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Chapter 13 Instructor Resources​

The Smallest Pathogens: Viruses and Viroids

Viruses are very small pathogens, composed of nucleic acid (DNA or RNA) with a protective protein coat. Viroids are even smaller pathogens, consisting only of pieces of RNA. Viruses and viroids cause many important plant diseases including tobacco mosaic, the “guinea pig” of early virus studies. Viruses are visible only with an electron microscope, can replicate (copy themselves) only when inside a living plant cell, and usually require an insect to move from plant to plant. Important molecular techniques now commonly used to detect many plant pathogens were first developed to detect plant viruses.

  • Chapter 13 Podcast

    Chapter 13 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 a new group of plant pathogens—the viruses—and clarifies the difference between symptoms and signs of a disease.

  • Group Discussions

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

    After this week, students will have studied the four major pathogen groups (fungi, bacteria, nematodes, and viruses) and the diseases caused by each of them. The following two activities are designed to pull together information from throughout the course.

    1) Pathogen Group Comparison

    Break the class into small groups of 3–5 students, and have each group fill in the following grid.

    Note: If students have been completing review grids throughout the semester, they should be able to fill in the categories in the top row and left column of this grid. If students are not used to the review grids, you can provide them the grid with these categories filled in and the rest blank. (See the completed Sample Grid for possible answers.)

    Prokaryotic or Eukaryotic           ​                   
    Example Disease
    Sexual Eggs
    Mosaics Yellowing Ringspots

    After 10–15 minutes, have the whole class fill in the grid with contributions from the small groups.

    Note: There is more than one correct answer for many of the boxes. To simplify the exercise, ask each small group to provide only one correct answer per box. During the whole-class discussion, the variety of correct answers will become apparent.

    Sample Grid:

    Pathogen GroupProkaryotic or EukaryoticReproductionDisseminationSymptomsSignsExample Disease
    FungiEukaryoticAsexual/Sexual SporesWind Water InsectsLesions Blight CankersHyphae SporesStem Rust
    BacteriaProkaryoticBinary FissionWater InsectsLesions Halos Soft RotoozeFire Blight
    NematodesEukaryoticSexual EggsSoil InsectsStunting ChlorosisEggs Juveniles CystsSoybean Cyst
    VirusesNeitherReplicationInsects Vegetative PropagationMosaics Yellowing RingspotsInclusion BodiesTobacco Mosaic

    2) Important Disease Comparison

    Break the class into small groups of 3–5 students, and have each group fill in the following grid.

    Note: You can include any diseases that you like. For example, you may choose classic diseases or diseases of local importance, which students are likely to see.

    DiseasePathogen Group (Be Specific!)Host(s)Common SymptomOverwintering SitePrimary InoculumCommon Method of Dispersal
    Crown Gall
    Fire Blight
    Late Blight
    Soybean Cyst
    Tobacco Mosaic

  • 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) Viruses Versus other Pathogens

    The viruses are the fourth group of biotic plant pathogens (although some people do not consider them alive). This group of pathogens is quite different from the other three biotic groups we have studied (bacteria, fungi, and nematodes). Describe two differences between plant-pathogenic viruses and one or more of the other groups of plant pathogens

    2) Virus Detection

    Many fungi and bacteria can be detected through visible signs or by isolation on growth media. However, viruses are typically more difficult to detect and identify. Briefly describe two methods used to detect and characterize (identify) plant-pathogenic viruses. For each method, indicate one benefit and one problem associated with its use.

    3) The Discovery of Virus Disease of Plants

    Whereas fungi and bacteria were identified as plant pathogens in the mid- to late-1800s, viruses were not identified as causes of plant diseases until the mid-1900s. Describe three characteristics of viruses or virus diseases of plants, and briefly discuss why these characteristics made it so difficult for scientists to determine the nature of this group of plant pathogens.