Diagnosis and Treatment: Pests, Diseases, and Disorders

Diagnosis and Treatment makes up 12% of the ISA Certified Arborist exam — about 24 of the 200 questions — and almost none of it rewards straight memorization. The exam wants to see a process: rule out abiotic causes (soil, water, mechanical, chemical) before reaching for a pest or pathogen, and don't name a specific culprit until the evidence points to one.

Domain Review: This topic is one of the 10 ISA exam domains you must master.

Working arborists tend to find this the hardest domain on the ISA Certified Arborist exam, and it's not because the content is obscure. Pruning has a right answer you can memorize. Diagnosis asks you to reason through a scenario the way you would on a real job: what do I see, what does it rule out, what do I check next. The underlying tree biology matters more here than in almost any other domain, because you can't reason about wilting or dieback without knowing what the tree's vascular system is supposed to be doing in the first place.

The Diagnostic Process: Start Broad

Don't look at a curled, browning leaf and jump straight to naming a disease. The diagnostic sequence the exam expects, sometimes called the Appropriate Response Process, runs in a specific order for a reason: each step rules things out before the next one gets more specific.

1. Identify the plant. You can't evaluate what's wrong until you know what species you're looking at, because plenty of problems are host-specific. Dutch elm disease only affects elms; a diagnosis that ignores the host is a guess dressed up as an answer.
2. Look for patterns. Is the damage confined to one branch, one side of the tree, the whole tree, or every tree on the property? Uniform damage spread across multiple unrelated species usually points to an abiotic cause, something in the shared environment. Damage that's random or confined to a single species growing next to healthy ones points toward something biotic and host-specific.
3. Check the trunk and root collar. Something like 70 to 80% of tree problems trace back to what's happening below ground: girdling roots, compacted soil, a grade change that buried the root flare. Rule that out before you go looking for an insect or fungus. Soil conditions cause more decline than most candidates expect walking into the exam.

Here's how that plays out on a real call. Say three trees along a driveway that was regraded and repaved last fall are all thinning out this summer, an oak, a maple, and a birch. That's the first clue. A pathogen or insect that hits oak, maple, and birch equally, in the same season, would be unusual. Three unrelated species declining together, on the side of the property that just had equipment on it, reads as abiotic before you've walked up to the first trunk. Now change the scenario: only the birch is struggling, and the oak and maple next to it look fine. Same property, same season, but now the pattern argues for something biotic and specific to birch, because whatever is going on isn't touching the other species growing a few feet away in identical conditions.

That's the whole process in miniature. Pattern first, specifics second. Construction activity near the root zone is one of the most common abiotic culprits behind multi-species decline, and it's exactly the kind of below-ground cause step three is built to catch.

Signs vs. Symptoms

This distinction shows up constantly, and it's also where candidates get caught by answer choices that sound reasonable but jump ahead of the evidence.

A symptom is the tree's reaction to a problem: wilted leaves, dieback, chlorosis (yellowing), stunted growth. Symptoms are real, but they're not diagnostic on their own, because the same symptom can come from a dozen different causes. Wilting shows up with drought, with root disease, with a vascular wilt, and with plain girdling roots. If a question describes only symptoms and then offers an answer choice that names a specific pest or disease, be suspicious of that choice unless the question also hands you a sign.

A sign is physical evidence of the causal agent itself: a fungal conk, frass, an exit hole, webbing, mycelium, the insect. A sign is what lets you move from "this tree looks stressed" to "this tree has this specific problem," because it's evidence of the agent, not just evidence that something is wrong. This is the trap the exam sets over and over: a scenario loaded with symptom language, and a correct answer that either identifies the one actual sign buried in the description or, when no sign is present, recommends further investigation instead of naming a culprit outright.

Signs vs. Symptoms

Term	Definition	Example
Sign	The causal agent itself	Fungal fruiting body, visible insect, mycelium
Symptom	The tree's response to the agent	Wilting, chlorosis, dieback, cankers

Biotic vs. Abiotic Disorders

Abiotic disorders come from non-living causes, and in urban and suburban settings they're more common than any single pest or pathogen. Soil compaction crushes pore space and suffocates roots. Drought and flooding both put the tree under water stress, and from the canopy the symptoms can look nearly identical even though the fix is the opposite. Chemical injury, especially herbicide drift, often shows up as twisted, curled leaf growth (epinasty) rather than obvious burn. Mechanical damage from mowers and string trimmers girdles the root collar slowly enough that by the time the canopy shows it, the damage has been happening for seasons. Arborists call it mower blight for a reason.

Biotic disorders come from living organisms, and the exam expects you to know the general categories rather than an exhaustive pest list. Insects break down by feeding guild: chewing insects leave visible holes in foliage, sucking insects like aphids and scales cause stippling or leave sticky honeydew behind, and borers like emerald ash borer tunnel galleries under the bark that you often find before you find the insect itself. Fungi cause everything from powdery mildew on leaf surfaces to root rots like Armillaria to vascular wilts like Verticillium. Bacteria cause their own distinct symptoms, fire blight's shepherd's-crook stem dieback being the classic example.

The distinction isn't academic. It changes what you recommend. An abiotic problem doesn't respond to a fungicide or an insecticide, because there's no organism to kill; you fix the site, not the tree. A biotic problem might not need treatment at all, which is the whole premise of the next section.

Plant Health Care and IPM

Plant Health Care is the proactive side of tree care: right tree in the right place, appropriate soil and water management, mulching, all aimed at preventing problems before they start. Integrated Pest Management is what happens when something shows up anyway, and it's built around one idea the exam tests hard: you don't treat every pest you find.

IPM runs on thresholds. A light population of aphids on a maple in June isn't automatically a spray job. What matters is whether the population is actually damaging the tree's health versus just being visible, whether natural predators like ladybugs are already knocking the population down, and whether the client can tolerate the aesthetic damage. Only once a real threshold is crossed, and cultural or biological options aren't enough on their own, does chemical control become the right call.

That threshold logic sits inside a control hierarchy the exam expects you to know in order:

1. Cultural controls first: proper watering, mulching, site correction. This is Right Tree, Right Place applied after the fact.
2. Biological controls next: encouraging or releasing beneficial predators rather than reaching for a product.
3. Chemical controls last: pesticides, used only when the first two options can't solve the problem and a real threshold has been crossed.

Exam Strategy

A few habits carry real weight on diagnosis questions. Look for host-specificity clues in the wording; a question that specifies "a birch tree has..." is telling you something on purpose. When a question asks for the best management response and offers both a cultural fix and a chemical one, the cultural answer is usually correct unless the scenario has already established that cultural options failed. And in any scenario describing general decline without a clear pest description, check the roots and site conditions before anything else, the same start-broad logic from the diagnostic process, just applied under time pressure.

It also helps to remember that this domain is only 12% of your score, and the exam has no per-domain minimum. A 76% overall passing score is the only bar you need to clear. That's not a reason to skip the domain, but if the reasoning process feels slower to click than a rules-based domain like Pruning, you have room to build that skill through repetition rather than panic. Scenario-based practice questions are the fastest way to get comfortable with exactly this kind of reasoning before exam day.

Frequently Asked Questions

How many ISA exam questions cover diagnosis and treatment?

Diagnosis and Treatment is 12% of the exam, roughly 24 of the 200 questions. It ties with Soil Management and sits just behind Pruning at 16% and Safe Work Practices at 15%.

What is the difference between a sign and a symptom?

A sign is the causal agent itself, a fungal fruiting body, mycelium, or the insect. A symptom is the tree's response, such as wilting, chlorosis, or dieback. The exam tests this distinction directly, often by describing only symptoms and seeing whether you jump to naming a cause anyway.

What is the difference between biotic and abiotic disorders?

Biotic disorders are caused by living agents such as insects, fungi, and bacteria. Abiotic disorders come from non-living factors like compaction, drought, road salt, or improper planting depth. Abiotic causes are more common in urban settings, and the exam expects you to check them first.

What is the correct diagnostic process on the ISA exam?

Start broad and narrow down: identify the plant, examine the whole site and root collar, look for patterns across multiple plants, then distinguish signs from symptoms. A problem affecting several unrelated species usually points to an abiotic site cause rather than a pest or pathogen.

How can you tell if a tree problem is abiotic or biotic in a scenario question?

Check how many species are affected and how the damage is distributed. Several different species declining together, especially on one side of a property or after site disturbance, points to abiotic causes. A single species affected while unrelated trees nearby stay healthy points toward something biotic and host-specific. A confirmed sign settles it either way.

What does IPM mean in arboriculture?

Integrated Pest Management is a strategy that uses monitoring and thresholds to decide whether treatment is warranted, favoring cultural and biological controls before chemical ones. It only escalates to chemical control once a population is doing real damage and the earlier options aren't enough.

What is Plant Health Care, and how is it different from IPM?

Plant Health Care is the broader, proactive program: right tree in the right place, proper soil and water management, mulching, all aimed at preventing decline before it starts. IPM is the reactive piece inside that program, the threshold-based decision process for handling pests and pathogens once they show up.

Why does the "signs vs. symptoms" distinction trip up exam candidates?

Because symptoms are easy to describe and signs often aren't mentioned at all. A well-built trap question loads the scenario with symptom language, wilting, thinning, dieback, then offers an answer choice that names a specific cause anyway. Without a sign confirming that cause, naming it is a guess, and the exam rewards recognizing that rather than committing to an answer the evidence doesn't support.

Why do arborists check the roots and site conditions before suspecting a pest?

Because the majority of tree problems, something like 70 to 80% of them, trace back to what's happening below ground: compaction, girdling roots, or a grade change that buried the root flare. Starting there rules out the most statistically likely cause before you spend time chasing something rarer.