Manual for Identification and Reporting of Palmer amaranth (Amaranthus palmeri) in South Africa
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Manual for Identification and Reporting of Palmer
amaranth (Amaranthus palmeri) in South Africa
Compiler: Prof Dr Charlie Reinhardt
Project leader: SAHRI (South African Herbicide Resistance Initiative)
University of Pretoria
Website: http://www.up.ac.za/SAHRI
Cell: 083 442 3427
Email: dr.charlie.reinhardt@gmail.com
Photo Credits
All photos appearing in this document, except two obtained from Dr Hugo Bezuidenhout, and one from
Purdue University (USA), were contributed by Prof Charlie Reinhardt
Text Credit
Text contributed by Prof C F Reinhardt
Copyright© in photographs and text: C F Reinhardt and the SAHRI
Historical overview of how Palmer amaranth was identified for the first time in South Africa • December 2017 – Farm managers on a particular farm in Douglas district (Northern Cape province) become aware they might be dealing with a “new” Amaranthus species, because “it looks different and responds differently to control measures known to be effective for controlling Amaranthus species”. • February 2018 – Prof Reinhardt of the SAHRI at UP (South African Herbicide Resistance Initiative) is requested to investigate the particular population. His preliminary prognosis, which is based on morphological traits alone, is that the weed in question probably is Amaranthus palmeri. • March/April 2018 – Identity of the weed is confirmed as Amaranthus palmeri through herbarium analysis conducted by SANBI national herbaria in Pretoria and Kirstenbosch, and by means of DNA analysis conducted in the SAHRI at UP.
Fact 1: Strategies for containment/management of any weedy plant that is the target of such efforts must
commence with correct identification, in conjunction with a survey to determine its current distribution.
Fact 2: Mistaken identification is most likely to occur between Palmer amaranth (Amaranthus palmeri) and Cape
pigweed (Amaranthus hybridus) due to similarities in morphology.
Fact 3: Other Amaranthus species that have hitherto attained weed status in South Africa (source: Henderson &
Anderson, 1966, “Common weeds in South Africa”)
• Amaranthus caudatus
• A. cruentus*
• A. deflexus
• A. hybridus*
• A. muricatus
• A. paniculatis
• A. spinosus
• A. thunbergii
• A. viridus (=lividus)
* Originally from the USA
Cotton field in Douglas district (Northern Cape Province, South Africa)
infested with possibly Amaranthus palmeri -- Date: 9 Feb 2018
Photo: H Bezuidenhout
9 February 2018: Prognosis made of “probably” Palmer amaranth (site: cotton field, Douglas district)
26 April 2018: Palmer amaranth in cotton field, Douglas district
Photo: H Bezuidenhout
Amaranthus hybridus (Cape pigweed) Note: Palmer amaranth can most easily be mistaken for this Amaranthus species, which is a common weed in RSA
Amaranthus hybridus varieties?
Amaranthus caudatus
Amaranthus lividus (syn. viridus) Note indentation at leaf tip, which is typical for this “Watermark” on leaves not same shape species, as well as for Palmer amaranth, albeit not visible and colour as for Palmer amaranth on all leaves at all growth stages of Palmer.
Amaranthus hybridus in alfalfa
Morphological characteristics that can be used to identify Palmer amaranth
– refer photos and text in slides that follow
Note: Such preliminary identification should be followed by confirmation of identity
through DNA and herbarium analyses
Reporting of sightings can be made to the SAHRI at UP
http://www.up.ac.za/SAHRI
Project leader: Prof Reinhardt
Cell: 083 442 3427
Email: dr.Charlie.Reinhardt@gmail.com
Note: Additional avenues of reporting will be announced in due coursePalmer amaranthus seedling from cotton field,
Douglas district
*Petiole is longer
than leaf blade
*In the USA this is reported as one of
the distinguishing characteristics of
Palmer amaranthPlant characteristics that can be used to distinguish between Palmer amaranth and other Amaranthus species: • Presence of hairs (trichomes) on leaves and stems – Palmer is devoid of hairs on leaves and stems • Single hair in indentation at leaf tip – visible on some leaves of some Palmer seedlings • Leaf shape – “assegai blade” shape, but can be ovoid in seedlings • Leaf stalk – Palmer leaf stalk length exceeds length of leaf blade, at least in case of some leaves on virtually all plants (can be observed from seedling stage to maturity) • Flowering parts – Palmer plants are dioecious (sexes on separate plants) • “Watermark” –Palmer has silver coloured chevron (V-shaped) on upper surface of some leaves on some plants
*V-shaped
indentation at leaf
tip of seedling
*In the USA this is reported as one of
the distinguishing characteristics of
Palmer amaranth*Trichome (hair) in
indentation at leaf tip of
some seedlings
*Chevron
watermark on
leaves of some
seedlings and
mature plants
*Trichome
*In the USA this is reported as
distinguishing characteristics of
Palmer amaranthSeedlings showing variable morphology
Seedlings already have obvious long petioles — petioles not always longer than leaf blade, but generally of either equal length, or petioles exceed leaf blade length.
Seedling comparison: different Amaranthus species in USA (source: Nebraska Extension Service)
Mature plant (origin Douglas district) Particular long petioles can be more common on some plants than on others, but can always be found on Palmer amaranth. Note V-shaped silver coloured chevron (“watermark”) on upper leaf surface (NB: not present on all plants).
Leaf stalk (petiole) is typically longer than leaf blade in the case of at least some fully developed leaves on
a particular plant.
NB: This is arguably the simplest way of distinguishing between Palmer amaranth and other Amaranthus
species – this characteristic is visible throughout its life cycle, from seedling to flowering stage.
For rapid test of petiole:blade
length ratio, the petiole is
folded over the leaf blade.Without physical support from crop plants the Palmer main stem can topple over in mature plants and grow horizontally, with side shoots developing vertically. NB: Is a particularly problematic growth habit in a low-growing crop such as alfalfa.
Flowering parts:
A. palmeri is dioecious, with sexes on separate plants
– this forces hybridization and high genetic diversity.
Male plantSexes are on separate plants, i.e. the species is dioecious. Male flower parts (cylindrical spikes) are soft to the touch, but female flower parts (cylindrical spikes) are stiffer and prickly to the touch. Flowers of 2 to 3 mm in length are carried in clusters along spikes (inflorescences). Cylindrical spikes can reach 60 cm in length on the central (main) stem. Flower colour is white/beige or white-green. The fruit is about 1.5 mm long. At maturity the top half of fruit fall off to reveal a single dark-purple seed of about 1 mm or less in diameter. Seeds germinate rapidly and plants have high growth rate when water is available. Because the species originates in the arid south-western parts of the USA, it is well-adapted to low rainfall (arid and semi-arid) environments.
Inflorescence of Palmer amaranth (Douglas district)
Female plant – anthers and
pollen absent; flower parts
are stiff and prickly to the
touch.
Male plant – anthers and
pollen abound on the spike
carrying small flowers in
clusters; inflorescence is soft
to the touch.
Note: The “touch test” also serves for id purposes in the USAFlowers arranged along spike of male
plant – ‘Douglas’ Palmer amaranth
Anther carried on filament –
pollen yet to be released
Pollen smear
Anther from which pollen has
been releasedFemale inflorescence in situ (cotton field, Douglas district) Female inflorescence (dried form), Douglas district
Female Palmer amaranth plant (in foreground) surrounded and outnumbered by male plants. Locality: Cotton field, Douglas district (26 Apr 2018)
Palmer amaranth in Douglas district
Stems
Amaranthus palmeri in USA
Note:
• Stems are glabrous (smooth/non-hairy)
• Leaves also glabrous with white-colored veins on
underside
Source: Extension Service, Purdue UnivStem coloration vary from green or red, to pale red, to reddish green.
Plant height is commonly 1 to 1.5 m, but height reached varies according to the crop it infests – plant height can exceed 2 m in maize, whilst in cotton it tends to grow about as tall or bit taller, but in latter case forming more side shoots, i.e. more “bushy”, whereas plants tend to grow less side shoots in relatively tall crops such as maize. Stem diameter at the base of large plants can be up to 10 cm (reported from the USA). Seed production can be about 100,000 to 500,000 per plant depending on plant size and number of female flowers per plant (USA reports). Seeds are small and can be carried long distances by wind, water, birds and herbivores. Notably, human activities such as crop production, movement of farm machinery and animal fodder contributes much to the ability of this weed to spread rapidly (USA reports). Seed longevity (dormancy) in soil can be 3 to 5 years (USA reports). Palmer amaranth seedling emergence occurs throughout the summer growing season, which stretches from spring to autumn – late-emerging plants can flower late-season and produce flowers and seed although barely 20 to 30 cm in height.
Could this be RSA farmers’ fate also?
Hand-weeding and physical removal from crop fields in the USA, and resistance
to certain herbicides from six important herbicide groupings:
glycine (glyphosate), triazine, dinitroaniline, HPPD inhibitors, PPO inhibitors,
and ALS inhibitors – source: http://www.weedscience.org/
Source: Extension Service, Purdue UnivBecause Palmer amaranth is dioecious (sexes on separate plants), outcrossing is forced, which
makes for high genetic diversity in this species. This trait makes it highly adaptable to diverse
environments and promotes rapid evolvement of herbicide resistance across herbicide modes-of-
action. In the USA, to date, resistance to six herbicide modes-action has been reported.What challenges await us in the RSA in dealing with such a formidable weed?
Fortunately, we can learn a lot from the USA experience!
Can we take over USA “lessons learnt” as they apply there, or is this species going to
prove unique over here, at least in some respects?Crucial for successful containment/management/eradication(?) of
this “wicked” weed is concerted, collaborative effort from as many
stakeholders as possible.
Reporting of sightings can be made to the SAHRI at UP
http://www.up.ac.za/SAHRI
Project leader: Prof Reinhardt
Cell: 083 442 3427
Email: dr.Charlie.Reinhardt@gmail.com
Note: Additional avenues of reporting will be announced in due
coursePhoto Credits
All photos appearing in this document, except two obtained from Dr Hugo Bezuidenhout, and one from
Purdue University (USA), were contributed by Prof Charlie Reinhardt
Text Credit
Text contributed by Prof C F Reinhardt
Copyright© in photographs and text: C F Reinhardt and the SAHRIBibliography • Various articles (scientific and popular) that originate mainly in the USA. • Website of the Weed Science Society of America which is managed by Dr Ian Heap: http://www.weedscience/org
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