Notes on Stone Fruit Diseases
Brown rot control

 

Development of resistance

Repeated use of fungicides from the same chemical "family" or group can lead to the development of fungicide resistant strains.

For example, Benlate and Senator are in the same chemical group. These products gave superior control of brown rot in the past. Frequent and sometimes almost exclusive use of Benlate in the 1970s created Benlate-resistant forms of the brown rot fungus in many stone fruit orchards in the Niagara region. This type of resistance has remained stable in the field for 20 years. Although Benlate is no longer registered for use in Canada, benomyl-resistant fungi are resistant to Senator.

Other products in different families of fungicides are available for brown rot control. The key to limiting the development of resistance is to know the chemical families of the products you use and, where possible, rotate use between different families and between different modes of action.

Rovral gives good control of brown rot on cherries and peaches. Rovral has some post-infection activity and also reduces new spore production on established lesions. Rovral-resistant forms of the brown rot fungus can be easily selected in the laboratory and resistance could occur also in the orchard. Rovral-resistant forms of brown rot have been found in the field in New Zealand but not in surveys conducted during 1995-96 in Ontario.

Vangard was registered in North America in 1998. It had been registered previously in Europe for use on grapes for botrytis bunch rot and brown rot on stone fruit. Botrytis has already developed resistance to this product in Europe and the risk of resistance development is expected to be similarly high for brown rot.

Indar is a relatively new brown rot fungicide in Ontario. This fungicide has consistently performed extremely well against brown rot in the United States for some time. Rotate Indar with fungicides from different chemical families. Follow best management practices for the reduction of potential resistance and to help sustain the efficacy of this fungicide for future use.

For more information on resistance management see the Pest Management chapter of Publication 360.

Management of resistance

In order to manage resistance it is important to:

  • minimize the number of applications per season per chemical group
  • rotate between different chemical families
  • apply fungicides before an infection period, not as a clean-up spray after an infection has occurred
  • Alternate fungicides to minimize resistance development.
  • Limit Rovral to only one to two applications per season to delay possible development of brown rot resistance to Rovral and cross-resistance to Botany and Ronilan.
  • Apply Rovral when the crop is most susceptible to infection, such as at bloom and immediately preharvest, and use less effective materials when there is reduced risk of infection.
  • Take extreme caution not to overuse Vangard. Limit its use to two applications per season and never use in consecutive or back-to-back sprays.
  • Use Indar in a managed fungicide rotation to lessen the chance of resistance development to this product.

Bravo, Sulfur, Captan and Maestro are protective fungicides. Brown rot has not developed resistance to these products. Captan and Maestro are particularly valuable in programs designed to delay the development of fungicide resistance. Alternate these products with other fungicides to limit the use of resistance-prone fungicides such as Rovral and Vangard.

Caution: Captan and Maestro can cause phytotoxicity to a few sweet cherry cultivars, Schmidt, Stanley prune and Japanese plums, for example. Captan and Maestro should not be used on any stone fruit within 10 - 14 days of an oil spray or phyotoxicity can occur. Sulfur is not widely recommended because it is a skin irritant and it kills beneficial mites.

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For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 17 May 2006
Last Reviewed: 17 May 2006