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
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
- 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
- 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
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