Agriculture Reference
In-Depth Information
better suppression of nightshade species than
S
-metolachlor (Hutchinson, 2012); however,
S
-metolachlor-based programs outperform di-
methenamid-P-based systems on other weedy
species, due to the much longer soil half-life of
S
-metolachlor (90 days) (Richardson
et al
., 2004).
Care must be taken when using
S
-metolachlor,
as crops grown in rotation with potato could be
injured, particularly wheat.
A single thiocarbamate herbicide con-
tinues to be used in potato. EPTC is used primar-
ily in mixture with other pre-plant-incorporated
herbicides. EPTC controls small-seeded broad-
leaf weeds and will suppress yellow nutsedge.
However, EPTC has to be incorporated within
24
h or efficacy will diminish due to volatiliza-
tion. EPTC can be applied pre-emergence or
after hilling, and incorporated with specific
incorporation equipment. In the Pacific North-
west potato producing region of the USA, EPTC
can be incorporated with irrigation. Incorpor-
ation may be accomplished either by chemi-
gation (herbicide injected into pressurized
overhead irrigation pipes) or via a ground
sprayer application that uses overhead irriga-
tion to incorporate the herbicide into the soil.
EPTC can also be applied in a hilling/layby ap-
plication by directing the herbicide spray be-
tween the rows and incorporating using the
aforementioned methods.
Flumioxazin, an
N-
phenylphthalimide herbi-
cide, is new by comparison with the previously
discussed chemistry. It controls weeds such as
nightshades, common lambsquarters, redroot
pigweed, and mustards (
Brassica
spp.). In contrast,
fomesafen is the most recent pre-emergence
herbicide to be labeled in potato. Fomesafen
controls nightshade species,
Amaranthus
species,
and suppresses yellow nutsedge. These herbi-
cides are protoporphyrinogen oxidase inhibitors
(or PPO, WSSA Group
14/
HRAC Group E) (Wilson
et al
., 2002). Fomesafen is most effective when
applied pre-emergence to potatoes to avoid in-
jury. Flumioxazin can only be applied between
hilling and potato emergence, or injury will
result. Unlike the previously discussed herbi-
cides that require considerable incorporation,
flumioxazin is most effective when concen-
trated in a narrower band of treated soil at the
soil surface.
Metribuzin, an s-triazine or triazinone, is
one of the oldest potato herbicides and is still
widely used in potato production. Metribuzin
controls a range of broadleaf weed species
and suppresses many additional grasses. Re-
sistance to triazine herbicides has developed
in
69
species worldwide, limiting activity and
requiring a combination of control practices
for these weeds. Additionally, metribuzin has
never provided effective control of weedy
Sola-
num
species.
Post-emergence herbicides are used primar-
ily to maintain a weed-free crop after the crop has
canopied. In extremely weedy fields, multiple
post-emergence applications may be needed;
however, most producers use secondary tillage
and hilling to minimize post-emergence herbi-
cide use. Herbicide options to control emerged
weeds in potato are limited to metribuzin, rimsul-
furon, sethoxydim, and clethodim, comprising
three herbicide families. Several herbicides with
residual activity only may be applied to newly
emerged potatoes; however, emerged weeds will
not be controlled.
Considerable research has been conducted
to understand how best to incorporate and
steward the post-emergence applied herbicide,
rimsulfuron, which controls broadleaf weeds in
potato (Eberlein
et al
., 1994; Blackshaw
et al
.,
1995). As a sulfonylurea herbicide, rimsulfuron
inhibits acetolactate synthase (ALS, WSSA
Group
2/
HRAC Group B). A considerable num-
ber of weeds have developed resistance to ALS
inhibitors. As a consequence, rimsulfuron is
usually applied in mixture with metribuzin.
Rimsulfuron can be applied pre-emergence by
chemigation, ground application, or by aircraft
after hilling, drag-off, or Dammer Diker
®
oper-
ations. Post-emergence applications are often
less successful, due to variation in the size of the
weeds. Ideally, post-emergence applications should
be timed when weeds are small, such as a cotyle-
don or in the two-leaf stage.
Metribuzin, as previously mentioned, is
widely used in potato production for weed man-
agement. It is often applied pre-emergence; however,
it controls many weed species post-emergence.
Metribuzin should be applied when weeds are
small and before potato row closure. Varietal
sensitivity to metribuzin has been documented
with visible symptoms consisting of chlorosis.
The only other post-emergence herbicides
available for use in potato are clethodim and
sethoxydim. Clethodim and sethoxydim are
