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Chapter 9 Instructor Resources​

The World Belowground: Soilborne Pathogens

Healthy roots are a requirement for healthy plants. Many plant pathogens live in the soil, such as the fungus that causes Panama disease of bananas and the nematode (tiny roundworm) that causes soybean cyst disease. Nematodes are the most numerous multicellular organisms on Earth, and some are very important pathogens of people and of plants. Sometimes, plants can create their own natural biological control against pathogens in the rhizosphere, the soil right around their roots. The understanding of soil microbiology also helps us protect groundwater from contamination by toxic chemicals that spill or leak into soil.

• Chapter 9 Podcast

  Chapter 9 Podcast

  Listen to the Podcast (mp3)
  

  The short podcast provided for each chapter includes a review of a major concept or issue, clarification of an important point that can be confusing to students, and questions for students to think about. This podcast introduces the nematodes—small roundworms that cause plant diseases—and contrasts the terms monoculture and genetic diversity.

• Demonstration

  Live Nematodes

  If any nearby researchers are working with ,em>Caenorhabditis elegans, they probably will be happy to share a few plates of worms for your class. The nematodes are visible to the eye, and with a dissecting microscope, they can easily be seen moving across the agar.

• Group Discussion

  Nematodes Versus Other Pathogen Groups

  Break the class into small groups of 3–5 students, and have each group complete the following grid:​
  

  
  

  	Plant-Pathogenic Nematodes	Similar to Fungi, Bacteria, Both, or Neither?
  Prokaryotic or eukaryotic
  Unicellular or Multicellular
  Individuals ever visible to the naked eye? (yes/no)
  Common survival structure
  How infect plants (directly or indirectly)
  Means of reproduction (sexual, asexual, both)
  All are obligate parasites? (yes/no)
  Means of motility
  Means of dissemination

  
  

  After giving the groups 10–15 minutes to work on the grid, have the whole class fill it out with contributions from the groups. Each point can be discussed and clarified, as necessary.

  
  
• Short Writing Assignments

  Notes: These assignments require each student to write a paragraph (introductory sentence, body, concluding sentence) and can be completed in 10–15 minutes in class. They provide a good way to check student comprehension and to improve student writing skills. See Chapter 1 for a simple grading system.

  
  

  1) Nematode Disease Cycles

  Compare the disease cycles of the soybean cyst nematode and the pinewood nematode. Describe one way these two cycles are similar and one way they are different. Next, propose a management strategy for one of these two diseases, and indicate what aspect of the disease cycle that strategy would primarily impact.

  
  

  2) Nematode Disease Management Strategies

  Both the soybean cyst nematode and the potato cyst nematodes are soilborne, root-infecting pathogens. The cycles of the diseases caused by these two pathogens are functionally monocyclic, with no significant plant-to-plant spread during the season. Based on what you know about monocyclic diseases, explain the general strategy used to manage these diseases. Also provide two examples of specific control methods based on this strategy that are used to manage soybean and/or potato cyst nematode.

  
  
• Longer Writing Assignments

  Using Fusarium Wilt to Control Drug Crops

  This exercise has three parts:

  1. To prepare for the discussion, have students read about Fusarium wilt in a commercial crop. This will help them understand more about the disease and what management approaches are available. Also have students read articles from the New York Times on the use of Fusarium wilt to control marijuana in Florida and the use of both Fusarium wilt and glyphosate to control coca in Colombia and Peru. Have students critically analyze the articles for accuracy.
  2. Next, have a group discussion to help students focus on accurate and inaccurate information and to help them develop an educated opinion on the use of glyphosate versus Fusarium wilt for control of drug crops.
  3. Ask each student to write a short paper expressing his or her opinion on which tool to use to destroy drug crops. Students should base their opinions on what they have learned about glyphosate and Fusarium wilt.

  
  

  Part 1 - Writing Assignment to Prepare for Group Discussion

  Fusarium wilt is caused by the fungal species Fusarium oxysporum. There are many subspecies of this fungus that are host specific. This means that they look identical but vary in what plants they can infect. These subspecies are designated by the Latin phrase forma specialis (f. sp.), which means “special form.” The plural of this phrase is formae speciales. For example, Fusarium wilt of tomato is caused by Fusarium oxysporum f. sp. lycopersici.

  This fungus infects plants primarily through the roots and then becomes a vascular wilt pathogen, which means it invades the xylem and prevents uptake of water and minerals. Note that the fungus produces three kinds of spores: macroconidia for infection of roots, microconidia to aid in distribution of the fungus in the xylem, and thick-walled chlamydospores for survival in the soil.

  
  

  Notes:

  Here are two possible sources to use:

  • APS Plant Disease Lesson: Fusarium Wilt of Watermelon
  • Fusarium wilt sections of various vegetable disease compendia published by APS—for example, Fusarium wilt of tomato in the Compendium of Tomato Diseases.

  
  

  Directions to Students:

  You will use this assignment for the group discussion, and then turn it in. Keep an extra copy for yourself for use in writing the next paper on using Fusarium wilt to control drug crops.

  For the Fusarium wilt disease you have been given to study, answer the following questions:

  1. Host plant (English and scientific names)
  2. Pathogen name: Fusarium oxysporum f. sp. ______________________
  3. Symptoms and signs (What does it do to the plant?)
  4. Means of dissemination (dispersal) and survival (Do you see references to macroconidia, microconidia, or chlamydospores or to water, air, soil?)
  5. Management: Are any of these methods used for the Fusarium wilt you read about?
  1. Cultural practices:
  • Crop rotation?
  • Modification of soil pH, fertilizers used?
  2. Resistant plants (cultivars) available?
  3. Chemical (fungicide) treatments recommended? If not, why not? (Hint: Think about how this pathogen enters the plant and where it is located in the plant.)
  6. Any other interesting or important information to note?

  
  
  

  Part 2—Group discussion of the use of Fusarium wilt for biological control of coca (source of cocaine) in Colombia and Peru and biological control of marijuana in Florida

  1. Read the following articles from the New York Times. Evaluate each article for accuracy, given what you know about Fusarium wilt, the fungal pathogen Fusarium oxysporum, and the herbicide glyphosate. Note any statements in the article that you doubt.
  1. Fusarium wilt to control marijuana: “A Fungus to Kill Marijuana Has Environmentalists Wary,” July 27, 1999
  2. Fusarium wilt to control coca: “Fungus Considered as a Tool to Kill Coca in Colombia,” July 6, 2000
  3. Herbicide (weed killer) glyphosate to control coca: “No Crops Spared in Colombia Coca War,” January 31, 2001
  2. Download a fact sheet on glyphosate from the Pesticide Action Network UK
  3. Answer these questions to prepare for discussing the articles:
  1. How would you establish the fungus in a coca-growing area to effectively kill the plants?
  2. What would be the advantages of using Fusarium wilt for biological control compared to herbicides (e.g., glyphosate)? (Consider ethical, biological, and environmental aspects.)
  3. What would be the disadvantages of using Fusarium wilt compared to herbicides?
  4. Discuss the articles with your group in class.

  
  

  Notes:

  • Students can likely find the New York Times articles themselves. Most university libraries offer registered students access to archived articles. Otherwise, the articles can be posted on a class website or printed and distributed in class. Students are usually quite amazed to find inaccurate statements in the New York Times.
  • Remind students about the significance of LD50 when discussing glyphosate.

  
  

  Part 3 - Writing Assignment

  Write an essay of 300–400 words (more if absolutely necessary). The essay must be typed. Answer these questions in the essay (don’t just answer the questions):

  1. How could you use Fusarium oxysporum (Fusarium wilt) to kill a drug crop such as coca or marijuana (i.e., how would you get the fungus to the crop for infection)?
  2. What are some advantages and disadvantages of using Fusarium wilt, rather than herbicides (weed killers)?
  3. What is your opinion about using Fusarium wilt for killing drug crops? Is this biological control or bioterrorism?

  
  

  Note: You may have the opinion that Fusarium wilt, herbicide, or neither should be used. Justify your opinion with scientific facts about both control options.