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Chapter 12 Instructor Resources​

Diseases of the Largest Plants: Trees

Trees live longer than any other organism on earth. This is because they are able to respond to attacks by pathogens and pests and continue to grow, some for thousands of years. However, trees also are subject to attack by pathogens. Two major U.S. tree disease epidemics of the 20th century, chestnut blight and Dutch elm disease, have killed millions of trees in forests and towns. The stresses of air pollution, salt spray, soil compaction, and monoculture also can shorten the lives of urban trees

• Chapter 12 Podcast

  Chapter 12 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 describes the meaning of the term vector in plant pathology and compares strategies used to manage two tree epidemics: chestnut blight and Dutch elm disease.

• Group Discussion

  1) Adaptations of basidiomycete fruiting bodies for spore production and dispersal

  Notes:

  • Provide various basidiomycete fruiting bodies from trees, agaric mushrooms from the grocery store (perhaps one per student), and spore prints prepared before class. (Place a mushroom cap on white paper and leave it there overnight, covered with a bowl.)
  • In observing mushrooms, students will usually notice the protective umbrella-like cap, the stalk, the multiple gills for more spore-producing surface, the location of the gills below the cap, and the membrane that protects young mushrooms when emerging. Students will notice other features on the other fruiting bodies depending on what is available, but they typically see the variations in spore-producing surfaces, such as pores and the new layers of perennial fruiting bodies. A short class discussion can follow the individual observations

  
  

  Directions to Students

  Mushrooms and other fruiting bodies of basidiomycetes are formed to help produce and disperse spores. Fungi are composed of a filamentous mycelium in soil or wood. A mushroom or other fruiting body is composed of mycelium.

  1. Examine the surfaces of the fruiting bodies available.
  2. Look at the spore prints from the mushrooms.
  3. Open up the mushroom and see how it is constructed. Look at the gills under the cap, and then compare these to the spore print.

  
  

  How is a mushroom (or other fruiting body) adapted for spore production and dispersal? List as many adaptations as possible.

  
  

  2) Review of Tree Diseases

  Diseases of trees have been discussed in many different sections of the course. This activity gives students the opportunity to pull together this information.

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

  
  

  Disease	Pathogen	Pathogen Group	Primary Inoculum	Means of Dissemination	Symptoms	Management Strategy
  Chestnut Blight
  Citrus Canker
  Coffee Rust
  Dutch Elm Disease
  Fire Blight
  Pine Wilt

  
  

  After 10–15 minutes, have the whole class fill out the grid with contributions from the small groups. Discuss similarities and differences among these tree diseases.

  
  
• 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) Roles of People in Tree Disease Epidemics

  Both Dutch elm disease and chestnut blight spread quickly across the United States, nearly wiping out American elm and American chestnut trees in their natural ranges. Millions of people were affected by the destruction caused by these introduced pathogens. Choose either disease and discuss (1) how people were involved in the introduction of the pathogen, (2) why people were unable to stop the rapid spread of the disease, and (3) how people’s lives changed as a result of the disease.

  
  

  2) Chestnut Blight Management

  Chestnut blight ravaged the entire population of American chestnut trees in the eastern United States. Briefly describe how the initial attempts to manage the chestnut blight epidemic in the early 1900s were similar to the procedures used to manage the citrus canker epidemic in Florida. Also explain why these procedures were ineffective for chestnut blight.