Disease Identification Guide

Tree disease diagnosis is rarely "spot the symptom, name the pathogen." It's a systematic narrowing — host, location, pattern, season, site, history — and it ends in a defensible recommendation, not a guess. This page is the field framework. It does not replace species-specific references (oak wilt, anthracnose, EAB, bark beetles each have their own pages); it gives you the diagnostic process you apply before reaching for those.

Quick Start

• Diagnosis is multi-factor. One symptom rarely gives you a pathogen — work the framework.
• The diagnostic question stack: Host species → Symptom location → Symptom pattern → Time of year → Site conditions → Recent stress → Lab confirm if needed.
• Distinguish biotic (a pathogen — host-specific, spreading) from abiotic (drought, salt, herbicide, gas — usually multi-species, geographic).
• When the recommendation hinges on the diagnosis (treat vs. remove), send tissue to a lab. Don't guess.

The diagnostic framework

1. Identify the host

   You cannot diagnose what you cannot name. Get the species — not just "an oak," but red oak group vs. white oak group. Many pathogens are host-specific (Bretziella fagacearum in red oaks, Discula destructiva in flowering dogwood, Diplodia in pines). The wrong host ID sends you down the wrong tree of pathogens.

   If you can't ID the species in the field, photograph leaves, bark, and overall form and confirm before writing the report.

2. Locate the symptoms

   Where on the tree are the symptoms? Leaves, twigs, branches, trunk, root flare, or whole-tree? Location narrows the candidate list dramatically — a leaf-spot fungus and a root-decay fungus produce different declines and require different responses.

   A common mistake: noting only the most visible symptom (browning leaves) and missing the upstream cause (basal canker, girdling root, herbicide line at the dripline).

3. Read the pattern

   Within the tree: is the symptom uniform, one-sided, scattered, concentrated at the top, concentrated at the bottom?

   Across trees: does it appear on one species only, or on multiple species in the same area? Single-species clustering points to a host-specific pathogen. Multi-species clustering on the same site points to abiotic stress (drought, salt, herbicide drift, gas leak, soil compaction).

4. Anchor it to the season

   Many pathogens have predictable timing. Anthracnose shows up after cool wet springs. Oak wilt symptoms in red oaks crash in mid-summer. Verticillium wilt expresses on hot July afternoons. EAB adult emergence is May-July. Late-summer scorch on uniform foliage is usually drought, not disease.

   A symptom pattern that doesn't match the calendar deserves a second look — it might be a different pathogen, or it might be abiotic.

5. Inspect the site

   Walk the dripline. Look for fill soil, recent grade changes, pavement creep, irrigation heads pointed at the trunk, herbicide spray patterns, gas service trenching, road salt accumulation. Half of "tree disease" calls are site problems with a secondary pathogen colonizing the stressed tissue.

6. Reconstruct the history

   Ask the owner: when did symptoms start? What changed before that? Construction, paving, lawn renovation, deep-water main work, the new neighbor's irrigation system, the herbicide application two summers ago that "definitely didn't drift." Tree decline is often slow — the trigger may be 18-36 months upstream of the visible symptom.

7. Confirm in a lab when it matters

   If the recommendation downstream of the diagnosis is significant — remove a 36-inch oak vs. treat it, condemn a row of street trees, trigger a quarantine — get lab confirmation. Field diagnosis without lab work is fine for low-stakes maintenance calls; it is not enough when six figures of liability or a quarantine designation is on the line.

Symptom-based decision tree (high level)

This is a starting orientation, not a key. Each branch sends you toward a category of cause; the framework above narrows from there.

Leaf symptoms

Spots, blotches, marginal scorch, curling, distortion, premature defoliation, tar spots, powdery coating, mosaic discoloration.

• Most common cause: foliar fungal pathogens (anthracnose, leaf spots, scab, rusts, powdery mildew). Cool wet springs amplify; a dry summer often resolves them.
• Less common: bacterial leaf scorch (xylem-limited bacterium, Xylella fastidiosa — chronic, marginal scorch, progressive year-over-year decline).
• Often abiotic: scorch from drought, salt, or root damage; herbicide drift (cupping, strapping, distortion); nutrient deficiency (interveinal chlorosis); air pollution.

If the pattern is uniform across the whole canopy and matches a stressful weather window, suspect abiotic before pathogen.

Twig and branch symptoms

Tip dieback, flagging (single-branch wilt), cankers (sunken or raised dead bark), oozing, gummosis, witch's broom proliferation.

• Cankers: localized infections (Cytospora, Nectria, Botryosphaeria) — usually opportunistic on stressed trees.
• Vascular wilts: oak wilt, Dutch elm disease, verticillium wilt — flagging on individual branches that progresses through the tree.
• Bacterial fire blight on Rosaceae (apple, pear, hawthorn, mountain ash) — shoots curl into a "shepherd's crook," tissue blackens.
• Chronic dieback is often multifactor: drought stress + secondary canker fungus + age. Don't pin it on one organism.

Trunk symptoms

Cankers, conks (woody fungal fruiting bodies), bleeding sap, bark cracks, sloughing bark, frass and exit holes.

• Wood-decay fungi: conks (Ganoderma, Inonotus, Phellinus, Laetiporus) indicate active internal decay. Conk size and location guide structural concern — see Hazard Rating with TRAQ.
• Phytophthora bleeding cankers at the lower trunk and root collar — dark stain, often weeping. Common on beech, oak, maple.
• Bark beetles: small exit holes, frass at base, blue-stain in sapwood. Bark beetles are usually the symptom of a stressed tree, not the cause.
• Bacterial wetwood / slime flux: weeping at branch unions or wounds — usually cosmetic, rarely the primary problem.

Root and basal symptoms

Mushrooms or conks at the base, missing root flare (buried or girdled), soft or hollow basal sound, recent lean, soil heave on the upwind side.

• Armillaria root rot: honey-colored mushrooms in fall, white mycelial fans under the bark at the root collar. Often advanced by the time it's visible.
• Ganoderma butt rot: shelf-like conks at or near the base. Structural risk — see Hazard Rating with TRAQ.
• Phytophthora root rot: no fruiting bodies, but progressive thinning canopy and root collar lesions. Common on overwatered or poorly drained sites.
• Girdling roots: not a pathogen, but produces classic "decline" symptoms — sparse canopy, tip dieback, a missing root flare. Always check before diagnosing a root pathogen.

Whole-tree decline

Sparse canopy, small leaves, premature fall color, year-over-year thinning, no single dramatic symptom.

Whole-tree decline is almost always multifactor: a primary stress (drought, construction damage, root loss, soil change) opens the door to secondary pests (bark beetles, opportunistic cankers) and the tree spirals down over 2-5 years. Diagnose the stress chain, not a single pathogen. The treatment is usually site repair (decompaction, mulch, supplemental irrigation, restoring the root zone) plus removing whatever secondary pest is accelerating the decline.

Biotic vs. abiotic — the key distinction

This is the call that decides whether you reach for a pathogen reference or a soil probe.

| Indicator | Suggests biotic (pathogen) | Suggests abiotic (environmental) | |---|---|---| | Host range | One species or one genus affected | Multiple unrelated species affected | | Spatial pattern | Spreading from a focal point over months/years | Sharp geographic line (property edge, prevailing wind, herbicide spray pattern) | | Symptom progression | Often gradual, with signs of the organism (mycelium, conks, oozing, exit holes) | Often sudden, follows a weather or site event | | Within-tree pattern | Concentrated on susceptible tissue (new growth, sapwood, specific branches) | Often uniform — whole canopy, uniform scorch, all leaves the same age affected | | Site correlation | Not strongly site-specific (other than environment favoring the pathogen) | Strong correlation with site features — fill soil, pavement, irrigation heads, gas service line, salt spray |

When to send tissue to a lab

Field diagnosis is sufficient for routine problems and low-stakes recommendations. Send tissue when:

• The recommendation downstream is remove vs. treat for a high-value tree.
• A regulated pathogen is suspected (oak wilt, sudden oak death, ralstonia, plum pox, citrus greening — quarantine implications).
• A first occurrence in your area for a known pathogen — confirmation matters for client and regulator communication.
• The diagnosis would trigger neighbor or municipal action (street tree program, HOA-wide treatment).
• The symptom doesn't match anything you've seen and your reference materials don't close the loop.

For low-cost maintenance work on a tree that's already a marginal candidate, lab confirmation may not pencil out — but say so explicitly in the report.

Sampling for the lab

Most diagnostic labs need fresh tissue at the margin of dead and live — the active edge where the pathogen is colonizing, not the long-dead center.

1. Collect at the active margin

   For leaf spots, take leaves with active lesions plus surrounding green tissue. For cankers, cut a sample that includes the canker edge and adjacent healthy bark and cambium. For root pathogens, take feeder roots with surrounding soil. The center of a dead lesion is colonized by saprophytes and tells the lab nothing.

2. Use paper, not plastic

   Paper bag or wrapped in newspaper. Plastic sweats; the sample arrives as a slimy mess of secondary mold and the lab can't read it.

3. Keep it cool, ship fast

   Refrigerate (don't freeze) until shipping. Overnight or two-day to the lab. Include the submission form completely filled out — host species, symptom description, site history, your tentative diagnosis. Labs work faster when you've narrowed the question.

4. Photograph everything

   Photograph the tree, the symptom on the tree, and the sample before shipping. Labs may ask follow-up questions; photos answer most of them without a return visit.

Where to send samples

Most US states have a university plant disease diagnostic lab — usually attached to the cooperative extension service. These are the right first stop: low cost, regionally calibrated, accept submissions from professionals and the public. Some states route through the department of agriculture for regulated pathogens.

For specialty work (DNA-based confirmation, microscopy beyond what extension labs offer), private and ISA-accredited diagnostic services exist. Find them through ISA chapter directories and TCIA member listings.

Don't list specific labs in client reports unless you've used them and trust the turnaround — submissions and results are part of your record.

Documenting the diagnosis

In a Tree Inventory AI report, every disease finding should include:

• The host species (genus and species, scientific name italicized).
• The symptoms observed with photos — tree, affected tissue, close-up of the diagnostic feature.
• The diagnosis — pathogen if confirmed, "consistent with X pending lab" if not, "decline of unknown cause" if you genuinely don't know.
• Confidence level — confirmed by lab, field-diagnosed by experienced arborist, suspected pending further inspection.
• The recommendation — treatment, monitoring interval, lab submission, removal, or no action.

A diagnosis with photos and a documented framework is defensible in a way that "the tree has a fungus" is not.

Common Questions

The tree looks bad but I can't pin down a single cause. What do I write? "Decline of multifactorial origin" is a legitimate diagnosis when the framework points at stress (drought, root damage, construction history) plus opportunistic secondary issues. Recommend site remediation and a follow-up inspection in 6-12 months. Don't manufacture a pathogen to make the report look authoritative.

How fast do diagnostic labs turn around? Most university extension labs run 1-3 weeks for fungal cultures, 3-7 days for visual ID and PCR-based tests. Plan accordingly — if the client needs an answer this week, advise them up front.

Should I treat preemptively if a neighboring property is infected? Sometimes — especially for oak wilt (root graft severance) and EAB (preventative trunk injection on high-value ash within the kill radius). For most pathogens, treat the trees you've documented with symptoms and monitor the rest. Preemptive treatment without justification is overselling.

What about AI-based identification apps? They're getting better but they're not diagnostic-grade for the calls that matter. Use them as a first-pass screening tool; confirm with the framework above and, when stakes are high, with a lab. Don't write a recommendation based solely on a phone app's identification.

Where do I learn the regional pathogens for my area? Your state university's extension service publishes free fact sheets on the most common diseases in your region. The ISA also publishes Best Management Practices for tree health care. Build a local reference set — most of your diagnostic work will involve the same 15-25 pathogens repeatedly.

Related

• Hazard Rating (TRAQ) — risk implications of decay and cankers found during disease assessment
• Seasonal Care Calendar — when to scout for which pathogens
• Pruning Best Practices — sanitation pruning for diseased material
• Capturing Trees — recording disease findings and photos