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​​Battling Black Sigatoka of Banana in Costa Rica

​​The Case​​​​​Background Information​​​

INSTRUCTOR NOTES

Overview

The case study introduces students to a real-life crop disease management problem faced by a field manager, Señor Oscar Granados, who works at a large banana plantation in Costa Rica. The plantation is experiencing losses from a disease called black Sigatoka, a major disease of bananas that occurs worldwide. In Costa Rica, black Sigatoka is controlled by a very intensive program of fungicide applications that has resulted in the development of resistance in the black Sigatoka pathogen, a fungus called Pseudocercospora fijiensis, to several classes of fungicides. As a result, the reduced effectiveness of these fungicides has forced Sr. Granados to use older fungicides that are more hazardous to human and environmental health. These older fungicides must also be applied more frequently, resulting in the need for 70 sprays per year, which accounts for a substantial part of the cost of fruit production. Unless Sr. Granados finds a way to significantly reduce the cost of fungicide control and maintain profitable yields, he may lose his job due to the competition from other banana-exporting countries.​

Objectives

Students are challenged to analyze several disease management strategies and devise a plan to help Sr. Granados cut the costs of the intensive fungicide application for controlling black Sigatoka disease. This case study will help students:

• Recognize black Sigatoka disease symptoms and understand how the black Sigatoka pathogen spreads.
• Understand the concept of the “disease triangle," which shows that three factors (host, pathogen, and environment) must interact to cause a disease.
• Understand how the risk of fungicide resistance is affected by the way fungicides are used.
• Evaluate the economic, environmental, and ethical tradeoffs associated with management practices for one of the world's most economically damaging crop diseases.​
  

Suggestions on how to use this case

The case study is designed for undergraduates at all levels and would be suitable for introductory courses in plant pathology. The discussion of the case study can be completed in a single 50- to 90-minute class period and is most suitable for class sizes of 10 to 50 students. We strongly suggest that students read the case study and background information before the class period. Additionally, students should complete a brief written assignment related to the introductory questions prior to the class period so that they are familiar with the case and prepared for in-class discussion.

At the beginning of the class, students can be divided into groups of 4 to 6 members. If desired, printouts of the case and background information can be distributed to each group and used as references for discussion. Before group discussion, we suggest that the instructor take 5 to 10 minutes to refresh students' memories of the case and its cast of characters while asking students to answer the questions in the pre-class assignment. It may be helpful to show images of black Sigatoka symptoms and review current fungicide spray practices in banana plantations. At this point, the instructor can ask the students to share their opinions on what makes the disease so challenging to control for Sr. Granados. Students should consider the climate in Costa Rica, the difficulties in breeding new, resistant banana cultivars, and the rise of fungicide resistance. After that, the instructor may wish to review the disease triangle concept and bring up the available strategies that could be used to manage black Sigatoka.

Following the introduction, we suggest giving students 5 minutes to briefly discuss, in their groups, which components of the disease triangle are targeted by each management strategy. This warm-up activity can prepare students to discuss the pros and cons of the available management practices. We suggest allowing 15 minutes for groups to weigh each management strategy and write up a plan with two or three management recommendations for Sr. Granados. After group discussion, a representative for each group can summarize the group's recommendations on the board. The instructor can then contrast the strategies among groups and facilitate class discussion by asking questions about which factors influenced students' decision-making. Students presenting cultural practices (e.g., improving soil drainage, removing infected leaves) can be prompted to discuss the possible economic constraints, such as the cost of labor. In the case of applying disease-warning systems to guide the timing of fungicide sprays, students should discuss the climate situation in eastern Costa Rica, the choice of fungicides, and the timing of fungicide spraying. Environmental and health tradeoffs of fungicide spraying should be mentioned during the discussion. Students presenting biological control strategies should discuss the effectiveness of the strategy and the time constraints associated with Sr. Granados's situation.

Based on our experience trying out the case study in an Iowa State University introductory plant pathology class of 33 students, a 50-minute class period is enough for the discussion questions listed below. We found that having students read the case study and background information beforehand facilitates the classroom discussion. Among these students, 72% thought that the classroom discussion contributed the most to their learning; they felt that the case study provided a fun and informative way of learning. Many students were also interested in discussing the potential risks and benefits of using genetically modified bananas with resistance to black Sigatoka. Thoughts that also emerged from the discussion include societal perceptions and regulation of genetically modified crops, public awareness of fungicide application, and concerns about food safety. Some students suggested to have more discussion about the health and environmental hazards of fungicides.

The case study is easily adaptable to a longer class period with more in-depth background introduction about the development of the disease and explanation of the principles of fungicide resistance and resistance management. More images and videos of the plantations can be shared to help students better envision the case. It is also possible to adapt this case to a classroom with graduate students. Before the class session, only the first part of the case could be given to students; students can read about Sr. Granados's situation and do research to seek potential management practices to help him. The second part of the case, which introduces the available strategies, could be distributed to students in class before group discussion. In this way, students would be challenged to think creatively and be prepared to participate actively in class discussion.​

Introductory questions

1. What are the symptoms of black Sigatoka on banana plants?
2. How does the black Sigatoka pathogen spread?
3. What environmental and agronomic conditions in banana plantations in eastern Costa Rica make the black Sigatoka fungus thrive all year round?
4. List the management strategies that are available to control black Sigatoka.
5. What makes conventional breeding of disease-resistant banana cultivars so difficult?

Discussion questions

1. For each disease management strategy listed below, what component(s) of the disease triangle does it mainly target?

            a. Spraying fungicides intensively

            b. Developing disease-resistant banana cultivars

            c. Improving drainage systems

            d. Red​ucing plant density

            e. Removing infected leaves

            f. Applying urea to accelerate decomposi​tion of fallen leaves and stems

            g. Applying disease-warning systems to guide fungicide spray timing

            h. Biological control​

2. Putting yourself in Sr. Granados's situation, discuss the pros and cons of the available disease management practices. Consider economic, environmental, and ethical implications of these practices. What management strategies would you c​hoose to help Sr. Granados reduce the plantation's disease control costs substantially within six months? Write down two or three strategies.
   
3. If a genetically modified banana cultivar with resistance to black Sigatoka was available, should it be adopted by growers? What are the risks and benefits?