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
- 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
Other resistance management practices:
- Make each fungicide application count. Be sure the sprayer is
calibrated, the correct rate is applied and spray coverage is
- 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. http://www.frac.info/index.htm.
FRAC guidelines were used for developing the information in Table
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
||Products included in this group
||Suggested use pattern for this group
||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.
||No more than one consecutive application, then rotate to a
different fungicide group. Botrytis resistance to Rovral has
been documented in several crops.
||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.
||Use once then rotate to a different group. No more than 30%
of total fungicide applications from this group.
||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)
||Use no more than 2 consecutive applications then rotate with
other fungicide groups. Maximum 4 applications per season.
||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
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