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I. Match the pathogen group in the left-hand column with the correct description in the right hand column. Maximum number of correct answers per description is 2, the minimum number is 1 (16 pts).

II. A cooperative farm raising organic tomatoes found that 70% of the roots had lesions caused by the root lesion nematode, Pratylenchus penetrans, at 100 days after planting. Prior to planting, the nematode testing laboratory had measured 10 P. penetrans/gram of soil. Knowing that there is no secondary inoculum with this disease, the cooperative set out to clean up the tomato debris at the end of the season with the objective of lowering the amount of disease the following year.

A. The life strategy of this nematode is that of a migratory ectoparasite. Describe this life strategy. (3 pts)

 

 

B. What is the key morphological trait that distinguishes pathogenic nematodes from saprophytic nematodes. (2 pts)

 

 

C. Disease severity rather than disease incidence was used to measure the amount of this disease in the tomato population. What is the difference between these two assessment methods? (4 pts)

 

 

D. Use the correct equation below to calculate the rate of disease progress (r or R) (3 pts). (SHOW YOUR WORK)

ln x/1-x = ln (xo/1-xo) + rt ln (1/1-x) = QRT

 

 

 

 

 

 

E. To what level would the primary inoculum need to be reduced to have 5% disease at 100 days after planting. (3 pts). SHOW YOUR WORK.

 

 

 

 

 

F. On the graph below, plot severity of root lesions versus time. Be certain to label the axes of the graph. (5 pts)

Date	Disease severity
August 5	1 %
August 12	8 %
August 19	22 %
August 25	53 %
August 31	76 %
September 7	93 %
September 14	96 %

 

 

 

 

 

 

 

G. Why does the curve flatten out at a high level of disease? (2 pts)

 

 

H. Now identify a disease management tactic that the cooperative might use to limit the amount of disease the following season. (2 pts)

 

 

I. Draw a dashed line on your graph in F to indicate the result of your management tactic. (2 pts)

J. The cooperative was successful in reducing the primary inoculum to this level, but sustained 30% disease the following year at 100 days. How would you explain this. Give two reasons. (4 pts)

III. Complete the following questions about virus and bacterial pathogens.

A. Insect vectors play an important role in dissemination of plant viruses. What are 2 other ways in which viruses can be transmitted from one plant to another? (4 pts)

 

 

B. Entry into the plant cell and uncoating of the virion is the first step in the disease cycle of plant viruses. The transmission of the virus to a healthy plant is the last. What are the 5 middle steps that occur in the cycle? (7 pts)

C. What morphological characteristic aids in attachment of bacterial pathogens to the host tissue? (2 pts)

 

 

D. In enumerating bacteria, cfu stands for what? (2 pts)

E. Many foliar bacterial pathogens exist as epiphytes on the foliage.

Define epiphyte. (2 pts)

 

 

 

 

F. Bacteria produce a number of enzymes and toxins in order to cause disease. Many of these compounds are not produced until "quorum sensing" occurs. Define this term and discuss how it might benefit a bacterium's ability to cause disease. (5 pts)

G. What are the 3 ways, other than mutation, a bacterial population might maintain genetic variability? NOW, define ONE of these terms. (5 pts)

IV. Dispersal of inoculum, both primary and secondary, is vital to a pathogen's ability to spread and cause disease. Most pathogens, when in contact with their host plant, are found in the still air layer on the plant surface. To insure long distance dispersal, the pathogen must get into the turbulent boundary layer.

A. What are 2 ways a pathogen propagule can "take off" from the leaf surface? (4 pts)

 

 

B. What size, shape, and color of spore is best at travelling long distances? (2 pts)

C. In order to cause a new infection the propagule must "land" on the host tissue. Name 2 ways this can occur? (4 pts)

 

 

D. Dispersal mechanisms vary in their ability to move propagules long distances from a point source of inoculum. Dispersal gradients illustrate these differences. For the dispersal gradients below, write the following mechanisms by the line that they best represent: rainsplash, insects, self-propulsion, wind, and cultivation. (5 pts)

E. For any one of the above mechanisms, draw a dashed line on the graph in D to represent a disease gradient. (2 pts)

F. Dispersal gradients are always flatter than disease gradients. Discuss 3 reasons for this. (6 pts)

V. Describe two ways to produce a transgenic plant. (4 pts)