Name _____________________





1.  (16 pts)  Define the following:





secondary inoculum









obligate parasite



            penetration peg






            horizontal resistance



2.  (12 pts)  List 2 enzymes that pathogens used to cause disease and the modes of action of the enzymes and 2 plant growth factors that pathogens use to cause disease and the symptoms associated with an excess of the plant growth factors.










3. (4 pts)  Phytotoxicity is a potential problem when using chemical control of plant diseases.  Give 2 factors that influence whether a chemical will be phytotoxic.

4.  (8 pts)  Using one of the 3 historical diseases discussed in class ( coffee rust, late blight, or southern corn leaf blight) discuss the disease in relation to the disease triangle.  What were the important factors contributing to the epidemic?  Give the specifics of each side of the triangle.

Would the disease be more common if the crop were grown in Arizona or West Virginia?

(For 4 extra credit points, answer the entire question for another of the diseases listed.)














5.  (10 pts)  Does an incompatible interaction end in disease?  What combination of host resistance and pathogen virulence will result in an incompatible interaction?  Which combinations give a compatible interaction. (Hint: Consider the gene for gene theory)









6. (6 pts)  Explain the difference between non-host specific toxins and host specific toxins.  Give an example of each type of toxin and explain how it works to cause disease.






7. (8 pts)  List four defense mechanisms, one of each type of  1) induced structural, 2) induced biochemical, 3) preformed structural, and 4) preformed biochemical that plants can use to prevent pathogen infection and explain how each works.





8.  (8 pts) Diagram a disease cycle and explain which steps in the cycle would differ between fungi and viruses.










9.  (12 pts)  A friend from the lower Rio Grande valley of Texas calls you with a frantic request to come look at his potatoes because he has noticed that the leaves have yellow and purple streaks and some plants have curled dried up leaves.  He has heard about a new disease of potatoes, Zebra chip, caused by Candidatus asiaticus, a non-culturable (biotrophic) bacteria vectored by psyllid insects. Confirmation of the disease in the potatoes will cause a huge problem, since a positive diagnosis will ruin his chance to sell his potatoes to FritoLay for potato chips due to the large dark stripes through the processed potatoes.  Alternatively, the symptoms on the potatoes might be due to infection with Fusarium (a common fungus) and purple top, a common disease caused by a phytoplasma vectored by leafhoppers. When you go to the potato field for the first time what will you look for in diagnosing the problem?   Since you need to be completely sure of your diagnosis, what steps will you need to go through to prove what is causing the problem?











10.  (10 pts)  Compare the use of plant resistance and cultural control for the next season if the turf has Fusarium  infection or Zebra chip.  Answer the question again (comparing the use of plant resistance and cultural control) for a different field of wheat infected with Fusarium.









11.  (6 pts)  What is vertical resistance?  List an advantage and a problem with growing plants with only this type of resistance.