Trees and Shrubs: Diseases, Insects and Other Problems
Despite providing our trees and shrubs with all their necessities, diseases and insects can still cause problems. In most cases, however, healthy mature trees and shrubs are able to defend themselves against minor insect and fungal disease exposure without intervention by us.
Trees and shrubs can tolerate substantial foliar loss caused by feeding by insects or foliar fungal disease. In fact, healthy plants can have every single leaf eaten off and still survive! Plants have latent buds which do not sprout into leaves unless needed. So, of a leaf is eaten away or extensively damaged by a disease, the latent bud sprouts to form a new leaf. If a leaf is only half eaten away, the remaining green tissue will continue to conduct photosynthesis and provide food for the plant. Because trees and shrubs have so many leaves, loosing a few, or parts of many leaves, is really not harmful.
However, if a tree or shrub is subjected to repeated defoliation by insects or disease, it can stress the plant and make it more susceptible to infestation by other diseases and insects. Severe, repeated insect or fungal infestation may warrant treatment.
Diseases and Insects are less likely to attack healthy trees and shrubs. However, a plant that is already under stress for some reason, such as drought, flooding, climate, poor pruning, etc., is much more likely to have problems with insects or diseases.
Young, newly planted trees may also be at higher risk for damage from diseases or insects. While large trees are usually able to handle the stress of defoliation by fungal diseases or insect feeding, young trees with few leaves are more susceptible to long-term damage. Because young newly planted trees are already stressed from being transplanted, insecticide or fungicide treatments may be warranted to reduce further stress.
Diseases and Environmental Problems
A wide variety of problems can afflict trees and shrubs, including diseases caused by microorganisms such as fungi and bacteria (biotic), or symptoms caused by environmental or cultural conditions (abiotic). Keeping a close eye on plants will allow you to detect problems early, when remedies will be most effective. If a plant is unhealthy, some detective work is in order to determine the cause.
Diagnosing Plant Problems
In many cases, similar symptoms can be caused by a wide variety of reasons. It is important to carefully assess an unhealthy plant to narrow down the cause of the problem. Often, a tree or shrub is suffering from more than one problem--a stressed plant is often the target of secondary invaders which may not have taken hold had the plant been healthy. In addition to insects, trees and shrubs are the victims of diseases, poor fertility, climate extremes, incorrect soil moisture conditions, environmental pollution (air, soil or water), and physical injuries.
"Symptoms" are displayed on the plant as a result of a biotic or abiotic problem. It isn't possible to see the organism itself, but you can see the plant's response to the problem. For example, wilting leaves may be the result of a root rot fungus that is preventing the roots from taking up moisture. You see the symptom--wilting leaves--but not the fungus itself.
"Signs" are direct evidence of the cause of the disease. You may see mushrooms growing around the base of the plant. You see actual parts of the fungus--mushrooms--that is causing the root rot.
When assessing an unhealthy plant, look for any and all symptoms and signs that may be present. Thoroughly observe the leaves, stems, branches, trunk and, if possible, roots.
Consider the history of the plant, since often symptoms do not appear until years after the cause. For example, construction damage to a large tree's roots may not cause dieback or tree death for several years, as the tree tries to hang on and grow new roots.
Leaves and needles
Leaves or needles on trees and shrubs are often indicators of the plant's overall health. Not only are there a variety of diseases that affect the foliage itself, but many root or vascular system problems often are reflected in the leaves or needles. Through photosynthesis involving green chlorophyll, leaves manufacture the food the plant needs, so it is important that they be healthy.
Because there are usually so many leaves on a tree or shrub, a fair amount of damages can be sustained before the overall plant's health is affected. However, if defoliation occurs season after season, the food reserves can be depleted making the plant more susceptible to other disease and insect problems.
Some common problems that occur with deciduous tree and shrub leaves include:
Off color or yellow leaves:
- Several things may cause yellowing leaves on trees and shrubs. On young leaves, if the leaves' veins remain green but the leaf tissue between leaves is lighter or yellow, it may be a micronutrient deficiency, such as iron chlorosis. An overall light green or yellow leaf appearance may mean a deficiency of major nutrients, such as nitrogen, potassium or phosphorus. Improper pH, root injury or construction damage could cause nutrient deficiency symptoms to occur. A soil test by a qualified lab is the first place to start if you suspect a nutrient deficiency or toxicity. If a soil test rules out a nutrient problem, disease possibilities must be investigated.
- Brown areas or spots on leaves are perhaps the most common symptoms found on trees and shrubs. They can be caused by fungal, bacteria, or insect attacks. They can also be caused by drought, pollution, herbicide or salt damage, or sun scorch.
- If the brown (necrotic) area is along the leaf margins or between veins, it is possible drought is to blame. Hot, dry weather can trigger these symptoms, as can a buildup of road salt in the soil. Newly transplanted trees and shrubs are highly susceptible to this type of injury.
Spots, irregular-shaped necrotic areas or bumps on leaves:
- This may signal fungal or bacterial attacks. Sometimes, spots may be rust colored, yellow, or red. Almost ALL plants will get brown areas on their leaves at some time in the growing season. Usually these symptoms affect just the leaves and cause no permanent damage to the plant, but are somewhat unattractive to people. If, however, a plant is severely infected year after year, control may be warranted due to stress being endured by the plant.
Twisted or curled leaves:
- This often is an indication of herbicide damage. Even if sprayed on calm days, herbicides can volatilize into invisible clouds after being sprayed, especially in hot weather. These "clouds" can float through the landscape, causing damage along the way. Pathogens such as fungi or insects can also cause curling leaves, as can unfavorable weather. Trees usually recover from twisted or curled leaves, because latent buds are not affected.
- Ragged leaves may be caused by insect feeding or by cold weather damage. Buds damaged by cold may produce tattered looking leaves. These symptoms can also be caused by leaf-eating insects, such as beetles or caterpillars. In any event, trees and shrubs typically recover just fine from this type of injury, and no treatment is necessary.
- Cold weather and frost can also cause leaves to suddenly turn black or brown. If no recent temperature extremes have occurred, a leaf or stem disease may be present. Check stems, twigs, branches or trunks for damage or signs of the presence of a pathogen.
Early fall color:
- In trees, this can be symptom of a serious compromise of the vascular system, through which nutrients and water are conducted throughout the tree. The vascular system may have been damaged by direct physical injury to the trunk (such as from a lawnmower, string trimmer or animal feeding), by a bacterial or fungal canker, by girdling roots (possibly because the tree was planted incorrectly) or by a fungal pathogen blocking the vascular system. Other symptoms caused by these problems include leaves that are smaller than usual, dieback of some branches, or a higher production of seeds.
Sudden leaf drop:
- Finally, a tree or shrub may suddenly drop a great many or even all of its leaves. There are a variety of things that may cause leaf drop, including weather extremes, insect feeding, foliar or vascular disease pathogens, drought, or even squirrel damage. Often, it is impossible to determine the cause of sudden leaf drop.
- Evergreens with needles that are browning starting at the tips are often suffering from drought. Unfortunately, the browning symptoms often don't appear until long after the drought conditions occurred. Too much moisture in the soil can cause the same symptoms, as waterlogged roots can't get enough oxygen and are unable to absorb moisture. Root rots can also compromise the root system, making it difficult for the plant to take up moisture.
- Random spots appearing in the mid-section of needles often signals the presence of a fungal disease. However, since most fungicides are preventative, once the symptoms have appeared, it is often too late to treat for the disease. Have the disease positively identified to determine if treatment is possible or warranted. There are also a variety of fungal diseases that cause shoot damage at the ends of branches, or the death of entire needles.
A - Bark; B - Phloem; C - Cambium; D - Xylem; E - Pith
The vascular system of a woody tree of shrub
The outside edge of the trunk is a corky bark, which serves to protect the inner part of the trunk. The next ring is the phloem, a thin ring that transports sugars produced in the leaves downward for the roots to use, or to store for winter. The next ring is the cambium, which is a ring of cells that is actively dividing. As these cells divide, the trunk or stem grows wider.
The next inner ring is the Xylem, which conducts water and nutrients from the roots up to the top of the plant, for use in leaves, twigs and buds. The xylem and phloem are produced by the cambium, which is the only actively growing area of a trunk. Finally, the center of the trunk or stem is the pith, which is technically dead cells that lend strength and structure to the trunk or stem. Eventually, these dead cells decompose which is why older trees are often hollow inside.
Any damage to vascular system (xylem, cambium, phloem) of a plant can have serious consequences to the plant's health. The following are some problems that can occur:
- This can have serious affects on trees and shrubs because it protects the important vascular system from diseases, insects, weather extremes, and moisture loss. Any physical damage, such as from lawnmowers, storm injury, animal feeding, or poor pruning, can open the plant up to a variety of threats.
- Girdling occurs when something damages the bark and inner vascular system of the plant around a significant portion of the circumference of the stem. Common causes of girdling are lawnmower or string trimmer damage, rope, wire or twine left around the base of the tree, animal feeding, or rubbing on a branch or trunk by other branches. Boring insects can also girdle a tree by getting under the bark and feeding on the inner wood. Diseases can cause cankers (holes in the bark and inner wood) that girdle the vascular system.
- Girdling symptoms include smaller than normal or fewer leaves, dieback of some or all branches (especially on the side of the tree where trunk damage has occurred), wilting and dropping of leaves, early fall color, or unusually high seed production. If these symptoms occur, inspect the trunk or branches for damage. Once a tree or shrub has suffered significant damage, there is not much that can be done to repair it. Trees and shrubs will often try to bypass the damage, and their success will depend on the severity of the problem. If cankers caused by disease pathogens appear on branches, they can be pruned away to prevent the pathogen from invading the rest of the plant.
Damage by animals:
- Animals, especially rabbits and rodents, commonly chew on bark and girdle young trees and shrubs. Finally, roots can grow in toward the trunk instead of outward, and if they come in contact with the trunk just below the soil surface, they can girdle the trunk over time. This is a common occurrence when trees are not planted correctly.
- Vascular wilts are caused by pathogens that get into the transport system of the plant (xylem and phloem) and plug it up, preventing moisture, nutrients, and carbohydrates from moving around in the plant. These diseases are usually fatal once they are in the main stem or trunk of a tree or shrub, and prevention is difficult.
- Common vascular wilts in trees are Oak Wilt, Dutch Elm Disease, and Verticillium Wilt. If you peel back the bark or cut into a twig of a woody plant you suspect has a vascular wilt, you will often find a discoloration of the inner wood just below the bark.
There are other trunk injuries that can cause problems trees and shrubs. There severity can range from mild, where the plant heals over and recovers, to severe, causing the tree to decline over time and die.
- This is damage to the bark and cambium caused by warm winter sun, which warms the trunk causing cells to become active only to be killed by cold temperatures once the sun sets. Smooth barked, young trees (such as maple and cherry) are especially susceptible to sunscald, and should be wrapped with a tree guard each winter for protection.
- Cracks can occur on tree trunks, usually at the site of an old injury. Over time, frost cracks can progress to the point of splitting the trunk and disrupting the vascular system. Typically, frost cracks close up in the warmth of summer, and put on a layer of scar tissue. Over time, frost cracks can be significantly raised from the buildup of scar tissue.
- Cultural conditions and occurrences such as lightning can also cause severe damage to a tree's vascular system. Some trees can survive or recover from lightening strikes, but it depends on how severe the damage is. Oaks struck by lightening have a better chance of surviving than maples, and other considerations (such as health of tree and severity of the initial damage) will determine if the tree makes it. Typically, the bark is split and becomes loose on a tree that has been struck by lightening.
- Fungal conks, or shelf fungi, or any type of mushrooms growing on the bark or at places where branches were previously removed indicate that the tree is hollow inside. This is normal, as decay fungi break down the pith in the center of the tree. These fungi do not harm the tree, but only live on already dead tissue in the tree. No action is necessary, and it is not recommended to fill hollow trees with any substance.
- Soil conditions:
- Besides girdling roots, the most common root problem is caused by wet soil conditions. Most trees prefer well drained soil, and if conditions are too wet, fungi can cause the roots to rot. Changes to the terrain around existing trees, such as from construction or re-grading, can cause the water drainage to change.
- Construction damage to roots is also a common cause of tree decline or death. Symptoms of dieback, early fall color, or small leaves may not appear for several years after the damage was caused, and there is no treatment once the roots are damaged. Adding soil over existing tree roots will also cause root death.
Galls and burls:
- Wood "galls", often called "burls", can form on woody plants and are usually caused by bacteria invading the plant and affecting DNA. These galls can be on tree trunks or on branches, and look like odd, corky balls. Pruning them away is the only cure. Because the galls are actually tree wood with a disorganized grain, they are often used to make furniture and other types of art. Treatment
Before any treatment can be done, it is imperative that a proper diagnosis be made. Because different causes create similar symptoms, it might be necessary to enlist the help of experts to assist with the diagnosis.
Start with your Regional Extension office for help. If necessary, they can assist you with the submission of a plant sample to a reputable plant disease diagnostics lab such as the University of Minnesota Plant Pathology Clinic. Pathology experts can isolate pathogens if they are present and provide you with a definite diagnosis. Certified commercial arborists, foresters, and plant health specialists can also be helpful.
Once the problem has been diagnosed and a treatment recommended, follow any treatment recommendations carefully. If the use of pesticides is warranted, be sure to follow all label instructions to protect yourself, the plant, and the environment.
For additional information on common diseases that affect trees and shrubs in Minnesota, Wisconsin and the upper Midwest:
Trees and Shrubs: Insects
At any given time during the growing season, if you look closely at your trees and shrubs, you will be able to find insects present. But, most of those insects are harmless, or even beneficial to the tree. Most insects cause damage for just a short time each year. However, some insects can cause serious damage or death if conditions are right.
If you see insect damage on your trees or shrubs, your first step is to inspect the plant for signs of insects. It is extremely important that you identify the insect. It's possible the insect you find is a beneficial insect that just finished consuming the insects that actually caused the damage. In some case, the insect that caused the damage has completed its lifecycle and is no longer present by the time the damage is noticed.
Because trees and shrubs can tolerate substantial leaf loss, consider the circumstances before deciding to implement insect control measures. Leaf loss late in the season, such as August or September, is less likely to impact the overall health of the plant because the plant is beginning to prepare for winter and lose it's leaves anyway. If the plant is a new transplant, or has been subjected to some other stress such as drought, flooding, or previous defoliation, insect control may be warranted.
Insect pests can be grouped into two major categories:
- those that feed on leaves
- those that invade bark and trunks.
- Leaf miners are very small insects that get between the upper and lower surfaces of a leaf. They feed on the leaf tissue inside of the leaf, and leave either serpentine "mines" winding throughout the leaf, or brown blotches where the inner leaf tissue has been removed. You may also see small black particles between the leaf tissue, which is the insects' fecal matter, or frass. There may be only a few leaves on a tree or shrub affected, or the entire tree can turn brown if there is a heavy infestation. Oaks, hawthorns and birches are most often attacked by leaf miners. Heavy or repeated infestations may not kill the plant outright, but may make it more susceptible to other problems.
- Chewing insects will eat entire parts of the leaf. They can feed on the edges of leaves, chew holes in the centers of leaves ("shotgun" holes), skeletonize the leaf (eat tissue between veins), or "window feed" (eat away only the upper or lower surface of the leaf.
- Most leaf eating insects are in their larval, worm-like stage. These caterpillars or worms can be as small as 1/4 inch long or as big as 4 inches long. They come in a variety of colors and shapes, often having stripes, spots, or hairs. They may also create shelter for themselves by spinning webs or rolling leaves around them. Keep in mind than many caterpillars are the immature stage of butterflies, so you may want to tolerate the damage caused by the caterpillars in exchange for having butterflies around later in the season!
- Many beetles also eat tree and shrub leaves. If you notice holes or leaf margins disappearing on your plants, check for leaf eating insects. If you don't see any present, try going out with a flashlight at night. June beetles, for example, can feed heavily on oak and birch trees but are only active at night.
- Sucking insects feed on plants by sucking the juices out of leaves or stems. Common sucking insects are aphids, scale, and mites.
- Heavy feeding by aphids may cause leaves to curl or crinkle, wilt, or take on a yellow or bronze color. You may also find a sticky residue on the plant, called "honeydew". Honeydew is a sugary substance excreted by the insects. Sometimes the honeydew becomes moldy and the plant takes on a sooty look. Ants may also be present, because they are attracted to the honeydew. Almost all plants can be attacked by aphids.
- Scale often feed on twigs and stems of plants, can cause the plant to look off-color and/or show early fall color. Scale come in a variety of sizes, shapes and colors. The adult stage is usually a somewhat hard-shelled or cottony "bump" on twigs that are immobile and can be often be controlled by dormant oil sprays. Young scale, called "crawlers", are present for a short time each year but are more susceptible to pesticides because they lack the hard shell protection adults have. Ladybugs are natural enemies of aphids and scale. Scale are common on maple, ash and oak.
- Mites attack almost all trees and shrubs. Plants that are stressed, particularly by heat or drought, are especially susceptible to mite infestation. Overuse of insecticides tends to increase mite populations. Mites are very small and usually feed on the undersides of leaves. Shaking a leaf over white paper may dislodge them, and you will see what looks like moving dust on the paper. They may be red, black, gray, or some other color. Leaves damaged by mites tend to look silver, bronze or yellow. Mites, especially on large shade trees, are typically controlled naturally, by predators or weather changes.
- Other sucking insects that may damage plants are lace bugs, plant bugs, and leafhoppers. Yellow or brown spots on leaves may indicate feeding by these insects, but can also be caused by fungal diseases.
- Galls are unusual growths that form on the upper or lower surface of leaves. They can be round or irregular shaped bumps, small pouches, or "spindles", and they are found in a variety of colors, including red, purple, green, black, and orange. These deformities are typically caused by feeding or egg laying of one of a variety of insects. While unattractive to us, galls rarely cause any harm to the plant. To reduce insect numbers for the following year, rake and destroy infected leaves in fall.
Bark and trunk invaders include borers that get into or below the bark and tunnel around in the wood. Most borers are not a problem on healthy trees, but act as "secondary invaders" attacking trees that are already weakened from some other reason.
Borers tunneling in the wood cause damage to the vascular system and typically cause wilting leaves and branch dieback. Thinning foliage and early fall color can also be a symptom of borer damage. Damage may appear suddenly, or the tree may slowly decline over several years.
"Galleries" from Elm Bark beetle under bark of dead elm.
Borers can be diagnosed by finding emergence holes on the trunk or branches. As the boring insect exits the plant, it will leave a hole depending on the size of the insect. Emergence holes can be pin-hole size to a quarter inch. Often holes are D shaped, and removing bark near an exit hole will reveal feeding tunnels.
Borer control is difficult because insecticides do not penetrate the bark. These insects must be controlled before they lay eggs on the bark. Keeping trees healthy is the best defense against boring insects.
Other insects may inhabit tree bark or live underneath it, such as beetles, ants, or maggots. These insects usually do not harm the tree, but are only there if the tree is already diseased or dying from some other cause. Carpenter ants may also live in trees, but typically nest in the already dead center of the tree, and don't really harm the healthy parts of the tree.
Again, before using pesticides to control insects on trees and shrubs, be sure to identify the insect causing the problem, and assess just how much damage is being caused. Unless the insect is currently present and is doing serious harm, treating with a pesticide is probably not warranted. If you see holes in leaves but do not see the insects causing those holes, spraying an insecticide will usually not be useful. Most insecticides must come in contact with the insects. The insects that caused the holes may have completed their lifecycle for the year and will not be causing any more damage. Not all pesticides are effective for all life stages of a particular insect.
Also be sure to always read pesticide labels thoroughly to be sure you do not cause damage to your plant or the environment. If your plant and the insect you are attempting to control are not listed on the pesticide label, it is dangerous, and illegal, to use it.
For information on a variety of disease and insects that affect trees and shrubs in Minnesota, Wisconsin and the upper Midwest:
Always use caution when using pesticides, and be sure to read and follow all label instructions. For information on available insecticides for use on trees and shrubs, see Insecticide Suggestions to Manage Landscape Tree and Shrub Insects
Home Landscape Insecticides