Diagnosis and Treatment: Pests, Diseases, and Disorders

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

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.