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Diagnosis and Treatment: Pests, Diseases, and Disorders

Master the diagnosis domain of the ISA exam. Learn the difference between signs and symptoms, biotic and abiotic disorders, and the Appropriate Response Process.

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Diagnosis is widely considered one of the hardest domains on the ISA Certified Arborist exam. It requires you to be a detective—combining knowledge of tree biology, local pests, and environmental conditions to solve a puzzle.

This guide simplifies the core concepts you need to master.

The Diagnostic Process: "Start Broad"

Don't look at a brown leaf and guess "Anthracnose!" immediately. Follow the Appropriate Response Process (ARP):

  1. Identify the Plant: You can't know what's wrong if you don't know what it is. (e.g., Dutch Elm Disease only affects Elms).
  2. Look for Patterns: Is the damage on one branch? The whole tree? All trees in the area?
    • Uniform damage usually suggests Abiotic (non-living) causes like soil or weather.
    • Random/Spotty damage usually suggests Biotic (living) causes like pests or fungi.
  3. Investigate the Trunk and Roots: 70-80% of tree problems start below ground (girdling roots, compaction, grade changes).

Signs vs. Symptoms

This is a guaranteed exam question. Know the difference.

  • Symptoms: The tree's reaction to the problem.
    • Examples: Wilted leaves, dieback, yellowing (chlorosis), stunted growth.
    • Tip: You can have the same symptom from many different causes.
  • Signs: The physical evidence of the causal agent itself.
    • Examples: Conks (mushrooms), frass (insect poop), exit holes, webs, insect bodies.
    • Tip: Signs definitive proof.

Biotic vs. Abiotic Disorders

Abiotic (Non-Living)

  • Soil Compaction: Reduces pore space, crushing roots and leading to decline.
  • Drought/Flooding: Water stress mimics many diseases.
  • Chemical Injury: Herbicide drift often causes twisted/curled leaves (epinasty).
  • Mechanical Damage: Lawn mowers, weed whips ("mower blight").

Biotic (Living)

  • Insects:
    • Chewing: Caterpillars, beetles (holes in leaves).
    • Sucking: Aphids, scales, mites (stippling or honeydew).
    • Borers: Emerald Ash Borer, bark beetles (galleries under bark).
  • Fungi: Powdery mildew, root rots (Armillaria), wilts (Verticillium).
  • Bacteria: Fire blight (shepherd's crook stems).

Plant Health Care (PHC) & IPM

The ISA promotes Integrated Pest Management (IPM).

  • Thresholds: You don't treat every pest. You wait until the population reaches a damage threshold.
  • Control Hierarchy:
    1. Cultural: Water, mulch, improve soil (Right Tree, Right Place).
    2. Biological: Release beneficial predators (ladybugs).
    3. Chemical: Pesticides (Last resort).

Exam Strategy

When answering diagnosis questions:

  • Look for clues about host specificity (e.g., "A birch tree has...").
  • Prioritize cultural controls over chemicals in "Best Management" questions.
  • Always check the roots first in scenario questions about general decline.