Be On Alert: Problematic Weed Moving Into Illinois
URBANA, ILL.
Amaranthus species are among the most
troublesome weed species in agronomic
production systems. Maintenance questions
regarding this species have tended to
focus on waterhemp. But Aaron Hager, U of I
Extension weed specialist, said move over, waterhemp,
Palmer amaranth is in town.
A panel of U of I specialists, including Hager,
provided the most
current information
about crop
production, pest
management, and
economics at the
2012 U of I Corn
and Soybean Classic
held on Jan. 12
at the I Hotel and
Conference Center
in Champaign, Ill.
Hager said Illinois
farmers have
become well acquainted
with waterhemp
and the
challenges caused
by this species,
but Palmer amaranth
is perhaps
the most aggressive
Amaranthus
species with respect
to growth
rate and competitive
ability.
“Palmer amaranth
is most
common in the southern third of Illinois but, it
may be expanding its range northward,” Hager
said. “The growth rate and competitive ability of
this species exceed that of other Amaranthus
species. Waterhemp can add close to one inch of
new growth per day under good growing conditions,
whereas Palmer amaranth can add multiple
inches.”
Recently, at least one population of
glyphosate-resistant Palmer amaranth was confirmed
in southern Illinois. Hager said this
should raise awareness among Illinois farmers
because this example provides additional evidence
that resistance to glyphosate can occur
in a summer annual weed species that is very
competitive with corn and soybeans. It occurs
in many fields in the southern portions of Illinois,
and Palmer amaranth has dramatically
changed farming practices in areas of the midsouth
and southeastern United States.
“This species has managed to drastically
spread across the southern United States, so
what says it won’t spread to northern Illinois as
temperatures increase,” Hager said. “This could
become a huge problem, especially as
glyphosate resistance is identified.”
He said the first step to managing this weed
is being able to recognize it. Accurate identification
of weedy Amaranthus species can be
difficult because many exhibit similar morphological
characteristics.
The cotyledon leaves of Palmer amaranth are
relatively long compared with other Amaranthus
species. As the plant matures, petioles
are two to three times longer than the leaf
blade and a watermark may appear. It also has
little to no pubescence. It is important to note
that Palmer amaranth is a dioecious species,
meaning plants are either male or female,
which introduces a great deal of genetic diversity,
Hager said.
Emergence, stress tolerance, and growth rate
are factors that make Palmer amaranth particularly
noxious. Hager said it tends to display
a prolonged emergence pattern with
higher germination rates at shallow seed
depths. Palmer amaranth can tolerate high
temperatures and dry soil conditions better
than any other Amaranthus species. It has an
excessive growth rate, which is evident in
southern regions.
These factors, coupled with resistance and
competitive ability, create a need for management
practices.
“The most consistent management for corn
and soybean involve an integrated approach
that utilizes soil-applied herbicides, postemergence
herbicides and mechanical,” Hager said.
“The management practices for Palmer amaranth
are very similar to those recommended
for waterhemp.”
Hager said time application can have a significant
impact on weed control. It is critically
important to remember that Palmer amaranth
can grow over one inch per day. Delaying applications
of postemergence herbicides is not
recommended. Δ