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Chapter 11 Instructor Resources​

Ancient and Present Day Foes: The Rusts

The rust fungi cause some of the most economically important plant diseases. They also have evolved complex life cycles involving two unrelated host plants and up to five spore stages. The ancient Romans made sacrifices to a rust god in hopes of protecting their wheat from rust. Modern scientific studies of fungal life cycles have led to better management of rust diseases. However, rust diseases still limit our ability to produce cereals (such as wheat), soybeans, apples, and trees for lumber (such as white pine). Rusts are common on many garden plants, including daylily, hollyhock, and rose.

• Chapter 11 Podcast

  Chapter 11 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 covers the topic of mycotoxins, including what they are, why they are problematic, and how they are different from the fungi that produce them.

• Demonstrations

  1) Rust Disease Samples

  If the class is taught in the fall, it may be possible to collect fresh examples of rust diseases. Plant materials with rust can easily be pressed and dried, but they are not as impressive as fresh specimens. Students might be asked to find examples of rusts and bring them to class. If the class is taught in the spring, it may be possible to find juniper galls of cedar-apple rust. These galls can be dried and then rehydrated the night before the class in which they will be used.

  
  

  2) Rust Disease Cycle Role-Play

  Make large signs containing the names of the different spore stages and the hosts of a dioecious, macrocyclic rust you study in class. Ask for volunteers who like to act, and give each student one sign. Have the students act out the rust disease cycle, demonstrating the order the spores are produced and the host each spore infects.

  Note: It may be simplest to begin the play with the overseasoning spore.

• Group Discussions

  1) Wheat Stem Rust Fungus Life Cycle

  Provide students with copies of the diagram of the life cycle of Puccinia graminis f. sp. tritici from the textbook (Figure 11.5) or the APSnet Education Center Plant Disease Lesson on stem rust of wheat. Have students work on the answers in small groups and then discuss their answers as a class.

  1. Match these five kinds of spores produced by rust fungi with their functions:
     ​

     ________ Pycniospores	A. Repeating Stage; Produced on Wheat and Infect Wheat; Dikaryotic
     _________ Aeciospores​	B. Thick-Walled Overwintering Spores; Where Karyogamy and Meiosis Occur
     _________ Urediniospores	C. Haploid Spores Formed from Meiosis; Infect Barberry
     _________ Teliospores	D. Dikaryotic Spores Produced on Barberry; Infect Wheat
     _________ Basidiospores	​E. ​Haploid Spores Produced on Barberry; Form the D. Dikaryotic Mycelium through Plasmogamy

     
     
  2. On the diagram, cover the two drawings near the word “Barberry.” Explain why the presence of only urediniospores and teliospores is not enough to complete all spores stage of the wheat stem rust pathogen life cycle on wheat in the midwestern states. Why does wheat rust still occur even in the absence of barberry?
  3. Cover the two drawings near the word “Wheat.” Explain why eradication of wheat would do a better job of protecting barberry from rust when compared with eradicating barberry to protect wheat. (Keep in mind that eradication of an alternate host is a human decision based on the economic value of the plant, not necessarily the best biological choice.)

  
  

  2) Rust Review

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

  Disease	Number of Spore Stages	Autoecious or Heteroecious	Telial Host (Primary Host)	Aecial Host (Alternate Host)
  Wheat Stem Rust
  Cedar-Apple Rust
  Coffee Rust
  Soybean Rust

  
  

  After 10–15 minutes, have the whole class fill in the grid with contributions from different small groups. Discuss the terms, as necessary, and compare the disease cycles of different rusts.

• Short Writing Assignment

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

  
  

  Rust in Your Backyard

  Suppose that while working in the yard, your mother notices that a red cedar tree has several large, orange, slimy globs hanging from its branches. Knowing that you are learning about plant pathology, she e-mails you a digital photo of the tree. You do some online research and discover that the plant is infected with cedar-apple rust, caused by a heteroecious rust fungus. Write a short message to your mother in which you explain (1) what this disease is and (2) the potential threat to the apple tree growing nearby. Include an explanation of the fungal life cycle in plain English.

  
  
• Longer Writing Assignments

  1) Rust Fungi

  Write a short essay of 250–300 words (no more). It must be typed.

  Answer these questions in your essay (not individual answers to the questions):

  1. Explain what the term heteroecious refers to for rust fungi.
  2. Explain how the repeating stage spores (urediniospores) are different from any of the other four kinds of rust spores.
  3. Explain why removing the host plant with the repeating stage is the most effective biological control for rust diseases.
  4. Speculate about why rust fungi may have evolved to have heteroecious life cycles.

  
  

  2) The Threat of Ug99

  You have recently graduated and are now working for the U.S. Department of Homeland Security (DHS). Your boss has asked you to write a report on the threat to the U.S. and world wheat crops from the new, virulent Ug99 race of stem rust, caused by Puccinia graminis f. sp. tritici. This fungus has been listed by DHS as a plant-pathogenic fungus that could potentially be used in agroterrorism, but the threat it poses has been minimized in recent years because there have not been any major outbreaks. DHS is considering shifting funds from other projects to make them available for research on Ug99.

  Should Ug99 be a priority for DHS, or should funding go to more immediate agroterrorism threats? Are there other reasons to fund research on Ug99? Write a short paper (~ 350 words) on this topic.

  Here are some questions to consider:

  • In the past and today, how important has wheat and stem rust been in the United States? In the world?
  • What is the source of primary inoculum in the United States, and how does the fungus survive and spread?
  • What are the current management strategies for wheat stem rust in the United States? In the world?
  • What is the origin of the Ug99 race, and what impact has it had?
  • What are the current predictions for the spread and impact of Ug99?
  • What is the likelihood that Ug99 will reach the United States, and what would its impact be?
  • What are the current and potential management strategies for Ug99?

  
  

  Notes:

  • Opinion papers such as this can be written in several drafts: Draft 1 (evaluated but not graded) summarizes the facts; draft 2 (also evaluated but not graded) incorporates the student’s opinion, supported by the facts; and draft 3 is a polished version of draft 2, which is finally graded. Students appreciate the opportunity to get feedback on their writing and thinking before being assigned a grade. This system also provides an opportunity for students to learn to separate facts from opinions and to evaluate the biases of different sources of information.
  • You can provide students with a reference list (including links), so that this is an exercise in evaluating and using information, rather than finding it. Extra credit can be given for sources with new information that students find and use in their papers.

  
  

  Evaluation of the Paper:

  Criteria Used in Evaluation can Include the Following:

  • Accuracy and completeness of factual information
    
  • Development and support of opinion
  • Overall organization and clarity
  • Mechanics (e.g., spelling, grammar, sentence structure)
  • Use of supporting references

  
  

  Important Note: This paper has no “right” or “wrong” conclusion. The student can take either side of the issue (pro–Ug99 funding or anti–Ug99 funding), include the pertinent facts, and use them to support his or her position.