I. Leaf spot of Poa turf grasses is caused by Septoria macropoda. Symptoms of the disease are stripes, spots or blotches near leaf tips, especially those that have been cut during mowing. Lesions vary in color from gray-green to dark purple. Lesions enlarge and typically fade with age to a straw color. The turf area may appear scorched.
The fungus survives unfavorable periods of winter and summer as mycelia or pycnidia in infected debris. The needlelike, septate conidia are produced and liberated from pycnidia during cool, wet periods in the spring and autumn. Conidia are splashed to healthy leaf tissue and may lie dormant on leaves for long periods when temperatures are below 10 C. Conidia germinate and penetrate the leaf cuticle if there is a film of water on the leaf surface. New pycnidia are formed on leaves within 3 weeks after inoculation. Periods of cool, wet weather, especially in spring and fall cause a rapid increase in the amount of disease.
1. Draw a generic disease cycle with the following stages: dormant period, dissemination, penetration, primary inoculum, colonization, symptom development, reproduction of pathogen, and secondary inoculum. (8 pts)
2. Now add the following terms to the appropriate corresponding stage in your drawing: Rainsplash, pycnidia, leaf spot, intercellular, conidia, direct. (6 pts)
3. Based on the above information, is this pathogen an Oomycete, Ascomycete, Deuteromycete, or Basidiomycete? Explain your choice. (3 pts)
4. What is the source of primary inoculum? (2 pts)
5. What is the source of secondary inoculum? (2 pts)
6. Draw a disease triangle and label each corner (one word is sufficient). (3 pt)
7. Now draw a polycyclic disease progress model. LABEL the axes of the graph and parts of the curve limited by each factor above. (4 pts)
8. Pick one of the factors from the triangle, and give a specific example of a management tactic that would limit this factor's effect on leaf spot development on Poa turf grass. (2 pts)
9. Draw a dashed line on your graph in (7) to indicate the result of your management tactic in (8). (2 pts)
II. Pathogen Biology. Match the terms in the left-hand column with the corresponding pathogen group in the right hand column. For each pathogen group there is more than one correct answer. (10 pts)
|1. haustorium ____________________
|2. planting stock ____________________
|3. necrotroph ____________________
|4. insects ____________________
|9. resting structures____________________
|10. direct penetration____________________
III. Scotch broom is considered an invasive weed in Oregon.
1. You've been assigned the task of engineering a necrotrophic fungal pathogen that will cause disease on scotch broom. You have identified several different toxins that can be put into your designer pathogen. As described in class, toxins produced by pathogens can be divided into two classes. What are these two classes? What class of toxin would you put into your designer pathogen? Why? (6 pts)
2. Your designer pathogen has been wildly successful at controlling scotch broom. However, 20 years later scientists have discovered a potent anticancer compound in scotch broom. You've now been assigned the task of making scotch broom resistant to your designer fungal pathogen. Name one preexisting and one inducible plant defense mechanism or molecule (not including SAR) that may be modified to improve resistance to this foliar pathogen. (4 pts)
3. You suspect that systemic acquired resistance (SAR) is one possible mechanism for minimizing disease caused by your designer pathogen. Describe a simple experiment that may help test this hypothesis. (BONUS 4pts)
IV. A potato grower near Klamath Falls, OR had severe losses from late blight in his potato crop in 1999. He is very concerned about losses on his farm again this year. The following is a list of potential sources of inoculum for this disease in 2000.
A. A large cull pile of potatoes to be stacked outside a processing factory 5 miles UPWIND from his field.
B. Seed potatoes purchased from a seed grower that also had late blight in 1999.
C. A neighbor 30 miles DOWNWIND who will plant his 120-acre field with the same potato seed lot as you are.
D. A major epidemic of late blight in the Columbia Basin (northcentral) Oregon.
List the above situations in order of risk (high to low) to his field. Explain your reasoning (6 pts).
V. Bean common mosaic is a seedborne virus of green beans. Programs to produce virus-free seed are in place in many bean-producing states. A technique commonly used is micropropagation.
1. In the space below, outline the process for producing virus-free bean seed in a large enough quantity for the 10,000 acres of commercial snap bean production in Oregon. (6 pts)
2. Give a definition of the following terms as they apply to pathogen-free programs. (10 pts)
b. Meristem tip culture:
e. Disease tolerance:
3. Bean common mosaic is vectored by aphids. A small proportion of infected seed is sufficient to provide enough inoculum to spread and infect many plants early in the season (first cycle of disease). If a seedlot has 10 infected seed per 10,000 seed, this can result in 30% disease 40 days after emergence. Assume one cycle of disease.
a. If we assume the infection rate does not change, to what level would the incidence of infected seed have to be in order to have only 2% disease incidence at 40 days after emergence. (6 pts) Show your work. (ln 1/1-x = QRT)
VI. In each of the questions below, there are two choices marked (A) and (B) with which to complete the statement. EITHER answer, BOTH answers, or NEITHER answer may be correct. You are to CIRCLE all the answers, if any, that correctly complete each statement; i.e. circle one, or both, or neither of the letters in each question. (20 pts)
1. The period between inoculation and sporulation is termed (A) latent period, (B) infectious period.
2. Long distance dispersal of fungal spores is a function of (A) pigmentation, (B) shape.
3. Disease increase in polycyclic diseases depends upon the (A) current amount of disease, (B) amount of initial inoculum.
4. The planting of a barrier crop around a susceptible host would be a form of (A) eradication, (B) exclusion.
5. The human eye recognizes diseased tissue most readily at (A) high disease levels, (B) low disease levels.
6. For inoculum to be transported long distances by wind, it must be placed into the (A) laminar boundary layer, (B) turbulent boundary layer.
7. Polycyclic pathogens are characterized as having (A) high birth rates, (B) high death rates.
8. (A) Rain tap, (B) rain splash dispersal occurs when raindrops hit leaves, thus mechanically jarring the propagules into the air.
9. With polycyclic diseases, the value of sanitation increases with a (A) increase, (B) decrease in the apparent infection rate.
10. With (A) polycyclic pathogens, (B) monocyclic pathogens, the effect of reducing initial inoculum is to delay the epidemic.