July 2021 - 6

Auxin herbicide problems crop up for growers
By Dean Peterson
VGN Correspondent
The continued surge in the numbers
of herbicide-resistant weeds and the
availability of crops resistant to the
auxin herbicides dicamba and/or 2,4-D
have made spray drift a serious issue for
specialty crop farmers.
The symptoms and negative effects
of dicamba and 2,4-D injury vary with
the crop, the amount of herbicide that's
moved off-target, and when in the growing
season the injury occurs, said Stephen
Meyers, Purdue University assistant
professor of weed science. Meyers spoke
on auxin herbicides at the recent Indiana
Horticulture Conference and Expo, held
virtually in 2021, and covered some key
points about auxin herbicide injury on
processing tomatoes, cucurbits and grapes.
In processing tomatoes, epinasty - the
twisting of stems and petioles - and leaf
cupping and strapping are the classic
symptoms of dicamba or 2,4-D injury.
While they both cause the same type of
injury, dicamba causes more yield loss.
Early season injury is less harmful in
processing tomatoes than later season
injury. However, injury in early bloom
results in flower drop, fewer fruit and
less yield, and is more harmful than early
season injury to vegetative tissue. Higher
rates of auxin herbicide also cause more
injury.
Stephen Meyers stands next to a Palmer
amaranth plant in sweetpotato research plots
in North Carolina. Photo: Purdue University
" Depending on the rate, processing
tomatoes may look fine weeks after
exposure, but if there's injury at bloom,
plants are more susceptible to flower loss
or delayed maturity, " Meyers said.
In cucurbits, cantaloupe and pumpkin
have about the same sensitivity to both
dicamba and 2,4-D, while watermelon is
more sensitive to dicamba. Epinasty, stem
callus and cracking, and adventitious
roots are classic injury symptoms.
As with processing tomatoes, the
time of injury and the amount of auxin
herbicide determine the amount of vine
length reduction and yield loss.
Grapes injured by dicamba show
different symptoms than grapes injured
by 2,4-D. Dicamba injury shows up as
elongated growth, upward leaf cupping
and fewer berries per cluster. Injury from
Herbicide drift injury on tomato.
Photo: Jenny Pollard/Mississippi
State University
2,4-D appears as shoot tip death, lateral
shoot growth, leaf strapping and fewer
berries per cluster. Vitis vinifera grapes
show more injury than the French
hybrids, which show more injury than the
American grapes.
Then there's the issue of pesticide residue
limits in the fruit.
Tomato, cucurbits, pome and stone
fruit, and grapes have a residue limit for
2,4-D of 0.05 parts per million (ppm) and
berries 0.20 ppm.
Asparagus has a dicamba residue
tolerance of 4.0 ppm and sweet corn 0.04
ppm. There is no allowable pesticide
residue limit for dicamba in tomato,
cucurbits, pome and stone fruit, grapes
and berries.
No allowable pesticide residue limit
essentially creates a zero tolerance.
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" What do you consider zero? " Meyers
said. " Labs can detect herbicide and
herbicide metabolite residue in parts per
billion. As the science advances and tests
increase in sensitivity, the threshold for what
is 'detectable' continues to be lowered. "
Pesticide residues, in part, are a function
of time and rate. Auxin herbicide exposure
earlier in the season is less likely to result
in herbicide residue than exposure later in
the season. Exposure from a low rate is less
likely to be a problem than exposure from
a higher rate.
The best starting point for growers
who suspect they have a problem is their
state's regulatory agency for pesticides. The
agency may be able to set up an analysis.
Resources are available from the North
Central IPM Center at www.ncipmc.
org. Dicamba and 2,4-D fact sheets for
specialty crop growers are available from
The Ohio State University at https://ipmdrift.cfaes.ohio-state.edu/dicamba-and-24d-fact-sheet-series.
The
North Central IPM Center is also
surveying specialty crop growers in the
North Central U.S. about dicamba and
2,4-D injury. The survey will collect
data, assess the frequency and severity
of herbicide drift damage and measure
grower concerns. This will identify
research needs to help address the issue.
This survey is administered by Ohio State
University and is available at
www.go.osu.edu\drift10. VGN
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http://www.ncipmc https://ipm http://drift.cfaes.ohio-state.edu/dicamba-and-24 http://www.go.osu.edu\drift10 http://www.philbrownwelding.com http://www.VegetableGrowersNews.com

July 2021

Table of Contents for the Digital Edition of July 2021