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In​effective Fungicides: A case study on problems in selection and use of fungicides for disease management

​Melissa B. Riley

Department of Entomology, Soils, and Plant Sciences
Clemson University

Riley, M.B. 2006. Ineffective Fungicides: A case study on problems in selection and use of fungicides for disease management. The Plant Health Instructor. DOI: 10.1094/PHI-T-2006-0522-01


In this case study a student and an extension agent analyze a disease management problem that has developed at a local peach orchard, and must recommend appropriate actions to help alleviate the problem. The student is working as a summer intern in a county extension office. A peach grower calls complaining about fruit losses that he has observed due to peach brown rot (Figure 1) and requests assistance in determining why the problem occurred. The extension agent asks the summer intern to obtain information on fungicides recommended for management of peach brown rot. The extension agent and intern then discuss what further information they need to determine what has caused the control failure. The intern and extension agent proceed to the grower’s facilities where they obtain information about the grower’s disease management strategy. They must determine why the disease management has failed and how the grower can alter his practices to prevent the problem from continuing or reoccurring in subsequent seasons.

Figure 1. Brown rot of peaches

Several aspects are integrated into this case on the selection of effective disease management strategies and the prevention of fungicide resistance. Students identify chemicals registered for management of brown rot disease of peaches and how these chemicals are used. The active ingredients of recommended products and their chemical class are determined from readily available resources. Once chemical classes of materials are identified, students must consider which and how often chemicals should be used to prevent resistance development.

The case can be used in different educational settings. The current form was designed for an undergraduate class of introductory plant pathology undergraduates majoring in horticulture, microbiology, crop sciences, biological sciences, agricultural education, and turf grass management. When used in a graduate class, the case can be written with additional questions and factors for consideration. For example, graduate students could be expected to study known resistance mechanisms of fungi and to obtain and screen isolates from the field for resistance to specific fungicides. Graduate students also could investigate previous fungicide resistance problems that have developed in management of peach brown rot, such as those that occurred in response to the use of benzimidazole fungicides (Zehr, et al. 1991). 



The overall goal is for students to learn important factors to consider when deciding which chemicals should be used and how they should be used to prevent the development of fungicide resistance. Specific objectives are:

  • Investigate factors important in deciding on appropriate fungicides to manage a specific disease
  • Understand the importance of knowing the chemical families of fungicides chosen to manage a specific disease
  • Identify important and useful information on pesticide labels
  • Illustrate how resistance of a pathogen population to a particular fungicide can be identified
  • Investigate disease management measures used in addition to fungicides possibly reducing the number of fungicide sprays needed as well as reducing the possibility of fungicide resistance development

All objectives may not be accomplished in every class. The depth of coverage is dependent on student level and the emphasis and direction of the specific class. 


Angie Hill: Undergraduate student majoring in Agricultural Education who is a summer intern in the Chesterfield County extension office. She is gaining experience by observing and by working on problems that come into the extension office.
Tony Watson: Extension agent for Chesterfield County who is Angie’s supervisor for the summer.
Harold Jones: Peach grower in Chesterfield County who has observed a significant increase in losses due to brown rot disease in his peach orchards this summer.
Jon: Farm manager for Harold Jones who is responsible for overseeing all pesticide applications in Mr. Jones’s peach orchards.
Mrs. Hobbs: Peach grower in Chesterfield County with farm adjacent to Harold Jones. She has observed no increases in her losses due to brown rot disease this summer.  

THE CASE (consists of two parts given to students in successive class periods)

Click here for part A

Click here for part B 


Case Summary

The focus of this case is the loss of disease management when inappropriate chemicals are combined into a disease management program. Since brown rot disease is a significant problem in peach orchards in the southeastern United States, the proper choice of fungicides to be used for disease management is vital (2006 Southeastern peach, nectarine, and plum management and culture guide). One must know the type of fungicide and whether it carries a risk that fungal populations develop resistance to that particular group, before deciding which chemicals to use in disease management. Currently there are few fungicides available for brown rot disease management because of the loss of the benzimidazole fungicides due to resistance development (Zehr et al. 1991). The loss of an additional family of fungicides could result in significant losses for all peach growers.

Suggestions on how to use this case

The two parts of this case should be given in consecutive class sessions or a combination of a class and a laboratory following the initial introduction of the case. Part A can be distributed to class members prior to a laboratory. Students must consider answers to questions at the end of the first portion of the case before coming to the second class session or laboratory. This allows students to obtain background information related to the disease, and information about fungicides that are available for brown rot disease management. The ability to obtain this type of information can be extremely important to the student’s future careers where they may have to select chemicals for disease management.

At the beginning of the second class or laboratory, the initial components of the case should be discussed along with possible answers to the questions associated with Part A. The last question is especially important since students should determine what questions to ask and what information they need to obtain in order to determine what caused Mr. Jones’s increased losses due to brown rot this season. Students must remember that when they are investigating a problem they need to ask many questions. The information they obtain can help determine what has caused increased disease losses. Although some of the answers that growers supply may not help to identify the cause, they may assist in eliminating some possibilities. Obtaining accurate information is necessary to determine the exact cause(s) of problems in disease management.

Once the initial portion of the case has been discussed completely, students can be given the second portion of the case (Part B) with data on the specific fungicides used by Mr. Jones and Mrs. Hobbs. The class should then be split into groups of approximately three to four students and allowed to work on the second part of the case. At the end of 30 minutes, groups discuss their ideas about what caused Mr. Jones’s problem. Once hypotheses have been suggested, the students should discuss how to prove or disprove their hypotheses.  

Possible adaptations

Laboratory exercises can be conducted year round with this case. Fungal isolations can be conducted from fruit in the summer and fall and from twigs and mummies during the winter and spring. Samples of peaches, plums, cherries or apricots with brown rot can be brought into the lab for observation and fungal isolation. After initial isolation on a medium containing no fungicides, isolates can be tested for growth on media amended with various fungicides at different levels. The sensitivity and occurrence of resistant isolates isolated by members of the class can be determined. If fresh Prunus samples are unavailable, canned peaches (or fresh apples) can be inoculated with Monilinia fructicola isolates and given to students. During the winter, mummies from the field (Figure 2) can be obtained and placed in warm, moist conditions to allow for isolation of the pathogen. Cankers on tree limbs (Figure 3) can also be brought into the laboratory for observation and possible isolation but it is more difficult for students in introductory plant pathology to isolate M. fructicola from cankers.

Figure 2. Mummies

Figure 3. Cankers

It is important for students to understand that only certain chemicals are labeled for the management of specific diseases. The use of chemicals that are not labeled for a specific disease and/or crop can result in fines, and confiscation and destruction of a crop. Much of the information on acceptable chemicals is available online from local land-grant institutions (2006 Southeastern peach, nectarine, and plum management and culture guide). It is important for students to realize that chemicals approved in one area may not be available for use in other areas, depending on the type of approval associated with the pesticide. Temporary exemptions may be available in areas with higher disease pressure. Chemical information also changes constantly and what is approved one year may be unacceptable the following year. It is extremely important for students to know where to obtain current information on approved chemicals. The recommended rates of application are another important factor to consider when deciding on chemical applications. The use of non-recommended rates of application, especially lower than recommended rates, can lead to development of resistance.

The case can be altered based on student interest. The plant and disease used should be one that is common in the area where the case is being used so that materials are available for use in the laboratory.

One alternative is gray leaf spot caused by Pyricularia grisea on perennial ryegrass where resistance to strobilurin (QoI) fungicides has been identified (Vincelli and Dizon 2001). Further studies of this pathogen have revealed two mutations in the cytochrome B gene (CYTB), which result in an increase in their resistance to QoI fungicides (Kim et al. 2003). This disease would be appropriate for students interested in diseases of turf grasses.

In areas where apples are a major crop, benzimidazole resistance of Venturia inaequalis, the cause of apple scab (Vaillancourt and Hartman 2000), could be used for this case. Benzimidazole resistance has been reported in many areas (e. g., Sholberg et al. 1989), and specific mutations resulting in resistance have been identified (Koenraadt et al. 1992). When the disease used in the case is changed, the fungicides associated with the case must be changed to chemicals approved for that specific disease and crop.


Background information on fungicide resistance and how it relates to the management of brown rot in Prunus species.

An important issue that should be discussed in plant pathology courses is the information that should be considered when deciding on appropriate chemicals for use in disease management. Development of fungicide resistance can be a huge problem when fungicides are used over a long period or when they are used inappropriately. Understanding the basis of how resistance develops and how the occurrence of resistance can be minimized are important lessons for students as well as other individuals choosing fungicides for disease management. People need to know about the resources that are available to assist them in making appropriate fungicide choices.

Many resources are available on fungicide use and resistance issues, including electronic resources, popular press, review articles, and scientific journal articles. Several general sources can serve as background information for students, For example, the fungicide resistance action committee (FRAC) has extensive publications available on its website (, including information on how to manage fungicide resistance in crop pathogens and the assessment of risk of fungicide resistance. The committee also provides a list of available fungicides sorted by their mode of action and their associated risk of resistance. Two articles located on the APSnet Education Center can be utilized as background information related to fungicide resistance (McGrath 2004, and Vincelli 2002). McGrath introduces fungicides in general – what they are, why they are needed, their role in disease management, application methods, fungicide resistance, and legislation and public health issues related to fungicides. This article can serve as a good starting point to introduce students to fungicides. Vincelli discusses a specific group of fungicides – the QoI fungicides (strobilurins) - and addresses the guidelines for reducing the risk of resistance development. Pesticide labels contain important information on these issues and students should obtain pesticide labels for all of the materials mentioned in the case during their investigations. Labels can usually be downloaded from manufacturer’s websites, from, or from the Crop Data Management System website at Labels may contain information on the procedures to be followed to reduce the possibility of resistance development. A good example of information on general resistance management strategies is from the website of Syngenta at Fungicide resistance can and has previously occurred with chemicals used for the management of brown rot of peaches caused by Monilinia fructicola, the disease utilized in this case. Weather conditions in the southeastern United States are extremely favorable for the rapid development and spread of the disease, including high humidity, frequent rainfall, and warm temperatures. Disease management with fungicides is critical, even when sanitation procedures are implemented (Ritchie 2000). Inappropriate fungicide use has been shown to alter the sensitivity within fungal populations and has resulted in resistance development to previously used fungicide groups such as the benzimidazoles (Zehr et al. 1991, Michailides et al. 1987), dicarboximides (Elmer and Gaunt 1993, Ritchie 1983) and demethylation inhibitor (DMI) fungicides (Zehr et al. 1999, Braithwaite et al. 1995, Schnabel et al. 2004). Current fungicide recommendations for the southeastern United States include the use of DMI fungicides such as Orbit and Indar, which are utilized in this case (2006 Southeastern peach, nectarine, and plum management and culture guide). The culture guide contains information on fungicide selection, approved fungicides, usage rates, and procedures to reduce the risk of fungicide resistance.

Making the appropriate choices of chemicals to use for disease management is a skill that should be discussed in plant pathology classes. This case can be utilized as an introduction to important sources of information that can be used to make decisions on chemical applications and guidelines and to prevent the development of resistant fungal populations (Damicone 1996). It can also serve to illustrate problems that can occur when decisions are not made appropriately.


Beresford, R. 1994. Understanding fungicide resistance.

Braithwaite, M., P. A. G. Elmer, D. J. Saville, K. S. H. Boyd-Wilson, and H. G. Whelan. 1995. Reduced sensitivity to DMI fungicides in Monilinia fructicola and the efficacy of DMI fungicides for blossom blight control. Proceedings of the 48th New Zealand Plant Protection Conference, pp 17-21.

Elmer, P. A. G., and R. E. Gaunt. 1993. Effect of frequency of dicarboximide application on resistant populations of Monilinia fructicola and brown rot in New Zealand orchards. Crop Protection 12:83-88.

Damicone, John. 1996. Fungicide Resistance Management. Oklahoma Cooperative Extension Service F-7663. (

Kim, Y.-S., E. W. Dixon, P. Vincelli, and M. L. Farman. 2003. Field resistance to strobilurin (QoI) fungicides in Pyricularia grisea caused by mutations in the mitochondrial cytochrome b gene. Phytopathology 92:891-900.

Koenraadt. H., S. C. Somerville, and A. L. Jones. 1992. Characterization of mutations in the beta-tubulin gene of benomyl-resistant field strains of Venturia inaequalis and other plant pathogenic fungi. Phytopathology 82:1348-1354.

McGrath, M. T. 2004. What are fungicides? The Plant Health Instructor. (

Michailides, T. J., J. M. Ogawa, and D. C. Opgenorth. 1987. Shift of Monilinia spp. and distribution of isolates sensitive and resistant to benomyl in California prune and apricot orchards. Plant Disease 71:893-896.

Ritchie, D. F. 1983. Mycelial growth, peach fruit-rotting capability, and sporulation of strains of Monilinia fructicola resistant to dichloran, iprodione, procymidone, and vinclozolin. Phytopathology 73:44-47.

Ritchie, D. F. 2000. Brown rot of stone fruits. The Plant Health Instructor. ( Updated 2005

Schnabel, G., P. K. Bryson, W. C. Bridges, and P. M. Brannen, 2004. Reduced sensitivity in Monilinia fructicola to propiconazole in Georgia and implications for disease management. Plant Disease 88:1000-1004.

Sholberg, P. L., J. M. Yorston, and D. Warnock. 1989. Resistance of Venturia inaequalis to benomyl and dodine in British Columbia, Canada. Plant Disease 73:667-669.

2007 Southeastern peach, nectarine, and plum pest management and culture guide. Bulletin 1171. D. Horton, P. Branner, B. Bellinger and D. Ritchie, eds. (Updated annually. Sections important to case include: Management Guide, Resistance Management, Pesticide Characteristics and Restrictions sections)

Vaillancourt, L.J. and J.R. Hartman. 2000. Apple scab. The Plant Health Instructor. (

Vincelli, P. 2002. QoI (Strobilurin) fungicides: benefits and risks. The Plant Health Instructor. (​

Vincelli, P., and E. Dixon. 2001. Resistance to QoI (strobilurin-like) fungicides in isolates of Pyricularia grisea from perennial ryegrass. Plant Disease 86:235-240.

Wong, Frank P. Fungicide resistance: past, present, and future outlook. Slide show on internet.​

Zehr, E. I., L. A. Luszcz, W. C. Olien, and W. C. Newall. 1999. Reduced sensitivity in Monilinia fructicola to propiconazole following prolonged exposure in peach orchards. Plant Disease 83:913-916.

Zehr, E. I., J. E.,Toler, and L. A. Luszcz. 1991. Spread and persistence of benomyl-resistant Monilinia fructicola in South Carolina peach orchards. Plant Disease 75:590-593.