Managing pathogen resistance to fungicides in berry crops

Resistance management is an important component in an integrated pest management program. As researchers are finding resistance to an increasing number of fungicides used in fruit production, it is becoming even more important to understand and follow sound resistance management practices. Researchers around North America are studying the development of resistance to several of the newer fungicide families, such as the sterol inhibiter fungicides (group 3) and the strobilurins (group 11). Researchers in Florida and South Carolina have reported Botrytis grey mould resistance to several fungicides used in strawberries. Researchers in Quebec, Michigan and Ontario are also studying this issue in berry crops.

Several factors affect the risk of a pathogen developing resistance to a fungicide. These include the biology of the pathogen, the properties of the fungicide, and the use pattern of the fungicide.

  • Biology of the pathogen: Fungi which produce lots and lots of spores, and/or fungi which are very host-specific, tend to develop resistance more rapidly than fungi which are slower growing or fungi which attack a wide range hosts. For example, resistance to fungicides can develop quickly in Botrytis, and in powdery mildew, partly because the fungi which cause these diseases produce so many spores.
  • Properties of the fungicide: Fungicides with a specific site of action are more susceptible to resistance than fungicides which attack a pathogen at multiple sites of action. For example, fungicides in group 3, loosely known as sterol inhibitors, target one specific process in cell metabolism, and are more susceptible to resistance compared to fungicides in group M which are toxic to fungi through several pathways.
  • Use pattern of the fungicide: Fungicides that are used repeatedly, have long persistent residues, or are used at excessive or deficient rates, can expose multiple generations of spores to the same fungicide and rapidly select for strains of the pathogen that are resistant to that fungicide.

Resistance management strategies include rotating products from different families and limiting the total number of applications per season of products from a single fungicide group. Specific knowledge is required for growers to manage resistance effectively.

  • Know the fungicide family groups. Choose fungicides from different family groups whenever possible. (Table 1. Fungicides, fungicide groups and resistance management strategies for botrytis grey mould in strawberries)
  • Limit the total number of applications of a particular fungicide family per season. Newer fungicides will have specific resistance management strategies on the product label. Remember that some labels, especially older labels, may provide a maximum number of applications per season based on factors other than resistance management.
  • Know which disease is targeted by which fungicide family.
  • For combination products, know which fungicide component is controlling which disease. For example, Pristine WDG is a combination of boscalid (group 7) and pyraclostrobin (group 11). Boscalid is strong against botrytis grey mould, and pyraclostrobin is effective against powdery mildew and anthracnose. Alternating Pristine with other fungicides in group 7 is not effective for resistance management of botrytis.

Other resistance management practices:

  • Make each fungicide application count. Be sure the sprayer is calibrated, the correct rate is applied and spray coverage is complete.
  • Apply fungicides before disease symptoms occur. Application after disease symptoms appear increases the risk of resistance.

Frequently asked questions:

What is the Fungicide Resistance Action Committee?

This committee, known also as FRAC, is: a technical working group of Crop Life International. Part of their mandate is to provide guidelines and advice on the use of fungicides to reduce the risk of resistance developing, and to manage it should it occur. FRAC guidelines were used for developing the information in Table 1.

How do I know which fungicides belong in which fungicide group:

Fungicide groups are provided on the upper right corner of each product label. Information on fungicide families is also provided in OMAF and MRA publication #360, Guide to Fruit Production.

How do biopesticides affect resistance management?

It depends on the mode of action and how effective they are. Biopesticides typically have modes of action that are unique from synthetic pesticides and do not rely on a single target site for efficacy. Properly used, biopesticides have the potential to extend the effective field life of all products by curtailing the development of resistant pest populations. However, if biopesticides provide suppression rather than control of plant diseases, they should not be relied upon for control in high pressure situations.

Table 1: Fungicides, fungicide groups and resistance management strategies for botrytis grey mould in strawberries.
Fungicide group Products included in this group Suggested use pattern for this group
Group M Captan, Folpan, Maestro, Bravo, Echo Products in this group can be applied repeatedly, according to the label. Fungicides in this group have multi-site contact activity against fungi and are generally considered low risk for resistance development. Use them in rotation with fungicides in other groups, to prevent resistance in these other groups.
Group 2 Rovral No more than one consecutive application, then rotate to a different fungicide group. Botrytis resistance to Rovral has been documented in several crops.
Group 7 Cantus, Lance, Pristine, Fontelis No more than 2 consecutive applications of Lance, Cantus or Fontelis, then rotate to a different fungicide group. No more than 1 consecutive application of Pristine. No more than 30% of fungicides applied per season should include a fungicide from this group.
Group 9 Scala, Switch Use once then rotate to a different group. No more than 30% of total fungicide applications from this group.
Group 11 Pristine, Flint No more than 1 consecutive application then rotate to a different fungicide group. No more than 30% of fungicides applied per season should include a fungicide from this group. (based on North American FRAC guidelines for strawberries)
Group 17 Elevate Use no more than 2 consecutive applications then rotate with other fungicide groups. Maximum 4 applications per season.
Biopesticides Actinovate, Serenade These products provide suppression rather than control of Botrytis. They may contribute to resistance management.

For more information on resistance management in fruit crops, see publication #360, Guide to Fruit Production, Chapter 12, resistance management.

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