Ask A Master Gardener – You’ve Got a Lot of Galls

Gardening season is getting off to a good start this year and soon the Dodge County Master Gardener Help Line will begin to receive inquiries about “bumps” on the leaves of maple and oak trees.  Many people assume the trees have a disease and want to know what is necessary to control the problem and restore the tree to good health.  Surprisingly enough, these noticeable bumps are not a disease, but are insect related and are called galls.

Galls are abnormal plant growth caused by not only insects, but by fungi, mites, nematodes, bacteria, and viruses.  Often, they do not affect the health of the plant, so treatment is unnecessary.

Galls form in late spring when new growth is accelerated, hence older, more mature parts of the plant are rarely affected.  Leaf galls can appear on both the upper and lower surface of the leaf.  Stem and twig galls will form either a swelling or a knot-like growth.  Flower galls will deform the flower and the bud.

Gall forming insects live most of their lives inside the gall, protected from any pesticide.  Therefore, timing is difficult for any type of pesticide application.  The best method of control is to simply tolerate their presence since they will not harm the tree.  Cleaning up any leaf debris in the fall will help limit the spread.

One of the most common inquiries that comes into the Help Line is red, green or black lumps on maple leaves, called maple bladder gall.  The lumps look like small warts and result from maple bladder gall mites.  The mites overwinter in the bark of the tree and once the tree buds begin to swell, they move to the new leaves to feed.  The tree develops galls in which the mites live, feed, and reproduce.  The galls begin as red “warts”, turn green, and then black.  The leaves can deform and drop early, but these galls do not harm the tree, even they do look a bit shocking.  No treatment is necessary; the mites will migrate back to the bark to overwinter and the process will repeat itself the following year.

Another common inquiry involves bumps on oak leaves, twigs and stems.  These are caused by tiny wasps that have an interesting life cycle.  The adult wasp emerges from stem galls in the spring to lay eggs the first year in new leaves.  The eggs hatch and the larva feeds along the leaf veins causing the leaf to respond by producing galls that are initially green, turning to dark brown.  Around mid-summer, adults emerge from the leaf galls and lay eggs back in the young twigs where more galls form.  After about a two-year period, those wasps emerge and the cycle repeats.  This stem gall formation will eventually girdle the stem and cause branch die-back.  Although there is no effective pesticide control, it is advisable to cut away all visible twig galls during the first winter.

There are more galls, but these are the two most commonly seen by the Help Line.  For further information about galls and their causes, please read the bulletins “Deciduous Tree Galls” from the University of Wisconsin Extension or “Insect and Mite Galls” from the University of Minnesota Extension.

Carol Shirk

Certified Master Gardener




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