Midwest Vegetable Guide - 2022 - 48

Disease Management Strategies
Chemical Control and Resistance
Management with Fungicides and
Bactericides
Disease control products (fungicides, bactericides, and
nematicides) are pesticides. Be sure to read the general
section about Pesticide Application and Safety for
information about safety, equipment, calibration,
formulations, storage, and other important topics. For a
disease control material to be effective, you must apply an
appropriate material at the right time, in the right
concentration, and in the right way.
Fungicides can be broadly classified as either contact or
systemic. Within those groups, however, there are many
active ingredients and multiple modes-of-action. Some
fungicides are allowed in organic vegetable production, and
some are designated as reduced-risk, including certain
biopesticides/biological controls.
Both contact and systemic fungicides are most effective if
you apply them before disease develops. Many diseases are
very difficult or impossible to control with chemicals once a
severe epidemic is underway. Throughout the crop-specific
sections of this book there are details about when you should
apply fungicides in order to be most effective (and
information about when they may be ineffective).
Pathogens usually require a specific temperature and
moisture range in order to cause diseases. For some diseases,
knowing those specific requirements can help you time
fungicide applications to coincide with disease risk. In some
cases, the guidelines are informal, and may simply make you
more aware that a wet season may require more applications
and a dry season may require fewer. In other cases, the
pathogen life cycle is understood well enough that you can
use a formal disease forecasting system. Here are some
resources for tracking weather and predictive models related
to diseases:
* enviroweather.msu.edu
* newa.cornell.edu
* melcast.ceris.purdue.edu
Contact fungicides (also called protectant fungicides) provide
a " coat " of protection on the plant's surface when applied
properly. These fungicides are designed to kill fungi oncontact
on the surface of plants - hence, the name. Most
contact fungicides have multiple modes of action, so fungal
pathogens are unlikely to develop resistance to all of these
different modes of action at the same time. For this reason,
alternating contact fungicides is unnecessary. Good coverage
is essential to maximize the efficacy of contact materials.
Systemic fungicides (sometimes called eradicant or curative
fungicides) don't merely coat the surface, they also enter the
plant. Once inside the plant, some stay relatively localized.
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Midwest Veg Guide 2022
Others move across to the opposite leaf surface, some move
upward in the plants, and a few move downward into the
roots. They can sometimes eradicate or cure a portion of
existing infections. Most systemic fungicides have a single
mode of action, so the risk of pathogens developing
resistance to these products is greater. The fungicides target a
very specific function of the pathogen's cells, and sometimes
the fungus develops methods to evade the activity.
Powdery mildews and downy mildews are particularly prone
to resistance development, but there are other examples. The
crop sections in this guide flag diseases that are at risk for
fungicide resistance. Always read and follow label directions
that list how to alternate systemic fungicides and/or combine
and rotate with contact fungicides to minimize the resistance
development.
The Fungicide Table lists several fungicides and their modes
of action to help in resistance management. Each fungicide
label is marked with a FRAC (Fungicide Resistance Action
Committee) code to designate a mode of action group and
help growers design a rotation plan. Try to avoid using
products with the same FRAC codes repeatedly to conserve
their efficacy against diseases.
Bactericides (copper and antibiotic compounds) can help
reduce the risk of early-season bacterial disease epidemics,
but are most effective when used with other control methods.
Copper compounds also are mediocre fungicides and are
handled similar to protectant fungicides. Antibiotics serve a
similar purpose in certain crops.
http://enviroweather.msu.edu http://newa.cornell.edu http://melcast.ceris.purdue.edu
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