ISA - ISA Certified Arborist Pest and Disease Diagnosis 2 — Questions and Answers
Question 1: An arborist is examining a crabapple tree where the tips of several new shoots have wilted and bent into a distinctive hook shape. The affected bark appears water-soaked and later turns dark brown. Which type of pathogen is responsible for this presentation?
- Fungus
- Bacterium (Correct answer)
- Virus
- Nematode
Correct answer: Bacterium
The shepherd's crook wilting of new shoot tips combined with water-soaked bark that darkens are classic signs of fire blight, caused by the bacterium Erwinia amylovora. Bacteria — not fungi or viruses — produce this rapid, scorched appearance in rosaceous hosts.
Question 2: An arborist is called to assess an oak tree that died rapidly during summer, with wilting and browning progressing from the upper crown downward. When branches are cross-sectioned, a ring of brown streaking is visible in the outer sapwood. Which disease best matches this presentation?
- Oak anthracnose
- Sudden oak death
- Oak wilt (Correct answer)
- Hypoxylon canker
Correct answer: Oak wilt
Oak wilt (caused by Bretziella fagacearum) is a vascular disease that interrupts water movement, producing the characteristic brown discoloration in the outer sapwood ring visible in cross-sections. The rapid top-down crown decline during summer is also a hallmark of oak wilt.
Question 3: An arborist inspects a honeylocust tree and finds fine silken webbing on the undersides of leaves along with tiny yellow flecks stippling the upper leaf surfaces. By late summer, the foliage has bronzed and is dropping prematurely. What is the most likely cause?
- Leafhoppers
- Spider mites (Correct answer)
- Lace bugs
- Whiteflies
Correct answer: Spider mites
Spider mites (family Tetranychidae) are the only common arthropod pests that produce silken webbing on foliage. Their feeding on individual plant cells creates the characteristic yellow stippling that, in heavy infestations, leads to bronzing and early defoliation.
Question 4: While inspecting an eastern hemlock, an arborist finds cottony white masses clustered at the base of needles on the undersides of branches, combined with yellowing needles and general canopy thinning. This presentation is most diagnostic of which pest?
- Elongate hemlock scale
- Hemlock woolly adelgid (Correct answer)
- Hemlock looper
- Pine bark adelgid
Correct answer: Hemlock woolly adelgid
Hemlock woolly adelgid (Adelges tsugae) produces the distinctive white woolly egg sacs (ovisacs) attached at the base of hemlock needles. This is a key diagnostic sign that distinguishes it from scale insects, which produce flat or waxy coverings, and from defoliating caterpillars.
Question 5: An arborist observes orange powdery pustules erupting from the undersides of apple leaves. In the context of tree disease diagnosis, how should these pustules be classified?
- A symptom, because they represent the tree's visible reaction to stress
- A sign, because they are physical structures produced by the pathogen itself (Correct answer)
- A symptom, because they cause visible damage to leaf tissue
- A sign, because they indicate the tree is mounting a defense response
Correct answer: A sign, because they are physical structures produced by the pathogen itself
Signs are physical evidence of the pathogen itself — spores, mycelium, fruiting bodies, or other pathogen structures. Orange pustules on leaf undersides are uredinia, spore-bearing structures of a rust fungus, making them signs rather than symptoms (which are the host plant's responses such as chlorosis, necrosis, or wilting).
Question 6: An arborist needs to collect a branch sample from a tree exhibiting a stem canker for submission to a plant diagnostic laboratory. Which portion of the tissue should be sampled to give the laboratory the best chance of identifying the causal organism?
- Healthy tissue well away from any visible discoloration
- The center of the most severely necrotic, dried-out tissue
- The margin between visibly healthy and diseased tissue (Correct answer)
- Only the outer bark surface scraped from the canker edge
Correct answer: The margin between visibly healthy and diseased tissue
The transition zone at the margin between healthy and diseased tissue contains actively growing pathogen and is the most reliable area for culturing or other diagnostic methods. Severely necrotic tissue in the canker center is often dead and colonized by secondary saprophytes, while healthy tissue may not yet contain the pathogen.
ISA - ISA Certified Arborist Pest and Disease Diagnosis 2
An arborist is examining a crabapple tree where the tips of several new shoots have wilted and bent into a distinctive hook shape.
The affected bark appears water-soaked and later turns dark brown.
Which type of pathogen is responsible for this presentation?