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​Hidden Killer: Apple Scar Skin Viroid

     
     
   

​​Case Study

​Instructor Resources​

Introduction

This case study focus on an important disease called apple scar skin viroid (ASSVd) in a major apple production region in Shaanxi Province, China. Students are introduced to an apple grower, Fuwang Li, whose orchard is facing a potential epidemic. like his neighbor Xilai Zhang, Fuwang could lose a great deal of money due to ASSVd.

In this case, students will learn to recognize disease symptoms and identify risk factors associated with the transmission of ASSVd. In addition, they will be challenged to develop a practical disease management plan to manage ASSVd and employ critical thinking as they weigh management alternatives.

This case study was initially conducted in an undergraduate class with 30 students in Introductory Plant Pathology (PLP 408) at Iowa State University, Ames, Iowa, USA. The majority of students were third-year majors in Agronomy, Seed Science, Horticulture, Microbiology, Biology, and Sustainable Agriculture.

Objectives

This case places students in the role of Fuwang Li, whose urgent mission is to somehow save his orchard from being further devastated by ASSVd. The grower in the case has found symptoms on apples in his own orchard, and faces a dilemma in trying to overcome the problem and sustain his orchard. The main goals are to engage students actively to:

  1. Understand how ASSVd can pose a serious threat to apple orchards, recognize its symptoms, and learn how the apple scar skin viroid is transmitted.
  2. Understand how viroid-incited diseases can be managed effectively in perennial crops such as apple.
  3. Recognize that each management strategy comes with its own advantages and disadvantages.
  4. Increase awareness of the critical importance of prevention strategies in management of viral diseases.

Case Summary

The case creates a scenario in which a viroid disease called apple scar skin viroid (ASSVd) threatens the sustainability of a commercial apple orchard operation in Shaanxi Province, China. When trees in an orchard become infected, these infections cannot be eradicated except by removing the infected trees, and infection by ASSVd will weaken and ultimately kill trees. Because viroid infections cannot be cured, ASSVd management efforts must focus on keeping the viroid from infecting the trees.

In this scenario, students are placed in the role of Fuwang Li and are facing a problem which is hidden and unpredictable. All trees in modern apple orchards are grafted, which means they have been produced by inserting an aboveground portion (the scion) into another apple variety from which the root system develops (the rootstock). Grafting involves creating wounding of the woody tissues, so there is a risk that ASSVd can spread on the knives that are used to graft trees. The risk of ASSVd transmission occurs when either the scion or the rootstock is infected with the viroid. The hidden aspect of ASSVd occurs because symptoms (visually apparent effects of the viroid on the plant) of viroid infection can remain latent (displaying no symptoms) for years, so that growers often discover that their trees and fruit are infected years after the infections began.

A valuable way to minimize the risk of ASSVd outbreaks is to make sure that apple orchards are free of the viroid from the beginning. The surest way to verify that young trees are viroid-free is to test them repeatedly during nursery production for the presence of the pathogen, using highly sensitive laboratory assays that detect extremely low concentrations of the nucleic acids in ASSVd. This process is called certification. A potential disadvantage of certification is that young trees produced by this process are likely to be much more expensive than trees that growers graft themselves.

Students are exposed to several management options that could help Mr. Li to keep the viroid confined. These include modification of his pruning practices, spraying insecticides in the orchard to reduce the possible risk of insect transmission of the viroid, and – as a longer-term option – planting only new trees that have been certified to be free from viroid infection. In addition to understanding how learning about the nature of a plant disease can help growers understand how it should be treated, students will also know the importance of communicating with neighbors in order to work together to prevent disease from transmission. The case underlines the central role of prevention and exclusion in managing viroid and virus pathogens in perennial crops that are clonally propagated.

Suggestions on how to use this case

This case study can be completed in a class period ranging from 40 to 90 minutes and class size ranging from 10 to 50 students. We urge that students be assigned to read the case and background before coming to class, and that they submit a short assignment or take a quiz before the class in order to prepare them to participate meaningfully in the class discussion.

At the beginning of the class, the instructor can spend 5 to 15 minutes repeating the basic facts of the case in order to refresh students' memories. Then the class can then be divided into groups of 3 to 6 students each for discussion of the case, lasting 15 to 20 minutes. Each group can be challenged to develop an ASSVd management plan for Mr. Li's orchard and explain why their plan is feasible in terms of the risks and benefits involved. The small-group discussion can be followed by a whole-class discussion of 20 to 30 minutes in which representatives of each group can explain their management plans (for example, by writing a few words about each strategy on the blackboard) and defend them in questioning by other class members. From our experience on the class, we strongly recommend that students write down their management recommendations, and that instructors encourage every group to share their reasoning. This exchange will others in the class to look the case from different angles and deepen their understanding.​