, bite or otherwise disturb my peaceful existence on a beautiful spring day. So, I pruned them off — this is the perfect time of year for pruning – tossed them into the burn barrel and turned to a few gardening books for elucidation.
Call me heartless, but gardeners – particularly old ones – learn to trust their instincts. Mine told me this looked like the work of bugs and not butterflies.
Hours later, I can report that what I found were galls, defined in one of my books as “an overgrowth of plant tissue, sometimes tumor-like,” caused by insects or bacteria. As with most topics in horticulture, the more you delve, the more fascinating facts emerge.
It turns out that most galls are caused by insects or mites. How do they do this? By producing chemicals that cause the plant to create a structure – the gall – in which the creatures live. (Now I feel like a murderer. But what about those manipulative bugs, forcing the tree to do their bidding?) Some galls contain only larvae, or immature bugs. Others house both adults and offspring.
Reading further, I came across the comforting news that galls caused by insects rarely do serious damage to plants and can be controlled easily by pruning off the affected part of the plant. Gardeners are advised to burn what they’ve cut off “quickly” or bag the material up for the landfill and not to put it in a compost heap.
That “quickly” made me feel a bit uneasy, as did the realization – gardener’s instincts at work again – that bacteria-caused galls might be a more complicated story. At any rate, if a gall is dried up and you see exit holes on the surface, it means the insects have matured and left home.
Further reading uncovered the advice that if pruning failed to stop further galls from appearing, a gardener could remove them, then spray the plant with organic insecticidal soap, hoping to kill any insects that remained. This should be done when new growth starts in the spring, meaning any day now.
Galls, whether caused by insects or bacteria, come in all sorts of shapes, not only round. They appear on twigs, leaves and roots, as well as on branches, and disfigure trees and shrubs, alike, both deciduous, like forsythia and oak, and evergreen, like azalea.
They can look like moss, lace or yarn, as is the case with mossyrose gall, found on rose leaves. They can crop up in the form of small bristly green balls stuck to a leaf — spiny rose gall, for example. They may even appear as bright red, swollen areas which turn out to be part of the original leaf, repurposed.
On shade trees, twig and stem galls are the work of gall wasps, according to the University of Maryland Master Gardener Handbook. This form appears most commonly on oak trees and should be pruned out, especially if the victim is a pin or willow oak. Gall can cause entire parts of a tree to die back.
Galls on leaves may show up as raised nodules in various colors and shapes. Eriophyiid mites, tiny creatures generally not visible to the naked eye, may be the culprits, as might gall midges and gall wasps. The Handbook says it’s not necessary to control these; but if you want to, pull off the galls when they’re green and destroy them. Then, spray the plant with horticultural oil and dormant season sprays, which might kill off the number of mites and insects that make it through the winter.
Turning to evergreen shrubs, exobasidium gall will cause swollen or puffy areas on newly expanded leaves and cannot be stopped with chemicals. The best policy is to prune out the infected parts. When buying new plants, gardeners should look for cultivars resistant to gall.
Stem galls on evergreens and deciduous plants come from crown gall, a bacterial disease which makes abnormal tissue grow along stems and can discolor large areas of the foliage. Gardeners should prune out the affected parts or remove the plant.
Crown gall, which lives in the soil and enters a plant through wounds, is a much more serious problem than insect-caused galls. On nut trees, for example, it causes warty growths on the trunks and exposed roots and leads to the tree dying prematurely. There is no way to control the disease, so gardeners should take care to avoid damaging the trunks and roots, for example when mowing.
Small fruit shrubs can get crown and cane gall at the base of the plants, as well as on the roots and canes. The disease stunts their growth and results in spongy, rough swellings that become woody and dark with age. The only thing to do is remove the plant.
Knowing that bacterial infections cannot be cured, the best course is to do everything possible to prevent them from spreading to other plants.
When buying plants and seeds – which you’re likely to be doing at this time of year – look for disease-resistant varieties. Check seed packets and talk to your local Extension Service for advice. In the meantime, happy gardening!
Editor’s Note: Laetitia Sands is a master gardener in Dorchester County.