CPH Plant Diseases and Pathology 2 — Questions and Answers
Question 1: Phytophthora root rot is most severe in landscapes when:
- Soils are sandy, dry, and low in organic matter
- Soils are poorly drained with prolonged saturation (Correct answer)
- Plants are grown in full sun with low humidity
- Soils are slightly acidic with high nitrogen levels
Correct answer: Soils are poorly drained with prolonged saturation
Phytophthora species are oomycetes (water molds) that produce motile zoospores requiring free water in saturated or flooded soils to infect plant roots.
Question 2: Black spot (Diplocarpon rosae) on roses is best managed through a combination of which two strategies?
- Soil drenching with systemic insecticides and deep watering
- Removing infected leaves and applying fungicides preventively (Correct answer)
- Overhead irrigation and foliar fertilization
- Heavy pruning in midsummer and applying copper sulfate in fall
Correct answer: Removing infected leaves and applying fungicides preventively
Black spot is managed by removing and disposing of infected leaves to reduce inoculum and applying protectant or systemic fungicides before infection occurs.
Question 3: Crown gall, which produces tumor-like growths at or near the soil line on woody plants, is caused by:
- Agrobacterium tumefaciens (bacterium) (Correct answer)
- Rhizoctonia solani (fungus)
- Tomato ringspot virus
- Pratylenchus penetrans (nematode)
Correct answer: Agrobacterium tumefaciens (bacterium)
Crown gall is caused by Agrobacterium tumefaciens, a bacterium that transfers a segment of its Ti plasmid DNA into plant cells, triggering uncontrolled cell proliferation.
Question 4: Anthracnose diseases of ornamental trees are primarily caused by which group of pathogens?
- Obligate bacterial pathogens
- Coelomycetous and hyphomycetous fungi (Correct answer)
- Phytoplasmas transmitted by leafhoppers
- Cyst nematodes in the root zone
Correct answer: Coelomycetous and hyphomycetous fungi
Anthracnose diseases are caused by fungi such as Colletotrichum, Apiognomonia, and Discula species that produce acervuli and infect young leaves and shoots in wet spring weather.
Question 5: Which of the following best describes a 'systemic' plant disease?
- A disease confined to a single leaf or stem lesion
- A disease that spreads throughout the entire plant via vascular tissue (Correct answer)
- A disease that only affects root tissue below the soil line
- A disease caused by multiple pathogens acting simultaneously
Correct answer: A disease that spreads throughout the entire plant via vascular tissue
Systemic diseases spread throughout the plant via xylem or phloem, meaning the pathogen or its products move beyond the initial infection site to affect the whole plant.
Question 6: The term 'necrosis' in plant pathology refers to:
- Abnormal cell enlargement resulting in gall formation
- Localized or widespread death of plant tissue (Correct answer)
- Yellow discoloration of leaves due to chlorophyll loss
- Distortion of leaves or stems from hormonal imbalance
Correct answer: Localized or widespread death of plant tissue
Necrosis is the death of plant cells or tissues, which appears as brown, tan, or black dead areas on leaves, stems, or roots.
Question 7: Copper-based fungicides and bactericides are commonly used in horticulture primarily because copper:
- Stimulates plant immune responses and strengthens cell walls
- Is toxic to fungal and bacterial cells while having low mammalian toxicity at label rates (Correct answer)
- Provides foliar micronutrient correction alongside disease control
- Degrades rapidly in soil, leaving no residue on harvested crops
Correct answer: Is toxic to fungal and bacterial cells while having low mammalian toxicity at label rates
Copper ions denature proteins and disrupt enzyme function in fungal and bacterial cells, making copper-based products broad-spectrum protectants with a favorable safety profile at registered use rates.
CPH Plant Diseases and Pathology 2
Phytophthora root rot is most severe in landscapes when: