What Are the Most Common Tree Diseases in Pennsylvania?
A Certified Arborist’s Guide for York and Lancaster County Homeowners By Specialty Tree Service | September 2026
September is one of the most revealing months of the year for tree health in York and Lancaster County. As summer’s growth slows and leaves begin to turn, the effects of a season’s worth of disease, pest pressure, and stress become unmistakable. What looked like a quirky discoloration in July is now clearly something more serious. What seemed like sparse growth in the spring is now confirmed crown dieback.
The good news: September is also one of the best months to act. Trees are heading into dormancy, which is the ideal time for certain disease treatments and structural work. And catching a disease now, before the tree endures another winter in a weakened state, can mean the difference between a recoverable situation and a tree you lose in the spring.
Our ISA Certified Arborists see these diseases regularly across York County and Lancaster County. Here is what you need to know about each one: what it looks like, what causes it, and what can be done about it.
Why September Is the Right Time to Assess Your Trees
Tree diseases do not take the fall off. In fact, several of Pennsylvania’s most serious pathogens are most active or most visible in late summer and early fall. Here is why September matters for disease management:
■ Symptoms are fully visible. By September, a full growing season of infection is on display in the canopy. Subtle spring symptoms are now obvious, making accurate diagnosis far easier.
■ Pre-dormancy treatments are more effective for certain diseases. Systemic soil applications and root flare injections applied in September travel efficiently through the tree before it fully shuts down for winter.
■ Oak pruning becomes safe again. September marks the beginning of the safe window for oak pruning in Pennsylvania. Oak Wilt risk drops sharply as beetle activity decreases in fall.
■ Early intervention prevents spring re-infection. Treating fungal diseases in fall removes the inoculum source before it overwinters and re-infects in spring.
The Most Common Tree Diseases in York and Lancaster County, PA
Each profile below includes severity, affected species, visible symptoms, and treatment options. If you recognize any of these on your property, call us for a free assessment. Oak Wilt Severity: Critical Affects: All oak species; red oaks are most susceptible and can die within weeks of infection.
Key Symptoms:
Rapid wilting and browning of leaves starting at the tips and margins, beginning at the top of the crown and moving downward. Leaves often drop while still partially green. Fungal mats may form under the bark.
Treatment:
No cure once a tree is infected, but the spread to nearby oaks through interconnected root systems can be stopped with trenching or root disruption. Preventive systemic fungicide injections protect high-value oaks in areas where Oak Wilt is present. Infected trees should be removed promptly to prevent spread.
Emerald Ash Borer (EAB) Severity: Critical
Affects: All native ash species (white ash, green ash, black ash). Pennsylvania has some of the highest EAB pressure in the country.
Key Symptoms:
Crown thinning beginning at the top, S-shaped larval galleries under bark, D-shaped exit holes approximately 1/8 inch in diameter, heavy woodpecker activity on the trunk, epicormic sprouting from the base of the trunk in advanced cases.
Treatment:
Systemic root flare injection with an approved insecticide. Highly effective when the tree retains at least 30 to 40 percent of its canopy. September is an excellent time to schedule treatment for trees not yet injected this season. Untreated ash trees in York and Lancaster County will not survive EAB. Bacterial Leaf Scorch (BLS) Severity: High
Affects:
Oak, elm, maple, sycamore, mulberry, sweet gum. Extremely common in central Pennsylvania.
Key Symptoms:
Leaf margins turn brown and scorch as if burned, progressing inward from the edges. A yellow halo often separates the brown from the green. Symptoms appear in mid to late summer and worsen each year as the disease progresses through the tree’s vascular system.
Treatment:
No cure. Antibiotic injections (oxytetracycline) have shown effectiveness in suppressing symptoms and slowing progression. Annual or biennial injections combined with good soil care and proper pruning can extend the life of a valuable tree by many years. Anthracnose Severity: High
Affects:
Sycamore, white oak, dogwood, ash, maple. Sycamore anthracnose is one of the most visible diseases in Lancaster and York County.
Key Symptoms:
Brown, irregular blotches on leaves following the veins; early and repeated defoliation; twig dieback and cankers; in sycamore, large-scale defoliation in spring followed by re-leafing. Trees weaken significantly after repeated annual infection.
Treatment:
Fungicide sprays applied at bud break in spring are most effective for prevention. Fall cleanup of fallen infected leaves removes overwintering spores. Severely affected trees benefit from systemic fungicide applications. Our arborists can assess whether a spray program or systemic treatment is appropriate for your species. Verticillium Wilt Severity: High
Affects:
Maple is most commonly affected in Pennsylvania; also affects ash, catalpa, tulip tree, and many ornamentals.
Key Symptoms:
Sudden wilting and dieback in isolated branches or entire sections of the crown, often on one side of the tree. Affected wood, when cut, shows a characteristic olive-green or brown discoloration in the sapwood. Can kill branches or entire trees in one to several seasons.
Treatment:
No cure for the soil-borne fungus itself. Management focuses on reducing tree stress through proper fertilization, watering, and mulching. Heavily infected trees may recover partially with vigorous support care. Severely affected specimens often warrant removal and replacement with a resistant species. Hemlock Woolly Adelgid (HWA) Severity: High
Affects:
Eastern hemlock. One of the most serious threats to hemlock trees throughout Pennsylvania.
Key Symptoms:
White, woolly egg sacs clustered at the base of needles (visible year-round but most obvious in fall and winter), yellowing and graying of needles, premature needle drop, reduced new growth, gradual thinning of the crown over multiple seasons.
Treatment:
Systemic soil drench or root flare injection with imidacloprid or dinotefuran. September and October treatments provide excellent uptake before dormancy. Hemlocks treated early and consistently can be preserved indefinitely. Untreated hemlocks typically die within 4 to 10 years of infestation. Cedar-Apple Rust Severity: Moderate
Affects:
Apple, crabapple, hawthorn (primary hosts); eastern red cedar (alternate host). Very common in York and Lancaster County orchards and residential landscapes.
Key Symptoms:
Bright orange or rust-colored spots on the upper surface of apple and crabapple leaves from late spring through summer; corresponding tube-like structures on the leaf undersides; premature defoliation in severe cases; orange gelatinous galls on cedar in spring.
Treatment:
Preventive fungicide sprays beginning at bud swell through petal fall in spring are most effective. Fall is the time to assess severity, remove heavily infected leaves and debris, and plan a spray program for next spring. Resistant varieties are available for new plantings. Fire Blight Severity: Moderate
Affects:
Apple, pear, crabapple, hawthorn, serviceberry, mountain ash.
Key Symptoms:
Branch tips that suddenly wilt and curve into a distinctive shepherd’s crook shape; blossoms that turn brown and cling to the tree; water-soaked lesions on bark that dry to a reddish brown; bacterial ooze in humid conditions.
Treatment:
Pruning infected wood at least 8 to 12 inches below visible symptoms during dry weather. Tools must be sterilized between cuts. Copper-based bactericide sprays applied during bloom period prevent new infections. September is a good time to prune out any remaining cankers from this season’s infection. Powdery Mildew Severity: Moderate
Affects:
Dogwood, oak, maple, lilac, crabapple, and many ornamentals. Extremely common across Pennsylvania in summer and fall.
Key Symptoms:
White or gray powdery coating on leaf surfaces, shoot tips, and young stems. Infected leaves may curl, distort, or drop early. Most noticeable in late summer and September as conditions favor the fungus.
Treatment:
Horticultural oil or fungicide spray treatments are effective when applied early. Fall cleanup of infected foliage reduces overwintering spores. Rarely fatal but significantly weakens trees over time and should not be ignored on young or already-stressed specimens.
Your September Tree Disease Action Plan
Here is what our arborists recommend for York and Lancaster County homeowners this month:
■ Walk your property and look up.
Take 15 minutes to examine the crowns of your trees from a distance. Look for areas of browning, dieback, unusual discoloration, or sparse foliage that stands out from healthy sections of the canopy.
■ Look for the specific symptoms in this guide.
Each disease has a visual fingerprint. Scorched leaf margins on oaks or maples may indicate Bacterial Leaf Scorch. White woolly tufts on hemlock needles mean HWA is present. Rapid wilting in a red oak crown in late summer is a red flag for Oak Wilt.
■ Schedule EAB injections for untreated ash trees.
If your ash trees have not been treated this season, September is your last reliable window for an effective systemic injection before dormancy. Do not wait until spring.
■ Begin fall cleanup of diseased foliage.
Raking and removing infected leaves from anthracnose, cedar-apple rust, and powdery mildew reduces the amount of pathogen that overwinters and re-infects next spring.
■ Book a fall arborist assessment.
Our ISA Certified Arborists can walk your property, identify active disease issues, and design a treatment and management plan tailored to your specific trees and landscape. September and October assessments set you up for the most effective spring treatments. September is one of the highest-impact months to invest in your trees. Call us today to schedule a fall disease assessment for your York or Lancaster property.
York: 717-755-7505 | Lancaster: 717-392-6306
Frequently Asked Questions
Q: What are the most common tree diseases in Pennsylvania?
A: The most common and serious tree diseases in York and Lancaster County include Oak Wilt, Bacterial Leaf Scorch, Emerald Ash Borer, Anthracnose, Verticillium Wilt, Hemlock Woolly Adelgid, Cedar-Apple Rust, Fire Blight, and Powdery Mildew. Each affects different species and requires a different treatment approach. An ISA Certified Arborist can accurately diagnose and recommend the appropriate response.
Q: Can tree diseases spread to other trees on my property?
A: Yes. Several of Pennsylvania’s most serious tree diseases spread aggressively. Oak Wilt travels through interconnected root systems, infecting neighboring oaks. Hemlock Woolly Adelgid moves on wind and wildlife. Bacterial diseases spread via insects and pruning tools. Early diagnosis and proper management, including removal of infected material and tool sterilization, are essential to protecting your entire landscape.
Q: Is it too late to treat Emerald Ash Borer in September?
A: No. September is still an effective treatment window for Emerald Ash Borer in Pennsylvania. Systemic root flare injections applied in September can still protect trees before winter dormancy. Trees that retain at least 30 to 40 percent of their canopy are good candidates for treatment. Call Specialty Tree Service for an assessment of your ash trees.
Q: What does Oak Wilt look like?
A: Oak Wilt causes rapid wilting and browning of leaves starting at the leaf tips and outer margins, progressing from the top of the crown downward. Leaves often drop while still partially green. In red oaks, the disease can kill the tree within weeks of infection. If you see rapid crown decline in an oak on your York or Lancaster County property, call an arborist immediately.
Q: How do I know if my tree has a disease or is just drought stressed?
A: Both drought stress and disease can cause leaf scorch, wilting, and dieback, which is why professional diagnosis matters. Drought stress typically affects the whole tree somewhat uniformly and improves with rain and watering. Disease tends to be more localized, may show a pattern that follows vascular pathways, and often worsens progressively rather than stabilizing. Our certified arborists use visual inspection and, when needed, bark and wood sampling to give you a definitive answer.
Q: How much does tree disease treatment cost in York or Lancaster PA?
A: Cost depends on the disease, the treatment method, and the size and number of trees involved. Specialty Tree Service provides free, no-obligation estimates. We also offer financing options for larger treatment programs. Call York at 717-755-7505 or Lancaster at 717-392-6306 to schedule your assessment.
Schedule a Fall Disease Assessment in York or Lancaster, PA
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