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Ineffective 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



Part A

  1. What is the causal agent of brown rot? How and where does the organism that causes brown rot survive from one season to the next season (overwinter)? What types of spores cause initial brown rot infections during a season? When and where (in what plant tissue) do these infections occur? The causal agents of brown rot are three species of Monilinia: M. fructicola, M. laxa and M. fructigena. M. fructicola is the most common species found in North America’s peach growing regions. Monilinia species overwinter in several places including in cankers resulting from infections during previous seasons, as mummies (desiccated rotten fruit, Figure 2) on the tree or on the orchard floor. Two different types of spores can be produced during the flowering stage. Conidia (asexual spores, Figure 6) are produced in cankers and on mummified fruit hanging in the trees, while ascospores (sexual spores, Figure 7) can be produced in apothecia found on mummified fruit (Figure 8) lying on or partly buried in the ground even though their production is rare. The initial inoculum of conidia and ascospores are spread to opening peach blossoms where they cause the initial infections that result in blighted blossoms that may become covered with secondary conidia (Figure 9). The conidia and ascospores are primarily spread by splashing water, wind blown rain, and the movement of bees during pollination of blossoms. Blossom infections can progress down into the twig and cause blighting of twigs (Figure 10). This infection can result in the production of a canker.

    Figure 2. Mummies

    Figure 6. Conidia

    Figure 7. Ascospores

    Figure 8. Apothecia

    Figure 9. Blighted blossoms

    Figure 10. Blighted twigs

    Figure 11. Blighted twigs

  2. What are the major disease management measures for brown rot of peaches based on reducing overwintering sources of inoculum?
    The major disease management measures for brown rot based on reducing overwintering sources of inoculum are pruning out cankers and blighted twigs, and removal of mummies from the tree and the orchard floor. Material removed from the trees should be removed from the orchard and burned. Orchard floors can be lightly cultivated to reduce development of apothecia but care should be taken to not damage tree roots during this cultivation.

  3. Can brown rot of peaches be managed strictly through the use of non-chemical measures? Why or why not? How do environmental conditions impact disease management?
    Blossom infection requires temperatures above 10°C (50°F) with at least 18 hours of wetness, but with increasing temperatures this wetting interval is decreased (24°C/77°F requires only 5 hours). Fruit infection increases with warm wet weather. As a result, the high humidity and frequent rainfall associated with the southeastern United States make brown rot management extremely difficult without the use of some type of chemical control. Organic producers commonly use frequent applications of sulfur. In areas with low humidity and rainfall, sanitation along with increasing air movement in the orchard to increase drying conditions (proper pruning) can be successful in managing brown rot.

  4. What chemicals are currently approved to manage brown rot of peaches? Are there important concerns with the use of any of these chemicals? What are these concerns?

    PesticideChemical ClassEffectiveness*Threat of Resistance
    AboundStrobilurin (QoI)++++ yes
    CaptanMulti-site toxin+++no
    SulfurMulti-site toxin+no
    EliteDMI (demethylation inhibitor)+++++yes
    Orbit or PropiMaxDMI+++++yes
    Topsin-M or Thiophanate- methylBenzimidazole+++yes
    PristineStrobilurin and anilide+++++low

    * The greater the number of plus signs the more effective the material for brown rot management.
    The chemicals listed in the above table are fungicides approved for use in South Carolina during 2006. The table contains the chemical class that the materials belong to as well as the threat of resistance development. Resistance development is one of the most important concerns.

  5. If you are going to use pesticides to manage brown rot of peaches, what are the important factors that you need to consider when deciding which chemicals to use? Why are these factors important?
    If you are going to use pesticides to manage brown rot of peaches you need to determine if the chemicals that you are using are at risk for resistance development. If they are at risk of resistance development they need to be rotated with a different class of pesticide – not just with a different material in situations where there are several pesticides that belong to the same class and have the same site of action. If materials have the same site of action the pathogen may develop cross resistance to several or all members of the class due to the same mutation(s). Determining the class of material and the site of action for materials are important when determining which materials should be used in an alternating spray schedule to prevent resistance development and to maintain good disease management which is the overall goal.

  6. Outline questions Mr. Watson and Angie should ask Mr. Jones when they go to the orchard and packing house. Why do they need to ask each question? Based on the information you currently have, can you determine what is wrong with Mr. Jones’s disease management program?
    The following are questions that should be asked of Mr. Jones in an attempt to determine the reason for the development of fungicide resistance: What fungicide materials are being used for management of brown rot? (which of the chemicals being used have known resistance problems?)
    What rates of the fungicide materials are being used? (are materials applied at the correct rate?)
    How frequently have fungicide been applied? (is the interval between fungicide applications correct?)
    Are you using any tank mixes? (incompatible materials may be mixed together which results in inadequate management due to deactivation of materials) At this point, students do not have enough information to determine the reason for the disease management failure.

Questions Related to Case – Part B

  1. What types of chemicals did Mr. Jones and Mrs. Hobbs use? What are the differences between the chemicals that the two peach growers used?
    Mr Jones was using Indar and Orbit which are both demethylation inhibitor fungicides which are susceptible to the development of resistance. Mrs. Hobbs is using Abound, a strobilurin, with Orbit, a demethylation inhibitor. Both of the materials are individually susceptible to the development of resistance but they inhibit different active sites so that when the two are used in an alternating fungicide spray program, they are much less susceptible to the development of fungicide resistance.

  2. What do you think Angie has identified as the reason why Mr. Jones has observed an increase in the incidence of brown rot in his peaches this year?
    Angie should have identified that Mr. Jones has been applying the same class of fungicides continuously for 3 years and as a result has developed a population of Monilinia that is resistant to the DMI fungicides.

  3. Do you think Mr. Jones has any basis for suing the chemical supplier? Why or why not?
    Mr. Jones has no basis for suing the chemical supplier. His problems are probably the result of improper usage of fungicide materials which has led to the development of resistance.

  4. What type of test could you do to determine if Angie’s hypothesis about the cause of the problem is correct? How could you set it up?
    Angie could possibly test various isolates of Monilinia isolated from Mr. Jones orchard to determine their sensitivity to DMI fungicides and to compare these with isolates obtained from an area where DMI fungicides have not been used. Sensitivity of isolates can be determined by plating isolates on culture plates containing various levels of DMI fungicides and observing the growth after a couple of days.

  5. Why did it take three years for the problem to develop at Mr. Jones’s orchard? What happened during that three-year period? Why did Mrs. Hobbs not have a similar increase in brown rot?
    The population of the Monilinia in the field has changed over the three-year period to having more resistance to the DMI fungicides. The presence of the fungicide caused a shift in the population’s sensitivity to this group of fungicides. Mrs. Hobbs did not see a similar shift in the population even though she did use a DMI fungicide because she was also using a strobilurin fungicide, which has a different active site. If you tested the sensitivity of pathogens isolated from Mrs. Hobbs’s orchard to the DMI fungicides there may be an increased resistance to this group but because a second type of material is being used, the pathogen is overall still being managed.

  6. Based on the information you have, what would you suggest as a disease management program for the next year?
    Mr. Jones needs to look at using fungicides other than those belonging to the DMI class of fungicides. Scholar, a phenypyrrole, which has a low risk of resistance development, might be a good material to incorporate into a program along with Abound, a strobilurin. The use of DMI fungicides could possibly be continued but the major change is that they need to be rotated with other fungicides that have different sites of activity against the pathogen. The starting material the next year should probably be something other than a DMI material. It would also be important to reduce the overwintering inoculum of the pathogen – reducing mummies on tree and ground which can be accomplished by a light cultivation of the orchard floor, and by pruning out blighted twigs/branches and cankers. All pruned material should be removed from the orchard and burned to eliminate the pathogen.