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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