20 Diagnosis & Treatment Practice Questions & Answers

1. 1. What is the correct first step when diagnosing a disorder on a landscape tree?

   • A.Apply a broad-spectrum treatment and watch for improvement
   • B.Collect a foliage sample and send it to a diagnostic lab
   • C.Identify the tree species and learn what is normal for it✓ Answer
   • D.Fertilize the root zone to rule out a nutrient deficiency

   The species determines which pests, pathogens and site sensitivities are even possible, and it defines what is normal, such as exfoliating bark, natural leaf shed or early fall colour. Without that baseline an arborist can easily treat a normal species trait as a disorder.

   Source: ISA Arborists' Certification Study Guide, diagnostic process: identify the plant firstReport a problem with this question

2. 2. An arborist cannot identify a tree disorder in the field. What is the appropriate professional response?

   • A.Treat for the most common pest of that host species
   • B.Log the case as seasonal variation and check it next year
   • C.Send a representative sample to a diagnostic laboratory✓ Answer
   • D.Tell the client that unknown disorders rarely need action

   Diagnosis beyond one's competence must be referred rather than guessed, and the sample must be representative, including the transition zone between healthy and affected tissue rather than dead tissue alone, because dead tissue is usually colonized by secondary organisms that mask the true cause.

   Source: ISA Arborists' Certification Study Guide, diagnosis: sampling and referral to a diagnostic laboratoryReport a problem with this question

3. 3. A shelf-like fungal fruiting body is growing on a tree trunk. In diagnostic terms this observation is:

   • A.A sign, because it is physical evidence of the fungus itself✓ Answer
   • B.A syndrome, meaning the whole set of observed changes
   • C.A secondary symptom of drought stress in the upper crown
   • D.A symptom, because it is the tree's response to the decay

   A sign is physical evidence of the causal agent itself, such as a conk, mycelial fan, spore mass or frass, while a symptom is the tree's reaction, such as dieback, wilting or chlorosis. Signs are more diagnostically reliable because many unrelated causes produce identical symptoms.

   Source: American Phytopathological Society, signs versus symptoms of plant diseaseReport a problem with this question

4. 4. Which of these field observations is a symptom rather than a sign?

   • A.White powdery fungal growth on leaf surfaces
   • B.Frass and boring dust at a bark crevice
   • C.Browning of leaf margins with twig dieback✓ Answer
   • D.Mycelial fans under loosened bark plates

   Marginal browning and twig dieback are the tree's own reaction to some stress, so they are symptoms; frass, mycelial fans and visible fungal growth are the organisms or their products and are therefore signs. The same symptom can follow drought, root loss, salt or infection, which is why it cannot identify a cause on its own.

   Source: American Phytopathological Society, signs versus symptoms of plant diseaseReport a problem with this question

5. 5. In one season, marginal leaf scorch appears on maples, lindens and pines all along one edge of a paved lot. What does this pattern most strongly suggest?

   • A.Damage from a wilt passed through root grafts
   • B.Damage from an abiotic factor acting site-wide✓ Answer
   • C.Damage from a fungal leaf disease spread by spores
   • D.Damage from borers moving between host trees

   Pathogens and insects are usually host-specific and spread progressively, so damage that appears at once on unrelated species and follows a line or a site feature points to an abiotic cause such as salt, drift, reflected heat or restricted rooting. The pattern, not the symptom, carries the diagnostic information here.

   Source: ISA Arborists' Certification Study Guide, damage patterns distinguishing abiotic and biotic causesReport a problem with this question

6. 6. New leaves on several unrelated trees beside a lawn are cupped, strapped and twisted while older leaves look normal. What is the most likely cause?

   • A.An iron deficiency induced by a high soil pH
   • B.A leaf-mining insect feeding between leaf surfaces
   • C.A canker fungus girdling the affected twigs
   • D.A growth-regulator herbicide reaching the tree✓ Answer

   Growth-regulator herbicides distort tissue that is actively expanding, so the injury shows as cupping, strapping and epinasty on new growth while mature leaves stay normal. Because drift or root uptake affects whatever is in the treated area, several unrelated species are hit at once, which is the abiotic signature.

   Source: Arboriculture & Urban Forestry, herbicide injuries to trees: growth-regulator symptomsReport a problem with this question

7. 7. An arborist manages a recurring foliar fungal disease by thinning for air movement, watering at the base instead of overhead, and removing fallen infected leaves. Which principle underlies this approach?

   • A.Disease develops only when host, pathogen and environment coincide✓ Answer
   • B.A pathogen is eliminated whenever the host species is resistant
   • C.Fungal spores cannot infect leaves that have been fertilized
   • D.Foliar pathogens die out once the tree has reached maturity

   The disease triangle holds that a susceptible host, a capable pathogen and a favourable environment must be present together, so removing any one leg prevents disease. Environment is the leg an arborist can most often change, which is why drying the foliage, improving air movement and sanitation are the primary cultural controls.

   Source: Oregon State University Extension, the disease triangleReport a problem with this question

8. 8. Why does a stem-girdling root typically cause decline on one side of the crown?

   • A.It compresses the conducting tissue at the point of contact✓ Answer
   • B.It carries a root-rot fungus into the buried root collar area
   • C.It blocks sunlight from reaching the lower trunk tissue
   • D.It releases a toxin that kills the roots on that side

   As the root and the stem both grow in diameter they press against each other, crushing the phloem and outer xylem where they touch, so water and food movement is restricted only in that sector and only the crown served by it thins and dies back. A buried root flare hides the defect, so the root collar must be excavated to find it.

   Source: ISA Best Management Practices for tree planting: root collar excavation and stem-girdling rootsReport a problem with this question

9. 9. The newest leaves of a tree are yellow between green veins while older leaves stay green, and the soil is strongly alkaline. What is the best explanation?

   • A.The soil contains almost no iron and must be replaced
   • B.Rain has leached nitrogen below the rooting zone
   • C.A vascular wilt fungus is plugging the youngest shoots
   • D.A high soil pH is making iron unavailable to the tree✓ Answer

   Iron is immobile in the plant, so a shortage shows first on the newest leaves as interveinal chlorosis with green veins, whereas mobile nitrogen is withdrawn from old leaves and yellows them first. Alkaline soils usually contain plenty of iron but hold it in insoluble form, so correcting or acidifying the soil condition outlasts simply adding iron.

   Source: University extension plant nutrition guidance on lime-induced iron chlorosis at high soil pHReport a problem with this question

10. 10. One species in a planting shows marginal leaf browning that began on scattered branches, worsens each year and returns even after wet summers. How does this differ from ordinary drought scorch?

    • A.Damage that recurs each year always traces to de-icing salt
    • B.Damage from drought occurs only where soils are compacted
    • C.Damage from drought appears first on shaded interior foliage
    • D.Damage spreads branch by branch yearly, showing an infectious cause✓ Answer

    Environmental scorch tracks the weather, appears on many species at once and does not advance in a fixed direction from year to year, whereas an infectious scorch confined to one host species creeps outward branch by branch and recurs regardless of rainfall. Progression over time plus host specificity is the biotic signature, and confirming the pathogen requires laboratory testing.

    Source: University extension plant pathology guidance on bacterial leaf scorch versus environmental scorchReport a problem with this question

11. 11. A mature tree over compacted, recently regraded soil shows dieback in the upper crown, and borer exit holes are found in the upper trunk. What is the most appropriate response?

    • A.Apply a high-nitrogen fertilizer to force out new growth
    • B.Relieve the site stress with irrigation and aeration✓ Answer
    • C.Remove all dead branches and consider the matter closed
    • D.Spray the trunk repeatedly through the growing season

    Most trunk and branch borers are secondary pests that colonize trees already weakened by root loss, compaction or drought, so killing the insect does not restore the tree. Correcting the underlying cultural problem, by deep infrequent watering, mulch over the root zone and relieving compaction, addresses the cause, while nitrogen pushed onto a root-damaged tree can deepen the decline.

    Source: ISA Arborists' Certification Study Guide, secondary pests and correcting the underlying stressReport a problem with this question

12. 12. A tree in a small compacted pit was heavily defoliated by caterpillars two seasons ago and now carries a canker fungus. Which is the inciting factor?

    • A.The species chosen for that difficult site
    • B.The defoliation two growing seasons ago✓ Answer
    • C.The compacted, undersized planting pit
    • D.The canker fungus now in the branches

    Decline is usually multi-causal: long-term site conditions such as the cramped compacted pit are predisposing factors, a short acute event such as heavy defoliation, drought or freeze is the inciting factor, and opportunistic organisms such as canker fungi or borers are contributing factors. Naming the level correctly determines whether the arborist treats the site, the event or the organism.

    Source: ISA Arborists' Certification Study Guide, predisposing, inciting and contributing factors in tree declineReport a problem with this question

13. 13. Leaves and parked cars beneath a tree are sticky, and a black film has developed on the leaf surfaces. What does this most likely indicate?

    • A.A root pathogen is forcing the sap out through the leaves
    • B.A leaf-spot fungus is infecting the upper leaf surfaces
    • C.A wood-boring beetle is pushing frass from its galleries below
    • D.A piercing-sucking insect is excreting honeydew that molds✓ Answer

    Aphids, scales, mealybugs and similar piercing-sucking insects feed on phloem sap and excrete the excess sugar as honeydew, on which sooty mould then grows. The mould is not a pathogen of the tree; it is a sign that a sucking insect is feeding above, so the insect population, not the black film, is what management addresses.

    Source: Connecticut Agricultural Experiment Station, insects and their injury to plants: piercing-sucking insects and honeydewReport a problem with this question

14. 14. Why does a vascular wilt disease often show as sudden wilting and browning of a whole branch rather than gradual thinning?

    • A.The pathogen releases a gas that scorches the foliage overnight
    • B.The pathogen digests leaf pigments so the branch loses its colour
    • C.The pathogen strips the phloem so sugars cannot leave the leaves
    • D.The pathogen and the tree's response plug the water-conducting xylem✓ Answer

    Wilt organisms grow in the xylem and the tree responds by walling off those vessels with gums and tyloses, so water delivery to everything above that point stops almost at once and the branch flags. Because such pathogens can also move between neighbouring trees through grafted roots, severing root connections is a recognized control.

    Source: ISA Arborists' Certification Study Guide, vascular wilt diseases and xylem occlusionReport a problem with this question

15. 15. Why is treatment for armored scale usually timed to the crawler stage?

    • A.Crawlers are the only stage that harms the foliage badly
    • B.Crawlers are the egg-laying stage of the scale population
    • C.Crawlers are mobile and lack the protective waxy cover✓ Answer
    • D.Crawlers feed on the roots and are reached by soil drenches

    Settled scales sit under a waxy shield that keeps contact materials off the insect, but the newly hatched crawler moves over the bark and foliage unprotected, so that is the vulnerable window. Timing treatment to the vulnerable life stage, using monitoring, degree-day models or phenological indicators rather than the calendar, is a core integrated management practice.

    Source: ISA Best Management Practices: Integrated Pest Management, timing treatment to the vulnerable life stageReport a problem with this question

16. 16. A large bracket fungus is fruiting at the root flare of a mature tree. What does this most reliably tell the arborist?

    • A.The decay is already well advanced in the roots and butt✓ Answer
    • B.The tree will compartmentalize and heal the decayed wood
    • C.The fungus has just begun colonizing the outer bark
    • D.A fungicide drench will cure the affected root zone

    A fruiting body is the reproductive structure of a fungus that has already been decaying wood internally, often for years, so it is a sign of established root or butt decay rather than a new infection. Trees compartmentalize and seal over wounds but never restore decayed wood, and there is no chemical cure for internal decay, so the finding calls for further assessment of the tree.

    Source: ISA Arborists' Certification Study Guide, wood decay fungi and compartmentalizationReport a problem with this question

17. 17. In integrated pest management, what does the action threshold define?

    • A.The number of inspections required in each season
    • B.The pest level at which action becomes justified✓ Answer
    • C.The lowest rate of product allowed by the label
    • D.The pest level at which eradication is achieved

    Integrated management aims to hold pests below a level of damage that the client and the tree can tolerate, not to eliminate them, so the threshold is agreed with the client and checked by regular monitoring. Below that level continued monitoring with no treatment is a legitimate recommendation, and every observation and decision is documented.

    Source: ISA Best Management Practices: Integrated Pest Management, action thresholdsReport a problem with this question

18. 18. After repeated broad-spectrum insecticide sprays, spider mites explode on a treated tree. What best explains this outcome?

    • A.The sprays washed protective wax from the surface of the leaves
    • B.The sprays killed the predators that had been holding mites down✓ Answer
    • C.The sprays stimulated the tree to produce extra young shoots
    • D.The sprays raised the nitrogen content of the treated foliage

    Broad-spectrum materials kill predatory mites, lady beetles and parasitoids along with the target pest, and those natural enemies usually recover more slowly than the pest, so a secondary outbreak or resurgence follows. Conserving natural enemies, spot-treating and rotating modes of action are the integrated answers to this problem.

    Source: ISA Best Management Practices: Integrated Pest Management, natural enemies and secondary pest outbreaksReport a problem with this question

19. 19. How does plant health care (PHC) differ from integrated pest management (IPM)?

    • A.PHC is the emergency response used after IPM monitoring has failed
    • B.PHC covers site, culture and client goals, with pest work in it✓ Answer
    • C.PHC replaces monitoring with a fixed schedule of treatments
    • D.PHC applies only to pesticide selection and to application timing

    IPM is the pest-management decision process, while PHC is the broader programme for the whole landscape: species selection and placement, planting depth, mulch, irrigation, soil management and structural pruning, carried out against the client's objectives and budget. Because a vigorous tree resists secondary pests on its own, prevention and monitoring, not treatment, form the core of PHC.

    Source: ISA Best Management Practices: Integrated Pest Management, scope of a plant health care programmeReport a problem with this question

20. 20. A plant health care programme calls for a pesticide application on a client's property. What governs how that material may lawfully be used?

    • A.The schedule set by the neighbouring property's contractor
    • B.The strongest rate the equipment can safely deliver
    • C.The label directions, followed by a licensed applicator✓ Answer
    • D.The written preference of the client paying for the service

    The label is a legally binding instruction, and using a product inconsistently with it is unlawful, while certification or licensing requirements for the applicator are set by the jurisdiction where the work is done and must be determined locally. An arborist who is not appropriately certified refers the application to someone who is, and records of each application are kept.

    Source: Pesticide label as legally binding directions and jurisdictional applicator certification; ISA Best Management Practices: Integrated Pest ManagementReport a problem with this question

Practice questions based on the published knowledge domains of the ISA Certified Arborist examination outline and on standard arboriculture references. ISA and Certified Arborist are marks of the International Society of Arboriculture; this site is not affiliated with or endorsed by ISA. Answers here deliberately avoid pesticide products and rates, standard clause numbers and the dimensions recalled from them, regional species recommendations and local ordinances, all of which change and vary by place — always follow the pesticide label and your jurisdiction's applicator licensing, the tree-care safety standards in force, and the requirements of the authority having jurisdiction. Confirm current exam requirements with ISA before testing. About the ISA Certified Arborist credential →