B.S.F. or B.Sc.F. Bachelor of Science in Forestry Bachelor of Science in Forestry: Forest Pathology and Disease Management 5 — Questions and Answers
Question 1: Which of the following correctly describes the difference between a primary pathogen and an opportunistic pathogen in forest disease ecology?
- Primary pathogens attack roots while opportunistic pathogens attack foliage
- Primary pathogens can infect and kill vigorous, healthy trees, while opportunistic pathogens typically only cause serious damage to stressed or weakened trees (Correct answer)
- Primary pathogens are exotic introductions while opportunistic pathogens are native species
- Primary pathogens are soilborne while opportunistic pathogens are airborne
Correct answer: Primary pathogens can infect and kill vigorous, healthy trees, while opportunistic pathogens typically only cause serious damage to stressed or weakened trees
Primary pathogens like Cryphonectria parasitica can kill healthy trees, while opportunistic pathogens like Cytospora species mainly cause severe disease only when host trees are predisposed by drought, injury, or other stressors.
Question 2: Dwarf mistletoe (Arceuthobium spp.) causes significant damage in western conifer forests primarily through which mechanism?
- Producing toxins that move systemically and disrupt photosynthesis throughout the crown
- Withdrawing water and nutrients from host xylem while stimulating abnormal host tissue growth, reducing growth and predisposing trees to bark beetles (Correct answer)
- Shading host foliage with dense mistletoe canopy, reducing net photosynthesis
- Introducing fungal pathogens into host vascular tissue through haustorial connections
Correct answer: Withdrawing water and nutrients from host xylem while stimulating abnormal host tissue growth, reducing growth and predisposing trees to bark beetles
Dwarf mistletoes are holoparasites that extract water and carbohydrates via haustoria embedded in host xylem, while also inducing witches' broom formation that reduces host vigor and increases susceptibility to secondary pests.
Question 3: In epidemiological modeling of forest diseases, the basic reproduction number (R0) represents:
- The number of root contacts needed for Armillaria to establish a new infection center
- The average number of secondary infections produced by a single infected individual in a completely susceptible population (Correct answer)
- The minimum inoculum dose required to cause infection in 50% of exposed hosts
- The ratio of susceptible to resistant tree genotypes in a given stand
Correct answer: The average number of secondary infections produced by a single infected individual in a completely susceptible population
R0 greater than 1 means an epidemic will grow, while R0 less than 1 means the pathogen will fade out; this threshold concept guides decisions about how much host density reduction is needed to manage a disease.
Question 4: Forest managers observing a 'type conversion' outcome of Phytophthora ramorum in coastal California tanoak-dominated forests are most concerned about which long-term consequence?
- Loss of timber value as tanoak is replaced by Douglas-fir which has lower market demand
- Replacement of the diverse tanoak understory with invasive shrubs, permanently altering forest structure and wildlife habitat (Correct answer)
- Increased soil erosion because tanoak has deeper roots than the shrubs replacing it
- Reduction in atmospheric carbon sequestration because shrubs have lower biomass than hardwood trees
Correct answer: Replacement of the diverse tanoak understory with invasive shrubs, permanently altering forest structure and wildlife habitat
Sudden oak death kills tanoak en masse, and the resulting openings are often colonized by invasive shrubs like coyotebrush and poison oak, permanently converting mixed forest to shrubland with major wildlife and ecosystem consequences.
Question 5: What is the primary reason that genetic resistance breeding for Dutch elm disease (Ophiostoma novo-ulmi) has been challenging compared to chestnut blight resistance efforts?
- Elms do not produce seed until 25 years of age, making breeding cycles extremely long
- Dutch elm disease resistance in wild Ulmus species is partial and quantitative, controlled by many genes, making resistance harder to transfer than the more discrete resistance in Asian chestnuts (Correct answer)
- The Ophiostoma pathogen mutates rapidly, overcoming resistance genes within a single growing season
- Elms are obligate outbreeders that cannot be selfed, preventing classical backcross breeding
Correct answer: Dutch elm disease resistance in wild Ulmus species is partial and quantitative, controlled by many genes, making resistance harder to transfer than the more discrete resistance in Asian chestnuts
Asian elms show moderate, polygenic resistance to Dutch elm disease rather than the high-level, oligogenic resistance seen in Chinese and Japanese chestnuts, making resistance introgression into American elm slower and less complete.
Question 6: Which abiotic factor most commonly predisposes western conifers to infection by Cytospora canker pathogens in managed forest stands?
- Elevated soil nitrogen from fertilizer applications
- Drought stress that reduces resin pressure and carbohydrate-based defense capacity (Correct answer)
- Excessively wet soils that promote root asphyxiation
- High light intensity causing photoinhibition of the outer crown
Correct answer: Drought stress that reduces resin pressure and carbohydrate-based defense capacity
Drought reduces the constitutive and induced resin-based defenses that otherwise repel Cytospora and other opportunistic bark pathogens, making moisture stress a key predisposing factor.
Question 7: When conducting a forest health aerial survey to map disease outbreaks, which symptom category is LEAST reliably detected by standard color-infrared (CIR) photography?
- Recent mortality with fully red or brown crowns
- Early-stage chlorosis before visible foliage color change occurs (Correct answer)
- Advanced defoliation resulting in thin or absent crowns
- Witches' broom formation from dwarf mistletoe infection
Correct answer: Early-stage chlorosis before visible foliage color change occurs
Color-infrared imagery detects chlorophyll content and near-infrared reflectance, so it captures established symptoms well but misses the very early physiological stress phase before foliage color changes become visible.
B.S.F. or B.Sc.F. Bachelor of Science in Forestry Bachelor of Science in Forestry: Forest Pathology and Disease Management 5
Which of the following correctly describes the difference between a primary pathogen and an opportunistic pathogen in forest disease ecology?