ISA - ISA Certified Arborist Pest and Disease Diagnosis 2

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?

Accepted Answer

Bacterium

Answer

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?

Accepted Answer

Oak wilt

Answer

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?

Accepted Answer

Spider mites

Answer

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?

Accepted Answer

Hemlock woolly adelgid

Answer

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?

Accepted Answer

A sign, because they are physical structures produced by the pathogen itself

Answer

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).

(ISA) ISA Certified Arborist Practice Test

(ISA) ISA Certified Arborist Practice Test

ISA - ISA Certified Arborist Pest and Disease Diagnosis 2