Page 7 - 16 Cotton SA December 2018

Superweed discovered in SA in cotton

, described as the USA’s number one weed

because of its strong competitive ability with crops and its ability to

thwart the best weed control measures and to establish rapidly
across diverse environments, was confirmed in April 2018 for the

first time in South Africa in a cotton field in the Douglas district.

(vernacular An exacerbating characteristic of
names: Palmer amaranth; is its ability to evolve
careless weed) is currently rated resistance in a relatively short time to
America’s top ‘wicked’ weed in a range of herbicides. At this stage in
terms of its detrimental impact on the US, the weed has developed
maize, cotton and soya bean resistance to five groupings of
production. Investigation on its herbicides, classified according to
current distribution in South Africa, their mechanism of action, namely:
its tolerance to various herbicides, protoporphyrinogen oxidase (PPO)
and management options is inhibitors, photo-synthesis (PSII)
underway in the South African inhibitors, acetolactate synthase
Herbicide Resistance Initiative (ALS) inhibitors, micro-tubule disrup-
(SAHRI) research programme at tors, and to glyphosate herbicide, an
the University of Pretoria. EPSPS enzyme inhibitor.

Because it origi- In the US,
nates from semi- different Palmer
arid regions in the amaranth popu-
USA, this weed lations show
has the ability to variable resis-
wreak havoc in tance to the
most parts of aforementioned
South Africa herbicide mecha-
where crops are nisms of action,
produced, per- with the most
haps with the common type of
exception of the resistance to ALS
winter rainfall inhibitors and
region. But then glyphosate. The
again, it might rendering inef-
happen that no fective of such
region in the major herbicide
country will be groups has had
spared this wicked weed. nearly incalculable crippling conse-
quences for especially maize, cotton
This weed reportedly produces as and soya bean production in the US.
many as 500 000 to 1 million seeds
per plant. Seeds can apparently
survive for at least three years in
soil and remain viable. The
photosynthetic efficacy of
is reportedly three to four
times greater than that of cotton,
maize and soya bean, and explains
its successful competition with A. palmeri female flower in Douglas cotton field.
these crops.
Photos - Charlie Reinhardt

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