CPH Plant Diseases and Pathology 1 — Questions and Answers
Question 1: Powdery mildew on ornamental plants is caused by which type of organism?
- Bacterium
- Fungus (Correct answer)
- Virus
- Nematode
Correct answer: Fungus
Powdery mildew is caused by obligate fungal pathogens in the order Erysiphales, which produce the characteristic white powdery coating on leaf surfaces.
Question 2: Fire blight, which causes a characteristic 'shepherd's crook' wilting of shoot tips on apple and pear trees, is caused by which pathogen?
- Erwinia amylovora (bacterium) (Correct answer)
- Botryosphaeria obtusa (fungus)
- Apple mosaic virus
- Phytophthora cactorum (oomycete)
Correct answer: Erwinia amylovora (bacterium)
Fire blight is caused by the bacterium Erwinia amylovora, which infects flowers and young shoots, causing the distinctive scorched, shepherd's-crook symptom.
Question 3: Dutch elm disease is caused by which type of pathogen, and is primarily spread by which vector?
- Bacterium spread by aphids
- Fungus spread by bark beetles (Correct answer)
- Virus spread by leafhoppers
- Oomycete spread by water splash
Correct answer: Fungus spread by bark beetles
Dutch elm disease is caused by the fungus Ophiostoma ulmi (and O. novo-ulmi) and is spread primarily by native and European elm bark beetles.
Question 4: Which of the following best describes a 'biotroph' in plant pathology?
- A pathogen that kills host tissue before extracting nutrients
- A pathogen that only colonizes dead plant material
- A pathogen that requires living host tissue to complete its life cycle (Correct answer)
- A pathogen capable of surviving indefinitely in the soil
Correct answer: A pathogen that requires living host tissue to complete its life cycle
Biotrophs are pathogens such as powdery mildews and rusts that must obtain nutrients from living host cells and cannot complete their life cycle on dead tissue.
Question 5: Verticillium wilt causes yellowing and wilting of foliage primarily by:
- Producing toxins that destroy chlorophyll in leaf cells
- Colonizing and blocking the xylem vessels, restricting water flow (Correct answer)
- Attacking root cortex cells, preventing mineral uptake
- Injecting proteases that digest cell walls throughout the plant
Correct answer: Colonizing and blocking the xylem vessels, restricting water flow
Verticillium dahliae colonizes the xylem vessels of host plants, where the fungal mycelium and produced gels physically block water transport, causing wilting.
Question 6: Which of the following symptoms is most characteristic of iron (Fe) deficiency chlorosis?
- Interveinal yellowing beginning on older lower leaves
- Interveinal yellowing beginning on young upper leaves (Correct answer)
- Uniform yellowing of all leaves simultaneously
- Purpling of older leaf margins and tips
Correct answer: Interveinal yellowing beginning on young upper leaves
Iron deficiency causes interveinal chlorosis on young, upper leaves first because iron is immobile in the plant and cannot be reallocated from older tissue.
Question 7: The disease triangle concept in plant pathology states that disease development requires:
- A susceptible host, a virulent pathogen, and a favorable environment (Correct answer)
- Heat, humidity, and a nutrient-deficient soil
- A vector, a wound site, and prolonged wetness
- Soil compaction, poor drainage, and a weakened host
Correct answer: A susceptible host, a virulent pathogen, and a favorable environment
The disease triangle model illustrates that plant disease occurs only when a susceptible host, a virulent pathogen, and a favorable environment all overlap simultaneously.
CPH Plant Diseases and Pathology 1
Powdery mildew on ornamental plants is caused by which type of organism?