If you have any hackberries around your house, you have probably noticed falling leaves that have little protrusions on them.
It looks strange and serious but rarely does it cause damage to the tree. What you are seeing is a type of gall. Referred to as spindle or nipple gall, the spindle-like protrusions on the leaves are caused by mites that gather around buds in the spring and enter the leaves as they unfurl.
As the mite enters the leaf it injects a substance that scientists believe to be a growth promoting hormone, which causes irregular tissue growth that ultimately protects her eggs.
Like the spindle gall mite, the bladder gall mite infects leaves in the spring, but the resulting galls are irregular shaped protrusions that, yes, look like bladders.
Since the infection takes place in early spring, you can use dormant oil just before bud break to smother the offending mites.
The galls rarely do harm to the trees they infect so the issue is really about appearance.
Galls come in many forms. There are root, crown, trunk, twig, flower and leaf galls. They are caused by mites, non-stinging wasps, flies, aphids, and fungi.
It really just depends on the plant because certain insects will only lay their eggs on certain plants and certain plant parts. For example, the witch hazel leaf gall aphid causes galls on witch hazels then moves over to nearby birch trees where it feeds all summer. It has a very specific habit which warrants its very specific name.
The galls that most people are familiar with are those golf-ball sized growths that appear on the branches of many oak species. These oak galls are caused by non-stinging wasps (called gall wasps).
There are different species of gall wasps that cause different shapes and sizes of galls.
Some galls are perfectly round; some are star-shaped, spiny or prickly.
The galls begin to form when the respective wasp or fly lays her eggs on the twig.
First she injects the growth stimulating chemical, then the eggs, and as the eggs hatch into grubs the gall forms around them as a hard, protective encasing. The grubs continue to feed and mature to either emerge to produce additional generations or over winter in the gall until the following spring.
These types of galls typically don’t cause damage to the tree but they may cause tip die back if the galls form on newer tip growth.
Our best bet it to cut away galls as soon as we see the first signs of them.
Years of going ignored can result in a tree covered in galls, as each new generation re-infests the tree. This usually will not kill a tree alone and is considered more unsightly than anything else.
Although most galls are caused by insects there are some exceptions. There are a few caused by fungi, including eastern gall rust. This disease causes the main trunk or main branches of oaks to swell.
The swellings caused by eastern gall rust disrupt the flow of nutrients in the tree, can weaken or stunt the tree’s growth and can encourage witches’ broom.
The fungus needs both pines and oaks to complete its life cycle (although the galls only infect the oak species) and it spreads by fungal spores so remove infected branches as soon as you see them.
Crown galls form at the trunk base of ornamental and fruit crops mostly.
Usually the bacterium that causes crown and root galls enter the plant through wounds; or the plant was infected before it was planted.
These types of galls disrupt nutrients and water flow so the plant’s appearance is usually compromised. All in all, there is not much to be done to control galls. Pruning them out may be reasonable in some cases but not in others.