Pests and Diseases

Keeping your trees and shrubs healthy means you have to address pests, weeds, and diseases at the onset. In this section, you’ll find information on weed management and control of trees and shrubs, including mulching, herbicides, and pesticides. Find tips on dealing with rot, scale, blight, mold, gall, canker, and insect pests such as moths, caterpillars, worms, beetles, borers, spiders, and lanternflies.

Common Tree and Shrub Diseases

Diagnosing diseases with trees and shrubs can be challenging. Some are very easy to identify, while others require expert knowledge, as there can be many look-alike diseases and hidden clues. There are also certain diseases that you can only confirm with laboratory analysis. The process for diagnosing problems with your trees and shrubs includes identifying signs and symptoms.

It is possible to learn how to scout for fungus and bacteria and manage diseases. But it’s not just diseases that can injure trees and shrubs. The weather can also be very unforgiving, particularly in the winter when heavy snow and ice storms can cause major damage to your trees and shrubs.

Some of the most common diseases that can affect trees and shrubs include:

• Anthracnose: Many deciduous hardwoods are particularly susceptible to this leaf disease, which is caused by various species of the fungus Apiognomonia.
• Cankers: Living organisms such as fungi and bacteria, or nonliving factors including excessive temperatures or hail can cause this disease.
• Fire Blight: This disease is caused by Erwinia amylovora and it attacks more than 75 species of trees and shrubs.
• Armillaria Root Rot: Various species of the fungus Armillaria are the cause of this disease. Shoestring root rot is another name for this disease.
• Phytophthora Root Rot: An accurate diagnosis of this disease requires laboratory analysis. It affects most Christmas tree species, true firs, Douglas-fir species, and eastern white pine.
• Black Root Rot: A soil-inhabiting fungus called Thielaviopsis basicola is the cause of this type of root rot and branch dieback on various woody and herbaceous plants.
• Gall Rusts: Two hosts are necessary for gall rust. Galls on pines release their spores, which can only infect the leaves of oak trees.
• Crown Gall: Bacteria belonging to the genus and species Agrobacterium tumefaciens are the cause of crown gall. It can infect a wide range of herbaceous and woody plants.
• Bacteria Wetwood or Slime Flux: Symptoms of this disease that affects many mature trees, including elms and oak, are large light or dark vertical streaks. These streaks found on the trunk are the result of a slimy liquid that oozes out of wounds or cracks.

We often think of lichens as a sign of disease, but they pose no danger to the trees on which they grow because they obtain their nutrients and water in other ways, rather than from the tree on which they’re found.

There are several popular trees and shrubs that are susceptible to certain diseases. For example, Beech diseases include bleeding canker, bark disease, and Laetiporus root rot.

Spruce trees can fall victim to two common fungal diseases that you can easily identify. Juniper diseases include cedar-apple rust, cedar-quince, cedar-hawthorn, and Japanese apple rust. Boxwood is prone to attack from the boxwood leafminer.

Tree Insect Pests

Insect pests are a concern not only for agricultural and horticultural professionals – they also cause damage to trees and woody ornamentals in residential landscapes. Some can cause significant damage, such as the spotted lanternfly. This invasive insect has spread throughout Pennsylvania since it was first discovered in 2014. All residents and businesses are obliged to follow the Pennsylvania Department of Agriculture quarantine guidelines to prevent the movement of this insect at any stage of its development.

Other tree insects that are more of a nuisance than a hazard include:

• Fall Webworm: Appears from late summer through early fall and feeds on many species of deciduous trees and shrubs.
• Oystershell Scale: This common armored scale insect causes injury to shade trees and shrubs.
• Lace Bug: In total, 28 species of this insect have been recorded in Pennsylvania; however, only a few cause damage to ornamental plants and you can generally find them on the foliage of trees and shrubs.
• Japanese Beetle: This insect causes significant damage to ornamental trees, shrubs, and flowers throughout the eastern United States.
• Gypsy Moth: Damage caused by this insect includes defoliation.

Trees and Shrubs Pesticide Application

Learning how to identify insect and disease problems in the landscape is key to introducing an integrated pest management program and for the effective application of pesticides. If you’re going to apply pesticides, pesticide applicator certification is required.

Penn State Extension runs a Professional Pest Managers School for anyone wanting to keep up to date with information relevant for applicators. If you use restricted-use pesticides on your farm or property, you have to pass the PDA private pesticide certification exam. You can also find training for this important certification in Spanish.

There are many factors to consider when applying pesticides. You should only use those registered with the EPA. This is because the agency has tested them for safety and efficacy. It’s also important to follow the directions on the label. Glyphosate is an active ingredient in a variety of herbicides that are used to manage weeds in landscape settings. However, you should be especially careful when you spray it near trees.