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Sooty Blotch and Flyspeck: Classroom Management​

By: Hafizi Rosli and Mark L. Gleason



The case study explores a common disease of apple fruit called sooty blotch and flyspeck (SBFS) that poses an unusual problem for apple growers. SBFS is a cosmetic disease; that is, it causes dark blotches on the apple peel but does not affect the eating quality of the fruit. In contrast, almost all other apple diseases also cause internal damage to fruit, leaves, stems, or roots. Despite the superficial nature of SBFS infections, most consumers of fresh apples refuse to buy fruit with SBFS blotches. As a result, these otherwise undamaged apples are typically diverted to making cider or pies, which are much less profitable than fresh apples. To protect against SBFS, apple growers in the eastern (more humid) half of the U.S. apply fungicides to their orchards every 10 to 14 days during most of the growing season - up to 8 applications per year. This intensive spray program is not only expensive and time-consuming, but can also be a health hazard to apple growers and their customers, since some of the fungicides can be harmful to people.

The case places students in the role of Umar Mukhlis, a small-scale commercial apple grower in Iowa. Umar’s customers, responding to news reports, suddenly become concerned that fungicide residues on his apples could endanger their health. Faced with declining profits, he needs to change the situation - and soon - in order for his business to survive. Students are asked to evaluate the options and recommend a management plan that could help Umar to overcome his customer relations crisis. The case enables students to learn the basic ecology of an important fruit crop disease, understand how weather impacts disease risk, consider the pros and cons of conventional and organic disease control, and strategize about how to get customers to buy smudged apples in exchange for less pesticide use.


The main goals of this case study are to engage students in:

  • Learning about a unique plant disease, SBFS, whose signs are only “skin deep.”
  • Understanding how the ecology of SBFS impacts a grower’s options to manage it effectively.
  • Discussing how having customers visit an apple orchard on a regular basis can affect how a grower controls crop diseases.
  • Learning how a disease-warning system functions, and understanding the tradeoffs for growers who are deciding whether or not to use such a system.
  • Considering how to persuade customers to accept superficially smudged apples as the price of potentially lower exposure to fungicides.


The case study dramatizes the real-life situation of a small-scale apple grower who runs a retail “pick-your-own” business in which his profit depends largely on his customers’ satisfaction. Umar’s customers raised concerns about the health hazards of fungicide residues on the apples due to frequent fungicide sprays, and many have abruptly stopped coming to his orchard to pick their own fruit. He fears that if he gives his customers what they are asking for, the cutback in fungicide spraying will raise his risk of outbreaks of SBFS, and that consequently he will harvest more dark-smudged apples. With help from Hannah Wilson, a plant pathology Extension specialist, as well as his family members who contributed some ideas, Umar has several strategies to consider in order to regain his customers’ trust and save his orchard from bankruptcy.

The case study is suitable for college and university students from undergraduate to graduate level. Prior knowledge about plant disease or plant disease management is not required for this case study.


The case study is designed for class periods from 50 to 90 minutes and class sizes from 5 to 50 students. Based on our experience in teaching this case, we suggest that students read the provided background information on SBFS at least one day before the class, and that they submit a short summary or written assignment before class so that they can participate meaningfully during in-class discussion.

The case study was tested in 50-minute-long class periods of Iowa State University (ISU) undergraduate courses in plant pathology and mycology. For a class period of this length, we suggest dividing the activity into three parts: for example, a 5- to 10-min presentation by the instructor, followed by 20 min of small-group discussion, and 20 min of open (whole class) discussion. The introduction can summarize the background information about SBFS and introduce the case study. From the ISU students’ feedback, the summary and images of the management strategies showed during the introduction strengthened their understanding of the case. Following this presentation, students can work in groups of 2 to 5 and confer on their answers to each of the 6 questions in the case study. Students can be asked to weigh the pros and cons for each disease management strategy during the open discussion.

Overall, students from the trial classes thought that the case study helped them learn about a unique disease of apple that they had never heard of. They indicated that they learned the most by exchanging opinions and judging the merits of different management strategies in both small-group and open discussion. This case study is suitable for use in courses such as plant pathology, mycology, horticulture, integrated pest management, sustainable agriculture, and other agriculture-related curricula.