December 2022 - 24

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Fighting herbicide resistance in vegetables
LYNN M. SOSNOSKIE &
THIERRY E. BESANÇON
Cornell AgriTech &
Rutgers University
Herbicide resistance is defined by
the Weed Science Society of America
(WSSA) as ''the inherited ability of
a plant to survive and reproduce
following exposure to a dose of
herbicide normally lethal to the
wild type. "
A less formal way of describing it
is " we used to be able to control this
plant with this rate of herbicide, and
now we can't. " Herbicide resistance in
weedy species is the result of adaptive
evolution in response to repeated
applications of an herbicide over space
and time (i.e. selection pressure). Plant
biotypes within a weed population that
possess naturally occurring mutations
that facilitate survival and seed set
following an herbicide treatment will
become more dominant over time as
susceptible individuals are controlled.
Resistance mechanisms can be
divided into two broad categories:
target-site and non-target-site.
Target-site resistance mechanisms
include genetic changes that alter
the structure of the protein that the
herbicide normally binds. Successfully
binding disrupts plant biochemical
processes and leads to plant death. If
the target protein is mutated, and the
herbicide can not attach as it normally
does, vital plant processes will not be
totally impeded. Another target-site
mechanism is the overexpression of the
target enzyme. Where this mechanism
is present, the target protein is
produced in such quantity that there
are not enough herbicide molecules
Lambsquarters in beans.
to bind to and inhibit them all. Nontarget-site
resistance is characterized
by mechanisms that prevent the
herbicide from reaching the target
protein in sufficient quantities. This
may include reduced herbicide uptake
and translocation, increased vacuolar
sequestration, rapid necrosis and
metabolic degradation.
Herbicide resistance is a global
phenomenon. As of October 2022,
the International Herbicide-Resistant
Weed Database (IHRWD) (www.
weedscience.org) indicates that there
are currently 514 unique cases of
herbicide-resistant weeds worldwide.
Resistance has been reported in 267
weed species (154 dicots and 113
monocots). Weeds have evolved 165
different herbicides across 21 sites of
action. While most resistance cases
have been identified in agronomic
systems, specialty crops are not
immune; the IHRWD documents 59
reports of resistance in vegetables
worldwide with 16 cases occurring in
the U.S. and 7 in Canada. Examples
of resistance in North American
vegetables include:
* Common groundsel (Senecio
vulgaris), Powell amaranth
(Amaranthus powellii), and
redroot pigweed (Amaranthus
retroflexus) with resistance
to photosystem II inhibiting
herbicides (WSSA 5) in asparagus.
Cultivators designed for
Blueberries, Strawberries, and
Vegetables in Plastic Culture
* Excellent weed removal provided by
rolling cultivators or disk gangs.
* Renovate Strawberries, cultivate
Blueberries
* Cultivate close to the edge of
plastic.
* Toolbars include Hydraulic Width
Adjustment
* Very adjustable for use on many
crops
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24 | VegetableGrowersNews.com
* Livid/purple amaranth
(Amaranthus blitum) with
resistance to imazethapyr and
possibly other acetolactate
synthesis-inhibiting herbicides
(WSSA 2) in cabbage, lettuce,
and onions.
* Powell amaranth, redroot
pigweed, common ragweed
(Ambrosia artemisiifolia), and
common purslane (Portulaca
oleracea) with resistance to
the photosystem II inhibiting
herbicides and giant foxtail
(Setaria faberi) and large
crabgrass (Digitaria sanguinalis)
with resistance to the acetyl
CoA carboxylase-inhibitors
(WSSA 1) in carrots.
* Fall panicum (Panicum
dichotomiflorum) with
resistance to nicosulfuron
(WSSA 2) in sweet corn.
* Goosegrass (Eleusine indica)
and American black nightshade
(Solanum americanum) with
resistance to paraquat, a
photosystem I electron diverter
(WSSA 22) in tomatoes.
Many vegetables are in rotation
with agronomic commodities and are
also threatened by the development
of resistance in those crops as well.
The buildup of weed seedbanks in
rotational crops could threaten the
sustainability of vegetable production.
The registration of new herbicides
or new herbicide modes of action
is relatively slow in specialty crops;
coupled with crop safety concerns
and rotational restrictions, this will
continue to limit the numbers of
chemical options available for weed
suppression.
Many best management practices
for herbicide resistance management
are already employed by vegetable
producers, such as weed scouting,
mulching, mechanical weed control
and hand weeding. Other strategies
include planting into weed-free fields,
managing manures and composts and
cleaning equipment to prevent the
movement of plant parts and weed
seed into fields, and controlling weeds
in fallow acreage and along field
margins. Novel technology, such as
vision-guided cultivators, precision
sprayers and laser weeders are being
adopted into vegetable production
systems and may be important tools
in a changing agricultural landscape
where the use of pesticides, plastic
and soil disturbance must be reduced,
and labor becomes more scarce and
costly. For a more extensive review
of the current state of herbicide
resistance in vegetable crops in the
U.S. and Canada, please see this
recently accepted article in the
journal Weed Science: Boyd et al.
(2022). Occurrence and management
of herbicide resistance in annual
vegetable production systems in North
America. Weed Science, 70:515-528.
doi:10.1017/wsc.2022.43. VGN
http://www.weedscience.org http://www.hillsidecultivator.com http://www.VegetableGrowersNews.com
December 2022

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