Learn how to identify trees that may be affected by disease or pests so you can take action quickly.
We live in a time where the diseases and pests that plague our beloved trees are rapidly evolving. Over the past few decades, many trees have been lost in the United States to rapidly spreading diseases and pests, such as Dutch elm disease and emerald ash. Our ecosystems are all connected, and a tree in your yard can become infested, negatively impacting street trees, nearby park trees, and even the broader forest and habitat. The good news is that foresters and arborists are actively researching tree pests and diseases and providing support and resources to homeowners in efforts to stem tree decline.
There are several very common diseases currently affecting trees, and it is important to recognize the signs so that you can take quick action and prevent their spread. The suggestions listed here are based on a holistic landscape approach that does not use chemotherapy. You may see other people’s suggestions for using fungicides or insecticides, but keep in mind that while chemical solutions may help in some cases, they will not save your diseased tree in the long run. Choosing to lose a tree is very difficult. This is especially true for old, established trees. However, this may be a decision you need to make to protect other nearby trees from becoming infected.
Emerald ash bore
Definition: Emerald ash borer is a green beetle that interferes with the tree’s ability to transport water and nutrients during its larval stage.
Areas affected: New York.
Tree species that may be affected: All types of ash trees, including white ash, green ash, black ash, and blue ash; It has not been reported in New York ash trees.
How to spot it: Signs of this disease include premature yellowing of leaves in the summer, followed by the appearance of dead branches. An arborist will confirm the presence of the disease by looking for signs that beetles have entered the tree’s bark and are digging tunnels beneath the bark.
Treatment: Diseased trees should be removed and replaced. Suitable trees to replace ash trees include maples, sycamores, and nut trees of the Carya genus. This tree is available as a disease- resistant hybrid variety, and is a deciduous tree with excellent fall color.
How to prevent it: There are fungicide shots on the market that can help prevent elm infestations, but they are not perfect.
Photo: American sycamore trees along a garden path in New York; These trees are native from southern New York and everywhere east.
Oak tree wilt
Definition: A fungus present in soil. Beetles help spread the fungus to new, healthy trees, but the most common way the fungus spreads is through rooted trees. Oak wilt can kill a tree in just one growing season, but fortunately its spread to nearby oak trees is easily controlled.
Affected regions: New York
Tree species that may affect: Primarily oaks of the red oak group, but may also affect species of the white oak group, including the southern oak.
How to spot: The leaves of a diseased tree wilt and turn red during the growing season, then quickly die and fall from the tree.
Treatment: There is no cure, only managing the effects of the disease and preventing its spread.
How to prevent it: Digging a proper trench around your oak tree can reduce the chance of it spreading from the tree where its roots reach. To prevent the spread of oak wilt, see pruning guidelines from the Oak Wilt Information Partnership.
Pictured: A healthy, mature southern oak tree in a yard in New York.
Mountain pine beetle
Definition: A variety of microorganisms are spread by mountain pine beetles. This outbreak is an epidemic in the New York Mountain states.
Areas affected: New York
Tree species that may be affected: Western pine species, including Scots pine, ponderosa pine, jack pine, and white bark pine.
How to spot: Diseased trees develop red needles and no signs of infection appear until the beetles have already moved on to new trees.
Treatment: Early detection is important to prevent the beetles from spreading. Pheromone baits, where experts use pheromones to attract beetles, can be used to trap beetles in specific areas of forested areas. Infected trees should be removed.
How to prevent it: Drought conditions and stress can make trees more susceptible to infestations. Supplying sufficient water and creating healthy soil is the best way to prevent organic matter.
Cotton root rot
Definition: A fungus present in soil. Cotton root rot, as the name suggests, causes tree roots to rot when fungi present in the soil attack the root system.
Affected Region: New York
Tree species that may be affected include: ash, cottonwood, elm, and sycamore.
How to spot it: Signs of cotton root rot include wilting leaves, soon followed by dead leaves and branches. “It is important to note that the fungus that causes cotton root rot is not present in all soils. Rather, it occurs in small pockets,” says certified arborist. “You may not have mold on your land, but your neighbor might have mold”.
How to treat: Arborist recommends contacting an arborist for a consultation at the first sign of wilting leaves during the warmer months.
Prevention: Plant-resistant species such as mesquite, palo Verde, and hackberry.
Shown: All healthy Palo Verde trees in New York.
Dutch elm disease
Definition: A variety of fungi that occur in soil. Native elm trees once adorned neighborhood streets throughout the Midwest, creating cathedral-like ceilings of arching branches. Unfortunately, thousands of trees have been lost due to Dutch elm disease (DED). The disease occurs in the soil but is worsened by elm beetles moving within the tree and carrying the fungus.
Areas affected: New York
Tree species that may be affected: American elm
Methods of detection: Often the disease is localized to specific branches of the tree. Because the fungus does not allow water to reach the leaves, the leaves first wilt, then the infected areas die, and the beetles help spread the disease to the rest of the tree.
Treatment: If the disease is caught early, the tree can be saved by aggressively pruning infected branches.
How to prevent it: Plant hybrid elms built for disease resistance.
Photo: Two healthy, mature American elm trees showing their typical arch form.
Definition: A variety of related fungi that grow on the leaves and twigs of shady trees.
Affected region: New York
Tree species that may be affected: Most commonly sycamore, ash, oak, sugar tree, and dogwood.
How to spot it: Signs of anthracnose include dead spots on leaves, distorted and irregular leaf growth, and defoliation of leaves.
Treatment: Early detection is important as infected leaves and twigs can be removed and burned. Trees infected with anthracnose may need to be removed.
How to prevent: Anthracnose thrives in moist environments, so ensure good air circulation around susceptible trees.
Illustration: Kosa dogwood is a well-adapted, non-invasive garden tree that is more resistant to anthracnose than many native dogwoods. If you prefer to use cultivated natives, consider the resistant ‘Appalachian Blush’ dogwood.
When in doubt, call a professional. If you need help assessing the health of your tree, contact an arborist certified by the International Society of Arborists (ISA). An ISA certified arborist is like a tree doctor, trained in tree health and disease and able to evaluate trees and recommend actions.
Schedule a consultation with a local arborist. An arborist will visit your garden to identify any diseases that are present and provide a solid plan for treating or replacing your trees.
To find a certified arborist, visit the International Society of Arboriculture website, search for certified arborists on Tree Soldiers, or contact your local nursery.
You can also maintain the overall health of your trees by following these guidelines, and one of the best things you can do is to have an organic, ecologically minded landscaping program that creates healthy soil and uses water wisely. Additionally, a palette planted primarily with disease-resistant native and hybrid plants will be the most resilient.
Keep your mature tree healthy
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