B.S.F. or B.Sc.F. Bachelor of Science in Forestry Bachelor of Science in Forestry: Forest Pathology and Disease Management 4 — Questions and Answers
Question 1: Chestnut blight, caused by Cryphonectria parasitica, kills American chestnut by producing which mechanism?
- Systemic toxins that move through the xylem and kill the root system
- Oxalic acid that lowers canker pH, killing cambial tissue while sparing roots (Correct answer)
- Blocking phloem transport by forming dense mats of mycelium throughout the bole
- Producing enzymes that digest the heartwood, causing structural failure
Correct answer: Oxalic acid that lowers canker pH, killing cambial tissue while sparing roots
Cryphonectria parasitica produces oxalic acid within cankers, creating a low-pH environment that kills cambial tissue but cannot penetrate below the soil line, allowing root sprouting.
Question 2: Which approach is the basis of the hypovirulence strategy for biological control of chestnut blight?
- Introducing CHV1 hypovirus into virulent fungal strains to reduce their pathogenicity (Correct answer)
- Breeding American chestnuts with genes from Chinese chestnut for blight resistance
- Applying systemic fungicides to protect the cambium from initial infection
- Removing infected trees to reduce regional inoculum levels
Correct answer: Introducing CHV1 hypovirus into virulent fungal strains to reduce their pathogenicity
CHV1 (Cryphonectria hypovirus 1) infects virulent Cryphonectria strains and reduces their virulence, allowing infected cankers to heal—a strategy that has succeeded in Europe but faces somatic incompatibility barriers in North America.
Question 3: In forest pathology surveys, a 'periodic detection survey' differs from an 'intensive delimiting survey' primarily in that it:
- Uses satellite imagery rather than ground-truthing
- Covers large areas to detect new or spreading pests, while delimiting defines the exact boundaries of a known infestation (Correct answer)
- Focuses exclusively on native pathogens rather than exotic introductions
- Is conducted annually whereas delimiting surveys occur every five years
Correct answer: Covers large areas to detect new or spreading pests, while delimiting defines the exact boundaries of a known infestation
Detection surveys cast a wide net to find new infestations across large landscapes, while delimiting surveys intensively map the precise extent of a known infestation to guide management response.
Question 4: Which fungal structure allows Armillaria species to spread through soil and infect new host roots at a distance from the original inoculum source?
- Sclerotia
- Rhizomorphs (Correct answer)
- Chlamydospores
- Appressoria
Correct answer: Rhizomorphs
Rhizomorphs are melanized, root-like mycelial strands that allow Armillaria to actively grow through soil and contact healthy roots, enabling spread far from the original infected stump.
Question 5: What is the epidemiological significance of asymptomatic carriers in forest disease spread, as seen with Phytophthora ramorum?
- Carriers are immune and thus serve as natural buffers that slow disease progression
- Some hosts like bay laurel can be heavily infected and produce massive quantities of spores without showing severe symptoms, fueling regional epidemics (Correct answer)
- Asymptomatic trees have neutralized the pathogen and can be used as sources of resistant germplasm
- Carriers allow early detection because they show subtle symptoms before neighboring trees become infected
Correct answer: Some hosts like bay laurel can be heavily infected and produce massive quantities of spores without showing severe symptoms, fueling regional epidemics
California bay laurel is a highly productive sporulating host for Phytophthora ramorum that shows minimal disease symptoms, making it a major driver of the sudden oak death epidemic.
Question 6: The CODIT (Compartmentalization of Decay in Trees) model, developed by Alex Shigo, explains how trees respond to wounding by:
- Producing systemic antifungal compounds that kill pathogens throughout the tree
- Forming four walls of chemical and anatomical barriers to limit decay spread in existing and new wood (Correct answer)
- Shedding infected branches through abscission to prevent decay from entering the trunk
- Rapidly replacing infected sapwood with heartwood to physically block pathogen advance
Correct answer: Forming four walls of chemical and anatomical barriers to limit decay spread in existing and new wood
CODIT describes four compartmentalization walls that trees form—blocking vertical, radial, and tangential spread in existing wood, plus a barrier zone in new wood—to limit decay following wounding.
Question 7: Which practice best reduces the risk of introducing Phytophthora pathogens into previously unaffected forest stands?
- Applying foliar copper-based fungicides before site entry
- Cleaning and disinfecting equipment and footwear before entering and upon leaving infested areas (Correct answer)
- Planting resistant cultivars as a buffer around the perimeter of forest stands
- Removing all standing water within 100 meters of the forest edge
Correct answer: Cleaning and disinfecting equipment and footwear before entering and upon leaving infested areas
Cleaning and disinfecting boots, tires, and equipment removes soil and plant material that harbor Phytophthora propagules, preventing their introduction to clean sites.
B.S.F. or B.Sc.F. Bachelor of Science in Forestry Bachelor of Science in Forestry: Forest Pathology and Disease Management 4
Chestnut blight, caused by Cryphonectria parasitica, kills American chestnut by producing which mechanism?