Common Tree Diseases in Pennsylvania

Pennsylvania's forests and residential trees face numerous disease threats. Some are manageable, others are devastating. Knowing what to look for helps you catch problems early—when treatment is more likely to succeed—and make informed decisions about trees on your property.

Widespread and Destructive

Oak Wilt

Affects: All oak species, but red oaks die fastest; white oaks are more resistant.

What it is: A fungal disease that clogs the water-conducting vessels in trees, essentially causing them to die of thirst.

Symptoms:

• Leaves wilting and browning from the edges inward
• Rapid defoliation (red oaks can die in weeks)
• Discoloration in wood when bark is peeled back
• Dead branches starting at the top of the crown

How it spreads:

• Beetles carrying fungal spores to fresh wounds (April–July is worst)
• Root grafts between nearby oak trees (underground spread)

What to do:

• Never prune oaks between April and July
• If you see symptoms, consult an arborist immediately
• Infected red oaks usually cannot be saved and should be removed to prevent spread
• Root barrier trenching may be needed to protect nearby oaks

Dutch Elm Disease

Affects: American elms and some other elm species.

What it is: A fungal disease that has devastated American elm populations since the 1930s.

Symptoms:

• "Flagging"—individual branches wilting and yellowing while others remain green
• Leaves turning yellow, then brown, but staying attached
• Brown streaking in the wood visible under bark
• Progressive dieback over one or more seasons

How it spreads:

• Bark beetles carrying fungal spores
• Root grafts between nearby elms

What to do:

• Avoid pruning elms mid-April through mid-July
• Early-stage infections can sometimes be treated with fungicide injection
• Severely infected trees should be removed promptly to protect other elms
• Consider disease-resistant elm cultivars for replacement

Emerald Ash Borer (EAB)

Affects: All species of ash trees (green, white, black ash).

What it is: An invasive beetle whose larvae feed under the bark, destroying the tree's ability to transport water and nutrients. This is an insect pest, not technically a disease, but it's killing millions of ash trees.

Symptoms:

• Thinning canopy, starting at the top
• D-shaped exit holes in bark (about 1/8 inch wide)
• Increased woodpecker activity (they feed on larvae)
• Bark splitting with S-shaped galleries visible underneath
• Sprouts growing from trunk (stress response)

What to do:

• Prevention: Valuable ash trees can be protected with annual or biennial insecticide treatments (trunk injection or soil drench)
• Once heavily infested, treatment is usually futile
• Removal is often the only option for symptomatic trees
• Do not move ash firewood—this spreads EAB

Common and Manageable

Apple Scab

Affects: Apple trees, crabapples, and related species.

What it is: A fungal disease causing leaf spots and fruit damage. Unsightly but rarely fatal.

Symptoms:

• Olive-green to black spots on leaves
• Leaves yellowing and dropping prematurely
• Scabby, cracked spots on fruit

What to do:

• Rake and destroy fallen leaves (fungus overwinters there)
• Improve air circulation with proper pruning
• Fungicide applications in spring for severe cases
• Consider resistant varieties when planting

Anthracnose

Affects: Sycamore, oak, maple, dogwood, and many other deciduous trees.

What it is: A fungal disease causing leaf spots, branch dieback, and defoliation. Often looks alarming but rarely kills mature trees.

Symptoms:

• Irregular brown or tan spots on leaves
• Dead areas along leaf veins
• Curled, distorted leaves
• Twig dieback on sycamores
• Early leaf drop

What to do:

• Usually no treatment needed—trees recover
• Rake fallen leaves to reduce next year's infection
• Water during drought to reduce stress
• Severe cases on valuable trees may warrant fungicide

Powdery Mildew

Affects: Many trees and shrubs; common on dogwood, oak, maple.

What it is: A fungal disease that appears as white powder on leaf surfaces. Unsightly but usually not serious.

Symptoms:

• White or gray powdery coating on leaves
• Leaf distortion or curling
• Premature leaf drop in severe cases

What to do:

• Often no treatment needed
• Improve air circulation through pruning
• Avoid overhead watering
• Fungicides available for severe or recurring cases

Bacterial Leaf Scorch

Affects: Oak, elm, maple, sycamore, and others.

What it is: A bacterial disease spread by insects that clogs water transport, causing progressive decline.

Symptoms:

• Marginal leaf browning (brown edges with yellow halos)
• Symptoms appear in mid to late summer
• Progressive year after year
• Eventually affects entire tree

What to do:

• No cure exists
• Antibiotic injections can suppress symptoms temporarily
• Good cultural care (watering, mulching) helps trees cope
• Eventually, removal is usually necessary

Species-Specific Concerns

Thousand Cankers Disease (Walnuts)

Affects black walnut and butternut. Caused by a fungus carried by walnut twig beetles. Creates numerous small cankers under bark. Currently spreading in Pennsylvania; no treatment available. Avoid moving walnut wood.

Beech Bark Disease

Affects American beech. Combination of scale insects and fungal infection. Creating dead patches of beech throughout Pennsylvania forests. Individual tree treatment is possible but impractical at scale.

Rhizosphaera Needle Cast (Spruce)

Common on blue and Colorado spruce. Causes needles to turn purple-brown and drop, typically starting at the bottom. Treatable with fungicides applied in spring, but requires multiple years of treatment.

When to Call a Professional

Consider professional consultation when:

• You're not sure what's affecting your tree
• Symptoms are progressing rapidly
• A valuable or significant tree is showing problems
• You're seeing symptoms that match oak wilt, Dutch elm disease, or EAB
• Multiple trees are affected
• You want to know whether treatment is feasible

A certified arborist or Penn State Extension can help diagnose problems and recommend appropriate action.

General Prevention Strategies

You can't prevent all tree diseases, but healthy trees are more resistant:

• Water during drought: Stressed trees are more susceptible to disease
• Mulch properly: 2-4 inches of mulch (not touching trunk) helps retain moisture and regulate soil temperature
• Avoid wounding trees: Lawn mower and string trimmer damage creates entry points for disease
• Prune properly: Correct pruning cuts heal faster than improper ones
• Time pruning appropriately: Avoid pruning oaks and elms during high-risk periods
• Don't move firewood: Many diseases and pests spread via transported wood
• Plant diverse species: Monocultures are more vulnerable to species-specific diseases

Resources

For disease identification help:

• Penn State Extension: Offers diagnostic services and extensive online resources
• PA DCNR Bureau of Forestry: Information on forest health issues
• Local certified arborists: Can diagnose and recommend treatment

The Bottom Line

Some tree diseases are cosmetic nuisances; others are death sentences. Knowing the difference—and recognizing problems early—gives you the best chance of saving treatable trees and making smart decisions about untreatable ones.

When in doubt, get a professional diagnosis. The cost of an arborist consultation is trivial compared to losing a valuable tree that could have been saved, or wasting money treating one that can't be.