INTRODUCTION
This case study involves an activity many people are involved in no matter their interest or present or future occupation. Many people have homes where they are attempting successfully and sometimes unsuccessfully to grow different plants. Often the need arises to determine what is wrong with a plant. This case study introduces students to the types of information needed for the proper diagnosis of plant diseases and where they can obtain information on plant diseases. It also introduces ideas about how to control plant diseases and can be used to introduce the concept of resistance that exists in plants to different plant diseases.
This case study was developed for the introductory laboratory in a basic Plant Pathology course designed for juniors and seniors in Horticulture, Agricultural Education, Agronomy, and Forestry. It could also be used in other courses such as Introductory Agriculture, Horticulture, Botany, and Biology, where there is an interest in the diagnosis of plant diseases. The level of the course could range from very basic to advanced, depending on the expectations of the instructor. The case was originally designed for use in a laboratory situation of approximately 20 students working in teams of two to three students (associated with a much larger class).
Students should have the ability to observe symptoms on plants using a microscope (samples of diseased leaves should be provided), access to making spore mounts from leaves, and access to a library and to the Internet. Access to microscopes and live samples is not absolutely necessary but makes the case much more interesting to the students and allows them to look for spores on the infected leaves. Many students will have previously seen the disease described in this case study and may even know what the disease is but they may have never pursued further information concerning the disease, its control, or what causes it.
The plant disease affecting Mrs. Cook's roses in this case study is Black Spot of Rose. Black Spot of Rose is the most common disease of roses, and its symptoms are commonly found even on roses that are sprayed for the disease. If leaves with symptoms of the disease can be obtained, it would be beneficial to use these for the students to observe. The leaves can also be used to observe the spores of the fungal agent, Diplocarpon rosae. The spores have a distinctive (2-celled) appearance, from which comes the scientific name for the pathogen. Procedures are included for the mounting of specimens. These procedures are simple and can be done in any laboratory with a minimum of supplies.
References related to general plant disease diagnosis are made available to students along with directions for making spore mounts for observations. There are references for the specific disease, Black Spot of Rose, included at the end of the teaching notes. This case can easily be altered for another plant disease.
OBJECTIVES
The overall objectives are to learn about the process involved in the diagnosis of plant diseases. Specific objectives are:
- Investigate the processes involved in diagnosing a plant disease.
- Illustrate how symptoms are used to describe a plant disease and how these descriptions are then used for comparison to previously described plant diseases.
- Learn how to make and use slides and microscopy for the identification of a plant disease.
- Examine information requested by clinics diagnosing diseases and determine why this information is needed.
- Investigate where information (books and Internet) concerning plant diseases can be obtained.
- Consider factors such as environment, soils, and specific cultivars of plants and how they can have an effect on a plant disease.
- Examine the control measures used in different situations for controlling a plant disease.
- Examine plant resistance and its effect on the development of a plant disease and how it is used in disease control and determine its limitations.
All of these objectives may not be accomplished in an introductory class. The depth of the coverage of these objectives is dependent on the level of the students.
MAJOR ISSUES
Any person who has been or will become interested in the growth of plants has or will at some time run across a plant disease or problem that is of interest. The major problem in trying to prevent or eliminate a plant problem involves the correct diagnosis of that problem. This is true for a homeowner as well as a professional grower. The correct diagnosis of a plant problem and controlling a plant problem involve several steps including:
- Being able to recognize and accurately describe the symptoms/signs of a plant problem.
- Knowing how different factors can affect plants directly and may be important in the development of plant diseases.
- Knowing where to obtain information concerning plant problems in the form of printed information or whom to contact.
- Knowing possible control measures and being able to determine which control measures could be used in different situations.
Answer to the Questions
- What is meant by symptoms and signs of plant diseases? What do these terms refer to and why are they important?
To diagnose a plant disease it is first necessary to make careful observations of the symptoms and signs of the disease. Symptoms have been defined as the visible reactions of a plant to a pathogen. These can be alterations from the normal appearance or physiology of a plant. Some examples of symptoms of plant diseases include: yellowing or chlorosis, death of new shoots or dieback, wilting of leaves or entire plants, abnormal coloration on leaves and bark, rotting of roots, stunting, and abnormal growth. It is vital to know what the normal healthy plant looks like in order to detect possible alterations in its normal growth and physiology. Signs are used to describe the actual physical evidence of the pathogen such as spores and fruiting bodies on the leaf surface. The combinations of symptoms and signs are used to describe the abnormalities that exist in or on the plant being observed. With some diseases, symptoms and signs are very characteristic and distinctive and can be used for a direct diagnosis of a disease. However, in many cases, further investigation and the possible isolation of the pathogen is necessary for the identification of a disease. Many nutritional and physical deficiencies and injuries can result in symptoms being exhibited by a plant. Careful observation of symptoms and a knowledge of the care of plants is necessary to determine if nutritional or physical problems exist. Chemical usage in and around plants can also have significant effects on plants.
- Describe the symptoms/signs of the rose plant provided in lab that was seen by Jason in Mrs. Cook's yard. How can the microscopic observations be important?
The symptoms of Black Spot of Rose include somewhat circular black to purplish spots approximately 1/2 inch or more in diameter. The edges of the spots when carefully observed are somewhat irregular appearing--almost like a fringe. The spots may coalesce and become irregular in shape. As symptoms develop further, the black spots become surrounded by yellowing, which may spread over the entire leaf. Leaves at this point may drop from the plant. Severely infected plants can become completely defoliated and flower quality is reduced. The asexual spores of the fungus that causes this disease (Diplocarpon rosae) are two-celled (Figure 1) and this is the reason for the genus name of the organism, while the species name comes from the rose plants it infects.
- What are some questions that Jason should ask his mother concerning the care and culture of her roses? Why are these questions important?
Many questions need to be answered in trying to determine the cause of symptoms on a plant. In some cases, such as this disease, the cause is fairly obvious, but in most cases the problem is not as easily identified. Jason would be expected to ask his mother if the two roses that she has are the same cultivar, if they have they been treated the same (fertilization, watering, pruning, etc.), and if there are any differences in the environment to which the two roses are exposed. Is the rose that is not affected in a much drier location, is it exposed to full sun, does it have good air circulation while the affected rose is in an area that is much wetter, does not dry as fast from the morning dew, is not in full sun, and has poor air circulation? All of these factors can be important to the development of the disease. Is the plant that does not exhibit symptoms a variety that is more resistant to Black Spot of Rose? Cleaning up old leaf debris can also be an important factor in the initial development of the disease since the debris can harbor spores that can reinfect new growth. Is there debris under one of the plants but not under the second plant?
- Take the plant problem clinic form and try to answer the questions requested. Why are these questions important?
Most land grant universities throughout the United States have a plant disease diagnosis laboratory, and the clinic form for the state where the case study is being used can be substituted for the generic form used in this case. It is important that students know if such a facility exists in their area and it would be beneficial to use the form associated with your area. The initial information on this form is requested so that a response can be sent to the appropriate individual once a diagnosis has been made. The grower type is requested because recommendations for control measures in a commercial situation may not be appropriate for a homeowner or residential operation.
The first question after the "Plant Problem Diagnosis" heading may give the person diagnosing the disease an idea of the resistance/possible problems associated with specific plants and cultivars. Nematodes and soil pH can be important in plants that are exhibiting nutritional deficiencies and tissue for nutrient analysis may be requested to confirm a diagnosis of a nutritional problem. Age and planting date can be important in diseases which are more common on plants of a certain age. Acreage and number of plants gives the diagnostician an idea of the area that may need to be treated. Some control measures are feasible for one plant but not for 100 plants. Previous crops can sometimes lead to problems in plants that are planted into the same area later. The location, soil texture, rainfall, irrigation, drainage, and lighting indicate the type of environment to which the plant is exposed and the environment can have a significant effect on the development and severity of diseases--some are favored by wet, humid conditions while others are favored by cool, dry conditions.
The symptoms, degree of injury, and distribution of the problem give the diagnostician an idea of the severity of the problem. Chemicals applied to a crop can be important because in some cases, such as when herbicides are used near plants, symptoms such as yellowing can occur. If a fungicide has been used, it may turn out that the fungicide used was inappropriate for the disease or the fungus may have developed resistance to that specific fungicide and an alternative fungicide would be recommended.
Overall, all of the questions are important--their purpose is to get as much information as possible so that a correct diagnosis and recommendation can be made to the grower. The more information that is available, the easier it will be for a correct diagnosis to be made.
- Is the problem found on the rose a disease, insect, or physiological/cultural problem? What is the basis of your answer? Are you positive? Why or why not?
If spores can be found and they are compared to the spores exhibited by the fungal agent thought to be causing the symptoms, this can serve as a good indication that the problem is a disease caused by that agent. The material should be observed for insects because these can cause problems and may also be important in the spread of disease. Physiological and nutritional problems often exhibit characteristic symptoms, but the symptoms observed in this case do not match the symptoms reported for various nutritional deficiencies and toxicities. In this case, because of the characteristic symptoms of Black Spot of Rose being exhibited, you can positively identify the disease that is present. To be absolutely sure of the diagnosis, you could isolate the fungus and then use this to infect a healthy rose of the same cultivar. The same cultivar should be used because of possible differences in resistance exhibited by various cultivars.
- Where would you find information on the diseases associated with roses, both on the Internet and in printed resources? Give three sources of information: at least one from Internet sources and at least one from printed resources.
There are many general gardening texts as well as a great deal of information on the Internet on Black Spot of Rose. At the end of these teaching notes, several print sources and selected Internet sites are listed. In our use of this case, most students have a tendency to only want to use Internet sites, but these often do not contain the more detailed information on plant diseases that can be found in printed texts. Students in general want to avoid having to go to the library to search for information. There are also publications available from various land grant universities and these may be available in local extension service offices.
- Based on the information that you have obtained, what is possibly affecting Mrs. Cook's roses? Is this the only possibility? Why or why not?
The disease used in the case study is Black Spot of Rose, which is caused by the fungus Diplocarpon rosae Wolf (ascomycete, anamorph Marssonina rosae (Lib.) Lind). D. rosae infection of roses results in symptoms and signs which are very distinctive and, due to this distinctiveness, a diagnosis of Black Spot of Rose in this case is the only diagnosis possible. In many other diseases with non-distinctive symptoms, further testing such as the isolation of the fungal agent would be necessary for confirmation of a disease diagnosis.
- What are the possible reasons why the problem is not occurring on the other rosebush in the yard?
The most likely reason for the difference in the plant that has not developed the disease is resistance but there are many different factors that may be involved in disease development. Black Spot resistance in roses is variable, with some varieties exhibiting good resistance. This is the reason that Jason would need to know the specific varieties of roses that were in the yard. The environmental conditions that the plant and pathogen are exposed to can also have a significant effect on the development of a disease and its symptoms. In the case of Black Spot of Rose, the environmental conditions that favor disease development include: wide temperature range (15-27° C) with optimum for conidial infection of 20±1° C, and wet humid conditions. Good air circulation around roses hastens drying and reduces the development of Black Spot and this may lead to reduced disease in one plant compared to another in a home situation.
- If you were Jason what kind of recommendations would you make to your mother? What are the advantages and disadvantages of the different measures? Would what you recommend be different if you were talking to a commercial grower of roses? Why or why not? What would you do if your mother does not want to use any chemical treatment measures?
Black Spot of Rose can be controlled through a combination of different measures. If Jason's mother is planting new roses, she should plant resistant varieties if possible. To control the disease on the plants that she already has in her yard, she needs to implement several different measures. Sanitation is the most important disease control measure. All infected leaves need to be removed from the area along with mulch that may be contaminated with infected leaves and spores. This should include any infected leaves on the plant. All infected plant materials including canes that have dark or reddish lesions should be pruned from the plant and destroyed. Leaves should be kept dry by avoiding overhead irrigation and bushes should be pruned to allow good air circulation for rapid drying.
Even with all of these measures, Black Spot may still develop and fungicide sprays may be necessary even on resistant varieties. If the next door neighbor does not control Black Spot on his roses, it can cause problems on Mrs. Cook's roses. Fungicide sprays will likely be necessary with susceptible varieties. Many different fungicides have been used for the control of Black Spot and it may be necessary to spray roses every one to two weeks depending on environmental conditions. It is a good idea to use at least two different fungicides from two different chemical classes and to alternate between these to prevent the development of resistance that may occur when using some fungicides exclusively. If cultural practices are carefully and diligently implemented especially over the winter and early in the year the use of fungicides can possibly be eliminated. In a commercial operation, the use of fungicides is a standard practice. The reasons include the ease of spraying a fungicide with minimal labor compared to the use of more labor intensive cultural control measures.
- What are the differences between diagnosing a plant disease and a human disease? Is it more or less difficult to diagnose a plant disease?
In some ways it is more difficult to diagnose plant diseases because you do not have the option of asking the patient how long he has had symptoms or where it hurts. You cannot ask a plant if anything unusual has happened--for example, you can't ask a plant if someone has run into it with a lawnmower. A diagnostician must make careful observations as to the environment and determine if there are connections between the conditions and symptoms. Of course, a plant is not likely to sue you if you make an incorrect diagnosis so in some respects it could be considered easier to diagnose plant diseases, but if you make an incorrect diagnosis in a commercial situation, the commercial grower may want to sue you. Correct diagnosis is based on careful observation and knowing what to look for in various situations.
CLASSROOM MANAGEMENT
This case was designed to be taught using problem based learning methods in laboratory periods where students have access to microscopes and work in groups of two to three people. If groups are much larger than this, it is difficult for students to work as groups due to the arrangement of our laboratories into rows of tables with minimal space between the rows.
This case has been used as the introductory laboratory and introduces many of the concepts and ideas that will be pursued further during the semester. It introduces the use of dissecting microscopes for closer observations of symptoms, how to make slides for the observation of possible spores, and how these spores can be used in the identification/confirmation of plant disease agents. It introduces students to library resources for locating information relating to plant diseases and also the availability of information on the Internet as well as possible limitations of the information found on the Internet. The case can also be used to introduce the control measures used with plant diseases and how these can be different in different situations.
The depth of the information and understanding of students was extremely variable depending on their background. The utilization of groups, even small groups, helped to balance the knowledge of the groups overall. In some cases, this was the first experience students had with problem based learning methods and the investigation of problems on their own.
The case is best taught over two laboratory periods (2-hour lab periods).
The case is given out to the students to read prior to the first laboratory period. Students have access via the class computer network to the general references on plant disease diagnosis they will need to answer the questions at the end of the case.
During the first laboratory period, the students and the lab instructor spend the first 30 to 45 minutes going around the building looking at different plants with various symptoms and discussing possible problems with plants such as environmental factors and damage from the activities of people. This is not a lecture activity but is a time for students to become aware of their surroundings and to make observations about the impact of various activities. A majority of students will become involved in this activity. Upon returning to the laboratory, students are expected to observe the symptoms of the plant disease associated with the case study. This includes observations using a dissecting microscope for the description of symptoms and a compound microscope for the observation of possible spores.
Some students may need an introduction or review on the use of a microscope and many have never made their own slides. The use of real samples, not just pictures, in the laboratory and having students make their own slides will help to better illustrate the procedures involved in diagnosing plant diseases and gets the students more involved. The case should be changed as needed if there is a disease that would be more appropriate/available in the area where the case is being used. Many different diseases can be used, but the disease should be a common disease for which students can readily find information on its symptoms and control and samples should be readily available for observation. It would also be good if the disease has signs such as spores or spore structures that can be easily observed on slides.
Prior to the second laboratory, students are expected to locate in the library and on the Internet sources of information related to rose diseases and to identify the possible causal agent of the disease and answer the questions associated with the case. The questions can be answered as a group or as individuals. During the second lab, a discussion of the case is led by the instructor and students discuss their answers to the questions as well as questions about the information they collected, where that information can be found, what further information would be useful, and why this information is needed.
The instructor might also ask other questions of the students in order to get more discussion. All individuals should be encouraged to participate and this can be handled by having each group answer one of the study questions and then having the other groups provide additional information. As questions are being discussed, additions can be made to the answers brought into class. The students' answers are turned in at the end of the discussion. A portion of the grade can be based on participation of individuals in the group discussion.
REFERENCES
General references related to diagnosis of plant diseases
Fox, R.T.V. 1993. Principles of diagnostic techniques in plant pathology. Wallingford : CAB International. Chapter 2, pp. 9-36.
Grogan, R.G. 1981. The science and art of plant-disease diagnosis. Ann. Rev. Phytophathology 19:331-351.
Maloy, Otis C. 1981. Diagnosis of plant diseases. Cooperative Extension Publication, Washington State University EB0751
Putnam, M.L. 1995. Evaluation of selected methods of plant disease diagnosis. Crop Protection 14:517-525. (Important portion for this case study includes pp. 517-519)
Wick, Robert L. 1986. Careful specimen selection helps plant disease diagnosis. American Nurseryman. 164:79-85. August 15, 1986.
References and web sites related to Black Spot of Rose:
Horst, R.K. 1983. Compendium of Rose Diseases. St. Paul, Minnesota : APS Press, pp. 7-11.
Rose Diseases. Clemson University Home & Garden Information Center HGIC 2106 http://hgic.clemson.edu/PDF/HGIC2106.pdf
Chatfield, Jim and Stephen Nameth. Black Spot of Roses. Ohio State University Extension Fact Sheet HYG-3072-96 http://www.ag.ohio-state.edu/~ohioline/hyg-fact/3000/3072.html
von Broembsen, Sharon. Diseases of Roses. Oklahoma Cooperative Extension Service Facts F-7607 http://www.ento.okstate.edu/ddd/diseases/roseblackspot.htm
Watkins, John E. Black Spot of Roses. University of Nebraska Cooperative Extension G91-1060-A
http://ianrwww.unl.edu/pubs/horticulture/g1060.htm
There are many other references which can be used and have information related to Black Spot of Rose and its control.
Glossary of plant pathology terms:
http://arneson.cornell.edu/Glossary/Glossary.htm
Clemson University. Technical Contribution Number 4672 of the SC Agricultural Experiment Station.
Acknowledgements: This case study was developed with support from The Pew Charitable Trusts and the National Science Foundation as part of the Case Studies in Science Workshop held at the State University of New York at Buffalo on May 22-26, 2000.
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