URBANA, Ill. – It's winter, and the deciduous trees outside are bare. It's a perfect time to examine your trees "au naturel", said Diane Plewa, a University of Illinois Extension diagnostic specialist.

"Because the trees are 'in the buff," it's a great time to look for damage to limbs and branches that become hidden once leaves cover the tree," she said

One common problem often seen with oaks is twig galls. These galls appear as round swellings on the branches of affected trees and are caused by a variety of parasitic wasps. There are a number of types of galls, caused by an even greater number of species of tiny wasps. Two very common oak twig galls are horned-oak galls and gouty oak galls, Plewa explained

Galls are tumor-like structures formed from tree tissue. Tiny, non-stinging wasps induce the tree to form the galls, which act as protection for the wasp eggs and developing larvae. While galls may not be aesthetically pleasing, they usually aren't harmful to healthy, well-established trees, she added.

"The galls can be pruned out of the tree, though that quickly becomes impractical as the tree grows," Plewa explained. "The general recommendation for oaks infested with galls is to maintain good tree health through watering during dry periods, fertilizing when needed, and responding quickly to other insect or pathogen problems."

Witch's brooms are another common problem. A witch's broom is a section of a branch or stem with a proliferation of slender, closely spaced twigs. This symptom can be seen on both herbaceous and woody hosts and can be caused by a wide variety of pathogens.

In trees, it's usually seen on sycamore trees and is caused by a pathogenic fungus. This disease, known as anthracnose, is a common disease that affects a wide variety of plants, though it doesn't always cause witch's broom in other hosts. On sycamores, anthracnose can also cause lesions on the leaves and cankers on branches.

"Twigs in witch's brooms tend to be thin and poorly spaced; as a result, they don't leaf out well and tend to break easily," Plewa said. "Much like oak galls, witch's brooms usually don't cause much injury to a healthy plant."

Management of these problems includes sanitation, or the removal of infected plant tissue (in this case, raking and bagging or mowing leaves, and removing twigs affected by the pathogen), and maintaining good tree health.

Plewa pointed out that trunks should be inspected for insect borer holes.

"There are a number of different borers in the state, some native and others invasive," she said.

"Probably the most (in)famous is the Emerald Ash Borer, an introduced pest that has decimated native ash trees across the Midwest. Emerald Ash Borer (or EAB) produces a distinct, D-shaped hole approximately 1/8 inch in diameter. Other native borers produce larger, O- or oval-shaped holes."

All American Ash trees should be scouted for EAB, according to Plewa. Additionally, EAB has been tentatively identified on white fringe trees in Ohio, a native tree to the United States. All fringe trees should also be examined for EAB holes.

Some systemic insecticides have been shown to be effective at protecting susceptible trees from EAB. Ideally, treatment is started before the tree is infested with the insect. If caught early, the insecticides can still be used to prolong the life of affected trees. The public can contact their local Extension office or the U of I Plant Clinic for a list of recommended insecticides.

Yellow-bellied sapsucker damage is often seen in winter. These migratory woodpeckers create large holes in trees, usually in evenly spaced lines around a branch or trunk. Initial holes are approximately 1/4 inch in diameter, and they may be enlarged over time. The damage looks dramatic and can negatively impact the health of the plant.

Plewa said hardware cloth or burlap can be wrapped around tree limbs or trunks to protect them against sapsuckers. Sticky bird repellants can also be used on trees to discourage sapsucker activity.

"Examining their naked trees can give owners a better idea of the overall health of the plants," she said. "This is also an excellent time to prune out dead or damaged limbs, branches that are crossing and rubbing against each other, and branches that attach to the tree at a small angle. So remember: don't be a prude; look at your trees nude!"