Why Does My Ash Tree Look Unhealthy?

Are the leaves yellow, branches, and leaves are covered with small bumps?

Does the trunk or the branches have small holes, limbs die, or break off?

Have the leaves been skeletonized or with large holes, branches may be webbed?

Do the branches have small masses of galls?

Do the leaves have spots on them?

Does the trunk or the branches have oozing lesions, branch tips die back?

Do the leaves have raised, rusty spots on their undersides?

Probably the most serious pests of ash trees are scale insects, with oystershell scale as the most important, especially in the Midwest. Putnam scale, San Jose scale, and soft scale also occur. Scrape off minor infestations with your fingernail. Prune out badly infested growth, or use a soft brush and soapy water to gently scrub the scales off the stems. Control scales by spraying insecticidal oil during the growing season.

Numerous borers attack woody plants, mining the inner bark and wood of branches and trunk. Prune off borer-infested branches. If you see borer holes in your trees, probe into them with a flexible wire or inject a solution of parasitic nematodes. After treatment, seal holes with putty.

These symptoms indicate caterpillars. Several kinds feed on foliage, including tent caterpillars and webworms. Control all of these pests by manually destroying their nests or egg cases, spraying leaves with BTK at the first sign of damage, or applying pyrethrin spray as a last resort.

Ash flower gall mites are tiny insects that attack male flower clusters on white ash (Fraxinus americana). Infested flowers develop abnormally, producing galls that dry and remain on the tree. Spray branches and galls with dormant oil in late winter for control.

A large number of fungi and bacteria cause spots on ash tree leaves, in a variety of colors, shapes, and sizes. Fortunately, the same controls are effective against many leaf spot diseases. Pick off infected leaves. Rake up and destroy fallen leaves and branches in autumn to eliminate overwintering spores. To prevent the spread of mild infections, spray leaves with Bordeaux mix; repeat twice at 10-day intervals.

Oozing lesions are caused by cankers. Cankers can be caused by several different kinds of fungi. Mildly affected plants will recover if you remove and destroy all affected branches. Heavily affected plants can't be cured and you should remove the whole plant. Avoid damaging your plant with lawnmowers, trimmers, or pruning tools because the wounds are the best place for cankers to start.

Raised, rusty spots on the undersides of the leaves are caused by a fungal disease Rust. Leaves may yellow and die. Rust usually doesn’t harm the tree much, except for detracting from the appearance. To control this disease, rake up and destroy fallen leaves. To prevent the spread of mild infections, spray leaves with sulfur early in the season.

Sawflies are known pests on ash trees. They are closely related to bees and wasps. The larvae are the most destructive stage, feeding on and skeletonizing plant leaves or completely defoliating the plant. Control larvae as soon as you spot them by handpicking or spraying leaves with insecticidal soap. Use pyrethrin for severe infestations.