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The Plant Health Instructor

Volume: 23 |
Year: 2023
Article Type: Lesson Plans
​Assessment of Foliar Fungal Diseases of P​lants​​​

​Eva Liantina Mulandesa,1 Galvin Alonzo2, and Brantlee Spakes-Richter3

1 Citrus Research and Education Center, University of Florida
2 Gulf Coast Research and Education Center, University of Florida
3 University of Florida, Gainesville

Date Accepted: 17 Jul 2023
 Date Published: 19 Oct 2023

Keywords: Epidemiology, Fungal Pathogens, Techniques

​This lesson plan is designed to guide instructors in a lecture class on disease assessment. It is designed to help undergraduate students understand the importance of assessing foliar fungal disease and how to select an appropriate rating scale. The lesson plan has two modules, with specific learning objectives for each. In Module 1, students will learn about the importance of disease assessment, different methods used to measure disease intensity, and how to select the appropriate tool to assess foliar disease. In Module 2, students will describe the weaknesses and strengths of using a specific rating scale and report on their own disease assessment plan. The lesson plan is designed for online delivery and would also be compatible with in-person delivery. For online instruction, students will need a computer with Internet access, camera, and microphone; online access to reading materials; and a camera to take photos of a foliar fungal disease for the graded assignment. The lesson plan includes suggested reading materials, sample lecture slides, a brief non-graded practice quiz, a graded assignment, sample disease rating scales, and an assessment rubric.


This lesson plan will guide undergraduate students in understanding the importance of assessing foliar fungal disease and learning the use of appropriate rating scales. It is most suitable for use late in an introductory plant pathology class (after students are familiar with fungal signs and symptoms) or early in an epidemiology class.​


At the end of this lesson, students should be able to:

  • Explain the importance of disease assessment.
  • Use an existing rating scale for disease assessment.
  • Select an appropriate tool to assess a foliar fungal disease.
  • Describe weaknesses and strengths of using a specific rating scale.

Suggestions are also included for use with graduate students, with the following additional learning objective:

  • Adapt an existing rating scale for use with a disease for which no rating scale exists.​


This lesson plan is designed to be delivered over two class meetings on consecutive weeks. The first meeting will be used to introduce the concepts, and the second will be used for students to present and discuss their assignment.

Week Module Learning objectives
Week 1
Methods to measure disease intensity
  • Explain th​e importance of disease assessment
  • Describe common methods used to measure disease intensity
  • Select the appropriate tool to assess a foliar disease
Week 2Using (and adapting) a rating scale for disease assessment
  • Use an existing rating scale for disease assessment
  • Describe the weaknesses and strengths of using a specific rating scale
  • Optional (advanced): Adapt an existing rating scale and justify its value to improve ratings for a selected disease​


Week 1


  • Online delivery: Access to a learning management system (e.g., Teams, Google, or Canvas). Camera and microphone for communication.
  • Reading: Chapter 2: Measuring Plant Diseases, by L. V. Madden, G. Hughes, and F. Bosch. 2007. The Study of Plant Disease Epidemics. American Phytopathological Society, St. Paul, MN.
  • Photos of a foliar fungal disease for the non-graded assignment (provided, but others may be substituted for regional relevancy).
  • Handout (provided).

Students will need a camera with a good lens to take pictures and standard note-taking supplies (notebook and pen/pencils). A smart-phone camera will suffice, if the user can obtain high-quality images at relatively close distance (e.g., 30 cm). If additional magnification is needed for a selected disease, students may wish to consider inexpensive clip-on lens options (reviewed in Cantonwine, 2017).​


Assigned reading materials, such as the suggested book chapter, should be made available to students several days prior to the first class to facilitate their preparation and organization. The materials listed above are necessary for the first class meeting and homework assignment.

During class, you will discuss plant pathology terms and concepts, such as disease assessment, incidence, severity, and intensity. You will cover the differences between categorical and continuous disease variables and when to use different assessment methods. Additionally, you will review visual assessment methods, including direct estimation, direct estimation with the use of disease scales, use of disease scales, and use of ordinal rating scales. You will emphasize the importance of using the appropriate method and provide examples of disease assessment for different diseases and scales (Supplement 1: Sample Introductory Slide Set).

Figure 1. Foliar fungal disease practice quiz. Observe the foliar disease carefully and rate for disease severity on a scale of 0–100%. A, Symptomatic leaf shown with no rating scale (as presented in round 1); B, symptomatic leaf shown beside rating scale (as presented in round 2).

A brief, non-graded quiz will be incorporated into the end of the presentation in the first class meeting to ensure that students are familiar with the concepts and methods used to assess foliar fungal diseases. The main goals of the quiz are to ensure that students can use an existing rating scale to rate a foliar fungal disease, demonstrate the value of visual guides, and generate curiosity about the subject discussed in class. The non-graded quiz for the class presentation consists of two parts. Part 1 displays 10 symptomatic leaves (powdery mildew on cantaloupe) individually on slides, and students have 12 s to rate each one as 0–100% infected area. Part 2 shows the same leaves randomly reordered—this time side-by-side with a visual rating scale (Fig. 1)—and students are encouraged to rate them using the rating scale, also with 12 s for each leaf. Students submit their ratings in a Google Form provided by the instructor, which allows for real-time data collection and analysis. Students are highly encouraged to participate; although we designed the quiz as an non-graded interactive component, instructors may wish to assign participation points for submitting the ratings.

After completing this exercise, students can analyze the difference between rating a leaf without a scale and rating the same leaf using a rating scale. The results will be presented, followed by a brief discussion led by the instructor. Students are encouraged to participate in the discussion via directed questions about the results. For example:

  • How accurate were your ratings without use of a rating scale?
  • Did your own ratings improve when you used the rating scale?
  • Was the class average more or less accurate when the rating scale was used?
  • How did variability in ratings change with the use of a rating scale?

The instructor may choose to emphasize one or more of the above aspects, using different preset graphs in Google Sheets to display the data in real time. To preset the graphs, go to the spreadsheet associated with the Google Form used to collect student answers, select as many empty data rows as there are expected students in the class, and insert the desired graph or table based on the empty rows. As students submit their answers, the rows will fill, and the graph will reflect the collected data.

Assignment Description (Graded)

The instructor will provide a brief description of the assignment, including the rubric and format, to facilitate student understanding. The assignment requires students to select a foliar fungal disease that is easily accessible to them (e.g., at home, at work, at school, or in their neighborhood). They will then conduct a literature review to find an appropriate rating scale for their case. Beginning students are expected to use a rating scale developed for a similar (or the same) disease to the one that they have selected; advanced students are encouraged to adapt the selected rating scale to customize it for their own application. All students are required to assess the leaves and provide a brief description of the symptoms (Supplement 2: Graded Assignment Examples).​

Week 2


  • Online meeting platform with screen-sharing capability; ensure that screen-sharing is enabled for all users.​


During the second session, students are required to submit their report, which should include a selected foliar fungal disease, a description of the disease symptoms, the rating scale used to assess the selected disease, and sample ratings obtained using the scale. In class, we will discuss any problems or situations that students may have encountered during their literature review and disease assessment. Students will be encouraged to share the strengths and weaknesses of their assessment methodology and recommendations. If needed, the following discussion questions may be used to facilitate the conversation and highlight important points about disease assessment ratings:

  • How similar was your disease to the one for which your rating scale was designed? How did it differ?
  • Was there anything about your disease symptoms that made it challenging to use your selected rating scale?
  • How accurate do you think your assessment was? How could you determine the true level of disease in order to evaluate your own ratings?
  • How would you train someone else to use your rating scale for your selected disease so that you would both consistently assign the same rating to the same leaf?

The final assignment will be graded based on an assessment rubric (Supplement 3: Sample Rubric).


This lesson plan was presented to a test audience consisting of graduate and undergraduate students who had at least some teaching assistant experience. Six participants completed a survey on the lesson plan and provided feedback and insightful suggestions for the lecture, which have been incorporated into the final draft. The demonstration class was copresented by authors Mulandesa and Alonzo. Having two instructors allowed one instructor to present the non-graded interactive quiz and collect the results, while the other instructor continued with the lecture on the graded assignment. Once the results were graphed, they were presented in class, and the instructor facilitated a discussion on the differences in evaluating disease with and without the aid of a diagram. If the lesson plan is intended to be taught by only one instructor, it may be necessary to allocate more time or to assign someone to help monitor and collect data while the instructor continues with the class. To benefit from having a Google Form with students' ratings, you will need to have real severity scores for the diseased leaves presented. By subtracting the students' ratings from the real severity values, you will get a score representing how much each student deviated from the true severity in each rating attempt. You can then sum the absolute value of all deviation scores for a student on each case (rating without and with a scale) and compare those values to detect any improvement in accuracy.

This lesson plan is intended for undergraduate students. To adapt it for use in graduate-level classes, we would suggest that the graded assignment require students to either significantly modify or develop their own rating scale from scratch, and a slide (e.g., PowerPoint) presentation could be included to present their justifications for the design of their assessment tool and to discuss the advantages and disadvantages of their scale relative to existing scales available in the literature.


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