The Most Important Plant Pathogens: Fungi and Oomycetes

Fungi are amazing organisms with a cell structure and other characteristics that make them the closest living relatives to animals. Many fungi are beneficial (e.g., yeast, mycorrhizae), but others are well adapted to cause plant diseases. A group of funguslike organisms, the Oomycetes, also cause important plant diseases, including downy mildew of grape which nearly destroyed the French wine industry in the mid-1800s. Both fungi and funguslike organisms have evolved a wide variety of reproduction, dispersal, and survival mechanisms that aid their ability to cause plant disease epidemics.

• Chapter 2 Podcast

  Chapter 2 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 discusses the characteristics of a major group of plant pathogens—the fungi—and the roles of sexual and asexual reproduction in their life cycles.

• Demonstration

  Fungus Party

  Note: With thanks to Dr. Robert Stack for many of these ideas

  
  

  This activity makes a nice class right before a vacation break, such as Thanksgiving. It works best toward the end of the semester, when students are familiar with much of the content. It is helpful to have a department kitchen near the classroom.

  Place various displays around the room to supplement the food items. Put some signs on the classroom door, and make it a party!

  Give menus to students, so they know what they are eating.

  
  

   

  
  

  Fungus Party Menu

  
  

  Appetizers

  Corn smut (Ustilago maydis) with corn chips
  Blue cheese (Penicillium roquefortii) and brie cheese (Penicillium camembertii)
  with crackers

  
  

  Entress

  Quorn mycoprotein (Fusarium sp.) nuggets
  Morels (Morchella sp.) in brown rice
  Bad (Saccharomyces cerivisiae—yeast)
  Sauteed agaric (Agraricus bisporus) and oyster mushrooms (Pleurotus ostreatus)

  
  

  Desserts

  (No fungal ingredients)
  Gingerbread (to avoid smell and taste of stinking smut, Tilletia foetida)
  Mushroom cake (based on a yule log)

  
  

  Preparation for Fungus Party

  Shopping:

  • Corn chips
  • Corn smut (Available at some Mexican grocery stores or can be ordered online; just reheat and serve)
  • Blue cheese (Two pieces: one for eating, one for display under a dissecting microscope; choose a piece with good holes showing the sporulation of Penicillium)
  • Brie cheese
  • Crackers
  • Bread
  • Agaric and oyster mushrooms
  • Dried morels (available online or in gourmet stores) or shiitakes (available online or in Asian grocery stores)
  • Mushroom cake (Ask a local pastry bakery to make a rolled yule log cake decorated with meringue “mushrooms” but no “holly”!)
  • Cola with citric acid (Check the label—citric acid from Aspergillus, not oranges)
  • Quorn (Available at health food stores, as frozen “fungus nuggets” in the meat-substitute section. Cook in a microwave just before the party, and cut into halves for tasting. Or buy the breaded patties and make Quorn parmigiana.)
  • Juice
  • Cups
  • Plates
  • Forks
  • Napkins

  
  

  To Make:

  • Morels or shiitakes and rice (Reconstitute mushrooms with broth and add to brown rice; sauté until cooked thoroughly; season to taste)
  • Gingerbread (Cut into small samples, depending on class size)
  • Sautéed agaric and oyster mushrooms (Buy fresh and sauté just before class)

  
  

  Possible Displays Around the Room:

  • Puffballs
  • Blue cheese under dissecting microscope
  • Birdsnest fungi
  • Wood ears (Buy dried and soak in water the night before)
  • Ergot (Collect on wild grasses)
  • Corn smut (On ear of corn and slide of spores with compound microscope)
  • Soy sauce
  • Truffle pictures
  • Botrytized wine (Purchase at most wine stores)
  • Stone-washed jeans (Produced by Trichoderma, not rocks)
  • Wild edible and poisonous mushroom photos

  
  
• Group Discussions

  1) What is a Fungus?

  This assignment can be preceded by a short in-class discussion in which small groups of 5–7 students try to write definitions of a fungus using the terms and characteristics that have been presented in class. After the group discussions, volunteers can read their groups’ definitions for discussion by the entire class to make sure everyone understands the terms and important characteristics that must be included. If students have grown Rhizoctonia solani from sclerotia on potatoes on water agar the week before, they can observe the feathery hyphae growing radially, without any “body,” as in plants and animals.

  
  

  2) Learning About Disease Cycles

  In small groups, ask students to list the key steps in a disease cycle (overwintering, dispersal, inoculum, infection, colonization, symptom development). They can use one or more specific disease cycles (e.g., late blight, downy mildew) to identify elements related to each step (site of overwintering, structure that overwinters, means of dispersal, inoculum structure, site of infection, example symptoms).

  
  

  Then, the whole class can be brought together to fill in this chart:

  Disease Cycle Stages	Late Blight of Potato	Downy Mildew of Grape
  Site(s) of Overwintering
  Overwintering Structure (Fungal)
  Methods of Dispersal
  Inoculum(a)
  Site(s) of Infection
  Symptoms

  
  

  This activity can be used to show similarities between Oomycetes and can later be modified and used to show differences between individual fungal groups.

  
  
  

  3) Terminology Worksheet

  This worksheet can be completed by small groups in class with discussion following, assigned as homework, or simply distributed for review.

  Define and differentiate each of the following groups of terms:

  1. Fungus, Mycology
  2. hypha, mycelium
  3. prokaryote, eukaryote
  4. autotroph, heterotroph
  5. DNA, genetic code
  6. haploid, diploid, dikaryotic
  7. sexual reproduction, asexual reproduction
  8. mitosis, meiosis
  9. plasmogamy, karyogamy
  10. pathogen, disease
  11. parasite, obligate parasite, saprophyte
  12. spore, sporangium, zoospore, oospore
  13. Disease cycle terms: overwintering, dispersal, inoculum, infection, colonization, symptom, sign

  
  
• 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) Characterisitcs of Ooomycetes

  Oomycetes were once classified with the true fungi (including ascomycetes and basidiomycetes), but taxonomists have now separated these two groups. Briefly describe two characteristics of oomycetes that make them similar to true fungi and two characteristics that distinguish oomycetes from true fungi.

  
  

  2) Disease Cycle Components

  You have now learned about plant diseases caused by fungal-like organisms called oomycetes. You have also learned that the progression of events that occur before, during, and after the disease develops can be organized into something called a disease cycle. List and describe the important components of a generalized plant disease cycle. Also, select a disease that you have learned about, and list the important components of its disease cycle.

• Longer Writing Assignments

  1) What is a Fungus?

  Most people know that mushrooms have something to do with fungi, but they have trouble understanding what kind of organism a fungus actually is. Write a short essay (~ 350 words) that explains to your nonbiologist friends what a fungus is

  Fungi are now in their own biological kingdom. What are their features? In other words, how do you know that you are looking at a fungus and not a plant, animal, or bacterium? Use the technical terms related to fungi and their characteristics that have been discussed in class.

  Be sure to incorporate answers to these questions in your essay:

  • How do fungi obtain nutrients and grow?
  • What are the components of fungal cells?
  • How do fungi reproduce? (Include information on their nuclei.)

  
  

  Do not answer these questions individually. Write paragraphs that include answers to these questions.

  
  

  2) Feeding the World with Fungi

  Quorn is a meat substitute composed primarily of the fungus Fusarium venetatum, which is produced by Marlow Foods in Great Britain. It is the best-selling retail brand of meat-free food in the world and the best-selling frozen meat-free brand in U.S. natural food stores. Marlow Foods claims that Quorn is an all-natural meat substitute of high nutritional value.

  Some people believe that food sources such as Quorn are the wave of the future, and will be useful in feeding the ever-growing world population. However, some people are concerned about the safety of Quorn for human consumption. Would you eat Quorn or recommend it to a vegetarian friend? Why? Write a short essay (~ 350 words) on this topic.

  Here are some questions to consider:

  • What is mycoprotein?
  • How is Quorn produced?
  • How are products such as Quorn tested for safety?
  • What are possible health advantages of eating Quorn or other mycoprotein?
  • What are possible health risks of eating Quorn?

  
  

  Notes:

  • You can make the audience for the assignment someone other than yourself—for example, a vegetarian friend of the student. The selection of audience should dictate the style of writing students use.
  • 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 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-Quorn or anti-Quorn), include the pertinent facts, and use them to support his or her position.