Managing Disease and Insects in Your Trees
After investing countless hours of hard labor and hard-earned dollars on tree landscaping, the last thing you need is to see your efforts ruined by an onslaught of disease and insects. The first step in protecting your valuable green assets is prevention.
Be sure to plant trees that are well suited to your location and that are resistant to common insect and disease infestations. Most insects and diseases take advantage of plants that are under stress so watering, mulching, and pruning are essential to preventing outbreaks.
Proper diagnosis is also key to treating the problem. The following are a few of the most asked-about tree diseases and insects in Ohio.
This disease makes your plants look as though they’ve been sprinkled with powder. Commonly found on crabapples, dogwoods, English oak, and catalpa trees, some mildews affect older leaves first. When new shoots are diseased, you’ll notice leaf curling, shoot stunting and twisting.
While this deforming disease definitely makes a tree less aesthetically appealing, the good news is that powdery mildew doesn’t usually kill. However, your tree may be less able to rebound during stressful periods like winter.
If you want to prevent powdery mildew from attacking your trees in the first place, keep water from standing on the leaves for long periods of time. Water only early in the day and thin surrounding vegetation to increase sunlight and improve air circulation around foliage.
Fungal leaf spots appear regularly in Ohio. They often cause homeowners endless worry, but their damage is usually minimal and fungicides are rarely necessary. These diseases generally winter on fallen leaves and then re-infect trees in spring. Tar spot and frogeye leaf spot are two of the most common.
Tar spot is very obvious as yellow-green circles on the leaves’ upper surfaces during spring. By mid to late summer, the circles become tar-like spots. Tar spot is found on maples, especially Amur, Japanese, red, and silver varieties.
Frogeye leaf spot affects most crabapples. Spots are roughly circular and develop into tannish spots with purple to red borders. Later in the season, the spots enlarge and the interior turns gray giving it a “frog eye” appearance. This is also a minor disease and treatment is usually unnecessary.
Since fungal leaf spots spread from the fruiting bodies on fallen leaves, the best protection is to rake up the leaves and remove them from the area.
These insects eat your tree’s leaves. Spring and fall cankerworm, tent caterpillar, gypsy moth, leafminers, and Japanese beetles are the usual suspects. Trees generally bounce back from an attack of these defoliators, but repeated infestations will weaken a tree and may eventually destroy it.
Boring insects tunnel into the stem, roots, or twigs of a tree. Some lay eggs and the offspring burrow more deeply into the wood blocking off the tree’s water-conducting tissues. If the infestation is serious, the upper leaves will be starved of nutrients and moisture and your tree could die. Be on the lookout for entry/exit holes in tree bark, small mounds of sawdust at the tree or branch base, and branch wilting and dying. Boring culprits are the Asian longhorn beetle, bronze birch borer, dogwood borer, two-lined chestnut borer, ash borer, and elm bark beetle.
These insects do their damage by sucking the liquid from leaves and twigs. One type of sucking insect called scales lives on the outside of a branch and forms a hard protective outer coating while feeding on the plant juices. Telltale signs of these pests are scaly formations on branches, leaf dieback and sticky, sweet excretions. These excretions often turn black when colonized by sooty mold fungus. Other sucking insects include, aphids, leafhoppers, spider mites, and thrips.
The best way to keep your trees healthy is to give Mother Nature a helping hand, or should I say “eye?” Visually inspect trees several times during the growing season and note any changes. Early detection helps prevent irreversible damage.
For more recommendations on treating diseases and insect infestations in your trees, check with your local urban forester, nursery, or county Extension office for advice, or go online to ohioline.osu.edu.