Department of Plant Pathology
University of Kentucky
Vincelli, P. 2005. An Inquiry-Based Approach to Teaching Disease Cycles. The Plant Health Instructor. DOI:10.1094/PHI-T-2005-0222-01
Most of the people who populate our discipline's introductory courses will take only one class in plant pathology in their lifetimes. Such beginning classes are a critical vehicle for teaching undergraduate students the fundamental concepts and principles of our discipline. However, introductory courses are more than this: they are also important opportunities--and often our only opportunity--to teach important cognitive skills, as well as to foster positive attitudes towards our discipline.
While inquiry as a teaching method is as old as Socrates (6), there has been increasing recognition in recent years of its value in promoting undergraduate learning (2,4,5,7,10,11,13). My own understanding of inquiry-based instruction was greatly enriched during a sabbatical leave with Jo Handelsman, a leader in biology education at the University of Wisconsin (9,10). One of the concepts I studied--constructivism--profoundly influenced my thinking.
The essence of constructivism is that knowledge is personally and idiosyncratically constructed; each individual actively creates her/his own unique knowledge structure as s/he tries to make sense of the world s/he experiences (8,12,15,16). As I dwelled on this notion, I became more and more appreciative of its significance: When I teach, I do not build students' knowledge structure; I simply share my own. No matter how hard I may try, I cannot construct my students' knowledge for them; only they can do so. As a teacher, I cannot really "convey knowledge"; all I can do is to create the conditions that facilitate students' own knowledge construction. As these thoughts dawned on me, I became committed to finding ways to enhance inquiry and problem-solving in my classroom.
Although teaching about disease cycles has always been central to my introductory course, I no longer give traditional lectures on them. I have not abdicated my responsibility to structure the learning environment and to lead in the classroom, but my current approach puts more of the work of learning squarely "in students' laps."
Pre-Class Preparation and Classroom Activity
Prior to covering the first disease cycle of the semester, I define the terms and explain the meaning of each of the nine bolded, underlined components of the section labelled "Disease Cycle" in Figure 1
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(Survival, Production of Primary Inoculum etc.). I follow this with an introduction to the concept of disease cycles using a whole-class inquiry. This involves listing these nine components in a scrambled order. Students are then asked to form self-selected small groups and spend four to five minutes arranging the components in a logical sequence. During that time I walk through the classroom to monitor the groups' work, answer questions, and offer comments and queries intended to facilitate progress. When most groups have completed the task, I regain the students' attention, pick a logical starting point, such as "Survival," and ask the class at large what comes next. My experience is that there are always students who feel confident enough to offer an answer, particularly since I provided some assistance during the small-group work.
For much of the remainder of the 14-week semester, I cover approximately 20 disease cycles in detail. When a disease cycle is covered in detail it is, along with disease management, a central topic for that 50-minute class period. Before we discuss a disease in class, students are required to review the assigned reading on the disease of concern and to come prepared to answer the questions posed in Figure 1. The assigned reading may come from George Agrios's text (1) or the Plant Disease Lessons available at the APSnet Education Center. (These sources both provide diagrams of disease cycles, but since they rarely are configured as outlined in Figure 1, students still must think independently about how to generate their own disease cycle.)
During the class period, I provide a brief introduction to the topic disease. I then select a student at random and call on him/her to provide the pathogen name. I'll call on another student at random to provide a brief description of the host range; another for a
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description of the survival phase, and so on, until we cover all the topics in Figure 1. During this exercise, students commonly offer only skeletal, often cumbersome explanations of components of the disease cycle, since they have merely read about it. Once a student has provided some explanation of the component in his or her own words, I expand on what the student has said, explain the event in detail, and add it to an emerging graphical illustration (Figure 2
) of the disease cycle on the chalkboard (admittedly an old-fashioned technology, but one that seems well-suited to a dynamic classroom environment). Upon completion of the disease cycle, I sometimes ask the class at large to identify one or more environmental factors that influence infection (or some other phase or event in the disease cycle).
After completing the disease cycle, student groups are required to spend several minutes discussing how they might manage the disease, including how the management practices they identify might influence the disease cycle (Figure 3) . During their discussions, I often circulate through the classroom to be available if they have questions. After allowing two to four minutes of discussion, I solicit possible management practices. In contrast to the queries on components of the disease cycle, inquiry on management is open to the class at large; I find I usually don't have to call on students for management ideas. As before, I elaborate on students' thoughts and explanations.
I always choose in advance a set of management practices I feel is important to present. Usually, the students offer up most of the practices on my list but, if not, I complete the list on my own. When students offer valid management suggestions that are not on my target list, I acknowledge those and may briefly discuss them, but I do not write them on the board. This assures that students know clearly which practices they need to understand for the exam.
I also show visual illustrations of important elements of the disease cycle: symptoms, survival and dispersal structures of pathogens, vectors, etc. This is done usually before (but sometimes after) the discussion of disease management. By the time we have completed the class period, students have had multiple representations of the disease: what they read prior to class, our disease cycle activity, and the projected images.
Early in my teaching career, I was reluctant to require attendance at lecture; it seemed rather "heavy-handed." I now consider this a fundamental component of this teaching approach. The possibility of being called on at any moment creates a mild sense of apprehension in the classroom. As a consequence, students are usually very self-motivated to give at least a passable answer in front of their peers. If attendance were not required, this peer pressure would probably be a motivation for some students to avoid attending class, especially when they do not feel adequately prepared.
I respect the fact that the unexpected can happen in students' lives, so the first unexcused absence from class does not affect their grade. I also respect that students sometimes need to be able to give themselves permission to come to class unprepared; thus, when asked a question, students are permitted to "pass" two or three times (the number has varied among semesters). When they do so, I accept their decision without judgement or comment and immediately note it in my ledger, so that they and the rest of the class know that such things are documented.
While this teaching approach generates a certain anxiety in students (as well as teachers!), students seem to appreciate it to the extent that they feel they are being treated with fairness and respect. I use a random numbers generator to select students from the roster in order to assure that my own biases do not influence my selection of students to answer questions. I announce in the first week of class that I don't expect them to be brilliant when they are called on, but that they should simply make an honest effort to come prepared and say what they know. I promise them that if they do this and they are struggling with an answer, then I will come to their rescue every time. And I do so. It is critically important, no matter what the teaching approach, to create a respectful, nurturing environment for young scholars.
It is imperative to deal with incorrect answers in as positive and constructive a manner as possible. I don't have any standard or recommended practice, but I always attempt to thank the student for his/her answer and encourage her/him to continue to participate in the future. However one deals with incorrect answers, it is important that students receive immediate and constructive feedback.
An important element of this teaching approach is that students use the same questions to investigate each disease cycle. While each plant disease is unique, there are common questions that can be asked for each. (Figure 1) This provides introductory students the repetition they need in order to become skilled at deriving disease cycles from written literature.
I teach about most principles and management practices within the context of our study of individual diseases. For example, when we discuss vertical vs. horizontal resistance, we do so during the management discussion of black shank (Phytophthora root rot) of tobacco. More than twenty out of thirty class periods are spent studying individual diseases. Since several diseases covered early in the semester are polycyclic, we initially discuss all the stages presented in Figure 1. As the semester progresses, students are introduced to the concept of a monocyclic disease, and for these they learn that the questions under the components "Production of Secondary Inoculum" and "Dispersal of Secondary Inoculum" (Figure 1) do not apply.
Why I Prefer This Approach to a Traditional Lecture
My experience suggests that there are many benefits to using this teaching approach. Foremost is that it teaches students more than facts; it teaches them an important cognitive skill—to analyze literature for key elements of the disease cycle. By promoting the students' analysis of literature and the synthesis of their own disease cycles, this teaching approach helps to develop higher-order thinking skills (3). I explain this to students on the first day of class, and I remind them of it often during the semester. Most students seem to appreciate this as the semester progresses. While some students struggle to develop their cognitive skills, a significant fraction of the intelligent, motivated students in the class begin in a rudimentary way to think like plant pathologists. It is very impressive to see, before the semester is over, students beginning to talk about sources of inoculum, modes of dispersal, sites of infection, and so on. Perhaps the best evidence that higher-order thinking skills have been developed is the fact that, in the final exam, the majority of students successfully prepare an acceptable disease cycle for a novel disease using only photocopied text provided from an APS compendium.
In addition to fostering the development of cognitive skills, I believe the inquiry-based approach helps students by providing an organizing scheme for making sense of the mass of detail present in the reading. Based on the questions in Figure 1, students know what is expected of them prior to class. They also have a cognitive framework on which to hang new facts. For example, if I tell them that contact fungicides can interfere with infection by powdery mildew fungi, they have a clearer sense of how that fits into the disease cycle.
This approach helps students in other ways. It provides a powerful motivation to remain abreast of the workload, and students appear to appreciate this enforced discipline. Because student verbalization is such a significant component of class time, students seem more likely to respond to open-ended, whole-class inquiry than in semesters when I relied exclusively on lecture. When I invite students to form small groups and brainstorm on disease management practices, it is not unusual for groups to offer interesting and creative ideas for disease management, ideas that can form a nice springboard for short discussions. Finally, the queries asked of students provide moments of mental refreshment, which help enliven everyone in the classroom, including the teacher.
This paper would not be complete without a presentation of limitations of the teaching approach described above, although in my experience there are few. The most significant one is that it is not possible to cover quite as much content as in a purely lecture-based classroom. Time must be allocated for students to do small-group work, and inviting student ideas for disease management is time that I am not presenting content. This is an important concern since, as the amount of knowledge in our discipline grows, we instructors commonly feel increasing pressure to present even more content to our students. However, in my judgement, the small reduction in content covered as compared to semesters when I have lectured is a reasonable sacrifice, since the inquiry-based teaching approach enhances students' cognitive skills and helps to foster a positive attitude towards our discipline. Furthermore, although I present slightly less content in class, students probably remember more of what is presented, as research shows a drop in concentration after 20-30 minutes of uninterrupted lecture even in highly motivated students (14).
Like all active-learning teaching methods, this teaching approach requires that the instructor is comfortable with both the course content and the occasional disorder of an inquiry-based classroom. No matter how good one's graduate education, most first-semester teachers of introductory plant pathology recognize a need to learn or re-learn important material. Thus, this teaching approach may not be suited for introductory instructors in their first semester of teaching. However, I would certainly encourage experimenting with it when one feels reasonably comfortable with the course content itself.
My personal experience with the teaching approach outlined here is consistent with the hypothesis that students in introductory plant pathology can learn more than substantial content; they also can develop important cognitive skills. I have found that most of my students have been surprisingly receptive to the high expectations for the pre-class preparation this teaching approach requires, as long as they feel they are being treated with fairness and respect. I also have found that engaging the students in active learning is more exciting and gratifying to me personally than when I lectured.
Like all teaching, this approach is an amalgam of many ideas from varied sources, tempered by my own personality. Teaching in the classroom is ultimately a very personal experience. In contrast to my writing style for research papers and Extension publications, this paper is written heavily in the first person, reflecting my belief that there are an infinite number of ways to successfully teach plant pathology. I hope that the ideas presented in this paper prove useful to others in the profession, in each of your unique journeys as teachers.
I am grateful for the chance to have studied teaching and learning with Jo Handelsman and Chin Sun while on sabbatical at the University of Wisconsin. I owe a debt to the many students and colleagues who have contributed to my ongoing growth as a teacher. Thanks also to David Smith for reviewing a previous draft of the manuscript.
Thanks are expressed to Tore Lindeman for valuable input in formulating Figure 2.
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Questions for Studying Individual Plant Diseases
For each assigned disease, please come to class prepared to answer the questions listed below. Develop brief answers to these questions by paraphrasing the assigned reading. It may help you to take written notes as you find answers. These questions will form the basis of much of the content of the exams.
What is the name of the pathogen?
Describe the host range in 10 words or less.
Which pathogen structures survive between seasons?
Where do they survive?
Production of Primary Inoculum: This is the stage when the pathogen produces a structure that can initiate infection.
After over-seasoning, which pathogen structures can infect a plant?
Where are they produced?
Dispersal of Primary Inoculum:
How do these structures disperse?
Establishment of Infection: This is the stage when the parasite initiates its food relationship with the plant.
Where on the plant (leaves, flowers, twigs, etc) does the pathogen penetrate?
How does it penetrate? (For example, does it penetrate wounds? Is it introduced by a vector?)
Colonization (=Invasion): This is the stage where the pathogen grows on/in the plant and feeds.
Where in the plant does the pathogen colonize?
How does it spread in the plant? (For example, via translocation in the phloem or xylem?
Mycelial growth within the tissue, upon the tissue? Through cell multiplication?)
In 20 words or less, list the key symptoms on all plant parts affected.
Production of Secondary Inoculum (if produced):
Which structures are produced that can initiate new infections?
Where on the plant are they produced?
Dissemination of Secondary Inoculum (if produced):
How do these structures spread?
Production of Survival Structures:
Where are survival structures produced in/on the plant?
Footnote: the policy outlined in my most recent syllabus is summarized as follows: Attendance in class and laboratory is required. Students will lose 2.5% credit for each unexcused absence from class or laboratory, up to the loss of one and one-half letter grades. Students will be granted one waiver for a missed class and no waivers for missed laboratories. With regard to in-class queries, each student is allowed to "pass" twice. After these "passes" have been used, the student will lose one point off the final grade for failing to provide an answer; answers that suggest a serious lack of preparation may also lose one point off the final grade.