ISA Pest and Disease Diagnosis 3 — Questions and Answers
Question 1: What is the characteristic sign of Dutch elm disease in an infected tree?
- White powdery coating on leaves
- Flagging of branches with brown, wilted leaves that curl inward (Correct answer)
- Mushroom fruiting bodies at the base of the trunk
- Bright orange cankers on the bark
Correct answer: Flagging of branches with brown, wilted leaves that curl inward
Dutch elm disease, caused by the fungus Ophiostoma, produces characteristic branch flagging where leaves wilt, turn brown, and curl inward. This often appears first in the upper crown.
Dutch elm disease (DED) is caused by the fungi Ophiostoma ulmi and O. novo-ulmi, spread primarily by elm bark beetles. The hallmark symptom is 'flagging' — individual branches wilt rapidly, with leaves turning yellow to brown and curling inward along the midrib. This occurs because the fungus invades the xylem, producing tyloses that block water transport. Diagnosis is confirmed by peeling bark from symptomatic branches to reveal brown streaking in the sapwood. Early detection and prompt removal of infected branches (at least 10 feet beyond visible symptoms) can sometimes save the tree.
Question 2: Which insect pest creates D-shaped exit holes in ash trees?
- Asian longhorned beetle
- Emerald ash borer (Correct answer)
- Bronze birch borer
- Mountain pine beetle
Correct answer: Emerald ash borer
The emerald ash borer (Agrilus planipennis) creates distinctive D-shaped exit holes approximately 3-4mm wide when adults emerge from beneath the bark of ash trees.
The emerald ash borer (EAB) is an invasive wood-boring beetle from Asia that has devastated ash tree populations across North America. Adult beetles create characteristic D-shaped exit holes (approximately 3-4mm wide) when they emerge from the tree. Other diagnostic signs include S-shaped serpentine larval galleries beneath the bark, crown dieback beginning at the top of the tree, epicormic sprouting from the trunk, bark splitting, and increased woodpecker activity. EAB larvae feed on the phloem and outer sapwood, disrupting nutrient and water transport, typically killing untreated trees within 3-5 years of infestation.
Question 3: What diagnostic technique involves removing a section of bark to examine underlying wood for discoloration patterns?
- Foliar analysis
- Bark tracing (Correct answer)
- Increment boring
- Root collar excavation
Correct answer: Bark tracing
Bark tracing involves carefully removing bark sections to examine the cambium and sapwood underneath for patterns of discoloration, canker margins, or insect galleries that aid in diagnosis.
Bark tracing is a fundamental diagnostic technique in arboriculture. By carefully removing sections of outer bark with a draw knife or chisel, the arborist can examine the cambium, phloem, and outer sapwood for diagnostic clues. Vascular discoloration (brown or green streaking) may indicate fungal wilt diseases. Clean wound margins versus diffuse staining help distinguish between active infections and old injuries. Serpentine galleries indicate boring insects. The technique is particularly valuable for diagnosing canker diseases, where tracing the canker margin reveals the extent of infection and whether it is actively expanding or contained.
Question 4: Which of the following conditions is most commonly misidentified as a disease but is actually caused by environmental stress?
- Fire blight
- Leaf scorch (Correct answer)
- Oak wilt
- Anthracnose
Correct answer: Leaf scorch
Leaf scorch — browning of leaf margins — is frequently caused by environmental stress factors such as drought, salt exposure, or root damage, and is often mistakenly attributed to disease.
Leaf scorch presents as browning and drying of leaf margins, often progressing inward. While bacterial leaf scorch (caused by Xylella fastidiosa) is a genuine disease, the vast majority of leaf scorch cases are caused by abiotic factors: drought stress, excessive heat, salt damage (road salt or irrigation water), wind desiccation, restricted root zones, or soil compaction limiting water uptake. Proper diagnosis requires considering site history, soil conditions, watering practices, and the pattern of symptoms across the tree and landscape. Laboratory testing for Xylella can confirm or rule out bacterial leaf scorch when abiotic causes seem unlikely.
Question 5: What is the primary vector for the spread of oak wilt from tree to tree over long distances?
- Wind-blown spores
- Nitidulid (sap) beetles carrying fungal spores (Correct answer)
- Root contact between adjacent trees
- Contaminated pruning tools
Correct answer: Nitidulid (sap) beetles carrying fungal spores
Nitidulid beetles (sap beetles) are the primary overland vectors of oak wilt. They are attracted to fungal mats produced on killed red oaks and carry spores to fresh wounds on healthy trees.
Oak wilt, caused by the fungus Bretziella fagacearum, spreads by two mechanisms: underground through interconnected root grafts between nearby oaks, and overland via nitidulid (sap) beetles. The fungus produces pressure pads called fungal mats beneath the bark of recently killed red oaks. These mats crack the bark and emit a fruity odor that attracts nitidulid beetles. The beetles pick up spores while feeding on the mats and can carry them to fresh wounds on healthy oaks. This is why pruning oaks during the growing season (especially April-July) is strongly discouraged — fresh wounds attract the spore-carrying beetles.
Question 6: When diagnosing a tree with progressive crown decline, yellowing foliage, and no visible trunk defects, what root-zone issue should be investigated first?
- Lightning strike damage
- Girdling roots (Correct answer)
- Bacterial wetwood
- Sapsucker damage
Correct answer: Girdling roots
Girdling roots compress the trunk at or below the soil line, restricting vascular flow. They are a common but often overlooked cause of progressive crown decline with no visible above-ground trunk defects.
Girdling roots are roots that grow laterally around the base of the trunk rather than radiating outward. Over time, they compress the trunk's vascular tissue, restricting the flow of water and nutrients to the crown. Symptoms include progressive thinning and dieback of the crown, undersized or chlorotic leaves, and early fall color on one side. The trunk base may lack a normal root flare, appearing to go straight into the ground like a telephone pole. Diagnosis requires a root collar excavation using an air spade or careful hand work to expose the root flare area. Treatment involves cutting the girdling root where it contacts the trunk, provided it does not represent the tree's primary structural root system.
ISA Pest and Disease Diagnosis 3
What is the characteristic sign of Dutch elm disease in an infected tree?