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​​Naughty Peat: A case study in plant pathology, with emphasis on Koch's Postulates and disease etiology

Don Mathre and Bill Grey

Montana State University
upldm@montana.edu

Mathre, D.E. and W.E. Grey  2002. Naughty Peat: A case study in plant pathology, with emphasis on Koch's Postulates and disease etiology. The Plant Health Instructor. DOI: 10.1094/PHI-T-2002-0301-01.Reviewed 2013

Trying to keep the students interested in introductory plant pathology classes has always been a challenge. Furthermore, getting students to understand the concepts of plant pathology can be frustrating. One approach that we have found useful both to catch the students' interest and to aid in their understanding of concepts is to use "case studies" periodically throughout the semester. Usually these involve situations where the instructor has had a personal involvement in a plant disease situation that illustrates one or more points about plant diseases and their impact on our everyday lives. Current, real-life situations seem to spark more student interest, even if they are not always "perfect" examples and often have unresolved aspects. It is good to emphasize to the students that problem solving in real-life situations is challenging because often the issues are unclear or the information available is incomplete.

In the following example, several questions can be posed to the students as the case is related. We normally do this orally, stopping the presentation while the questions are discussed. This can be done if the student numbers are not too great, e.g. no more than 20-30. If the class is larger, you might want to save the questions for a time when the class breaks into smaller groups, e.g. during the laboratory portion of the class. We normally do not write the student responses on the board since this takes too much time and detracts from the ongoing discussion. The questions are based on material that has previously been presented to the students. For instance, we normally cover Koch's Postulates early in the term so that questions that rely on students' understanding of these postulates can be asked. We suggest that you try to complete the presentation and questions within one class period so that the information and conclusions don't have to be carried over and possibly lost. An alternative would be to pass out the questions in written form, and to ask the students to individually prepare written answers.

Questions that can be asked relatively early in the term might be the following: "How would you go about determining if an organism is a pathogen? What are possible effects of environmental conditions? What other factors could result in stunted plant growth? How would you determine which factors are involved? Each instructor will need to develop his or her own set of questions to go along with each case study. In the following case study, questions for students to answer are interspersed within the "story". Each instructor will need to decide how and when to present the questions that she or he would like the students to cover. Our experience is that students really are intrigued with case studies and use of them does enhance the students' understanding of underlying concepts, such as Koch's Postulates.

CASE STUDY. One of us (DEM) received a phone call early during one autumn semester in which the caller asked whether I knew anything about the fungus Fusarium. I assured the caller that "Yes", I did, since I had worked on Fusarium root rot of cereals for a number of years. However, the caller did not indicate the reason for the question and hung up. Later that autumn, we received another phone call from an insurance agent wanting to know if he could bring a flat of pine tree seedlings up to our lab to determine why they were not growing normally. Although we did not work on forest related disease problems, we rarely turned down the chance to be involved in disease diagnosis. It turned out that the first caller who asked if we knew anything about Fusarium was a university employee who was a friend of the insurance agent.

When the insurance agent brought the flat of pine seedlings by, it was obvious that most of the 100 seedlings in the flat were suffering from some disease problem. They were stunted and yellow, while a few seedlings were 3-4 times taller and dark green in color (Figure 1). The agent told us that the plants were growing in peat moss so they shouldn't be infected since "peat moss is sterile." They came from a greenhouse in northern Montana. The greenhouse owner had contracted with a private timber producer to grow 250,000 seedlings for reforestation purposes.

Figure 1. Pine seedlings growing in Montana-produced peat moss, showing several tall healthy plants and many stunted diseased plants.

At this point, the following questions might be posed to the students:

1. Do you believe the statement that "peat moss is sterile"? Why or why not?

2. How would you determine the accuracy of this statement? What is the difference between "sterile" and "pasteurized"? Do you think a peat growth medium would remain sterile after it was bagged for sale?

3. How would you go about determining if the pine seedlings growing in the peat moss were infected with a pathogen?

4. How would you determine if the peat moss was the source of the pathogen?

5. What if several different fungi were isolated from the pine seedlings. How would you determine if each one is a saprophyte or a pathogen?

As it turned out, isolations indicated that both Pythium ultimum and several Fusarium species could be easily isolated from the roots that were dark brown (Figure 2). We relayed this information to the insurance agent and suggested that additional work needed to be done to determine whether these isolated fungi were actually pathogenic to the various conifer seedlings. At this time, no lawsuit was indicated or planned. The insurance company just wanted to know what the problem was and who was responsible for it. So we agreed to perform "Koch's Postulates" using a number of different conifers as hosts, and the various Pythium and Fusarium isolates we had collected. We also requested a sample of the peat moss to determine whether it was "sterile", as the insurance agent had suggested. Isolations from the peat moss were made by dilution plating on selective media, and produced both Pythium and Fusarium spp. from the peat.

Figure 2. Diseased (left) and healthy (right) pine seedlings. Note the lack of fine roots on the diseased seedling.

In previous years, the greenhouse operator had used Canadian sphagnum peat moss with no problems, but recently he had been approached by a Montana peat moss producer who asked him to try his product. As it turns out, this peat was not at all similar to sphagnum peat. It was a peat that was very dark brown in color with a high pH, in contrast to the lighter color of sphagnum peat that had a much lower pH. Pathogenicity tests with the isolated Pythium and Fusarium spp. were then conducted to determine if the strains we isolated from the pine seedlings were pathogenic.

Questions to consider:

1. How would you actually go about determining the pathogenicity of the fungi you isolated?

2. What growth medium would you use to carry out your tests? Could the growth medium possibly have an impact on the tests? If so, why or how might this impact occur? How would you actually infest the growth medium? Would the type of inoculum used have any potential effect on the results?

3. What environmental conditions would need to be regulated or monitored during these tests?

4. Could pH perhaps be a part of the problem? If so, how would you go about testing this? If there is a pH effect, could it be on the host or the pathogen, or both? How would you determine this?

We did find that both Pythium ultimum and the Fusarium spp. were indeed pathogenic to a number of conifer seedlings.

End of story? Not quite. Over the months that it took us to do the peat moss isolations and the pathogenicity tests, the greenhouse operator decided to file a lawsuit against the Montana peat moss producer for providing him with a "defective product" since the peat appeared to be the source of the pathogens, and hence his production problem. Since the insurance agent who first contacted us represented the peat producer, we thought that our information would be devastating to his case. A trial before a judge eventually occurred, and the peat producer won!!

Questions to consider:

1. Why do you suppose the greenhouse grower lost his case when it appeared that the growing medium he purchased from the peat producer was contaminated with pathogenic fungi?

2. What things might the greenhouse grower have done to accentuate his problem?

How could this decision against the grower be based on the evidence that we produced that the peat was not sterile and that it was loaded with plant pathogenic fungi, which could cause great reduction in growth of infected seedlings? The decision of the trial judge was based on the fact that the peat producer had offered a free sample of his peat to the greenhouse producer for a very small test to see if it would be suitable for conifer production. The greenhouse operator, wanting to save money, decided to forgo a small test, and instead purchased enough peat to grow enough conifer seedlings to fill his entire greenhouse. Thus, the judge ruled that even though the peat was "defective," the greenhouse operator had not used good business judgment in forgoing the small test with the peat.

The greenhouse operator eventually went bankrupt, possibly as a result of this case as well as other problems in management.

Now, is this the end of the story? No!! Remember that one of us (DEM) was contacted by an insurance agent to determine why the pine seedlings were not growing well. As it turned out, the greenhouse operator greatly objected to the insurance agent contacting university employees to perform a service that might adversely affect the grower. He contacted the governor's office, which then phoned the university president, who then visited with the Dean of Agriculture - all asking the question of why university employees were involved in this lawsuit. Needless to say, none of the above was happy with the situation. Our response was that our service to the insurance agent began as an innocent attempt to use our expertise to provide help to someone in need. Furthermore, our main university responsibilities were for classroom teaching and for research on wheat and barley diseases, certainly not forestry problems. As a result, we looked upon this request as outside of our normal sphere of duties. While the university administration eventually backed us up, this situation certainly created some hard feelings in some individuals.

Questions to consider:

1. What is the responsibility of an employee of a public land grant university to help solve a practical problem such as that described above?

2. Are there situations where such an employee should say no to a request such as that made by the insurance agent?

Additional Background Information for Instructors:

For those not familiar with forestry or conifers, the following background information may be helpful. Conifer seed is usually collected from the same ecological zone where it is to be planted, e.g. lodgepole pine growing at 5000 ft. To get the best reforestation, timber companies or agencies like to use seedlings either grown outside in nurseries, or in greenhouses using cone-shaped containers filled with a growing medium, often Sphagnum peat moss. Since most forest soils are acidic, it helps to use a growing medium with a similar pH, and Sphagnum peat does have this characteristic. It usually takes at least 6 months to more than 2 years to produce a seedling that will withstand the rigors of transplanting. Therefore, disease-free conditions must be maintained for a considerable length of time. Avoidance of disease is preferred to controlling disease once it is established. Hence, the growing medium plays a key role in seedling health.