Presentations were made primarily by research and extension specialists from the Southeast
region of the US, but also included a Research Botanist from Washington D.C. with expertise in
the Commelinaceae. Each presenter supplied an abstract of his/her presentation,
and a copy of their presentation. The following is a summary of the presentations
from notes that I recorded at the meeting.
Mr. Tim Flanders (Grady County Extension Coordinator, University of Georgia, Cairo)
detailed his tropical spiderwort experiences with growers in southwest Georgia. Mr. Flanders
was among the first county agents to bring this weed to the attention of University of Georgia
(UGA) and USDA-Agricultural Research Service (ARS) personnel. Mr. Flanders’ hands-on
extension and research experience in Grady County have provided him with a greater knowledge
of this weed than anyone else in Georgia. He initially began to receive questions from growers
about this dayflower in 1998. As of 2000, it was the most troublesome cotton weed in the county
and a year later the most troublesome peanut weed. Mr. Flanders estimates that 60 to 70% of the
fields in Grady County have tropical spiderwort. He hypothesizes that tropical spiderwort has
increased as a problem and has rapidly spread through the county due to: 1. increased use of
conservation tillage, 2. large corn acreage that is not managed for tropical spiderwort following
harvest, allowing tropical spiderwort to grow unchecked for nearly three months until a hard
frost, and 3. introduction and widespread use of glyphosate-resistant cotton, coupled with
changes in herbicide-use patterns (i.e. elimination of soil applied herbicides with residual activity
against tropical spiderwort (e.g. fluometuron), and 4.) aggressive growth and reproductive
characteristics of tropical spiderwort.
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Dr. Mike Burton (Assistant Professor, North Carolina State University, Raleigh) followed with
a presentation titled: "Demography of tropical spiderwort in the Southeast US". His presentation
focused on 1.) tropical spiderwort propagation, 2.) tropical spiderwort dispersal, and 3.) the
current distribution of tropical spiderwort in the US. Dr. Burton presented information
concerning the ability of tropical spiderwort to survive and vegetatively propagate following
disking. He also presented data to support the hypothesis that tropical spiderwort may have
multiple generations per growing season and summarized preliminary data on tropical spiderwort
seedbank longevity. Dr. Burton proposed several different mechanisms for tropical spiderwort
dispersal, including movement with: 1.) equipment, 2.) plant material and soil, 3.) field disposal
of gin trash, 4.) nursery and livestock, 5.) wind or water associated with floods and hurricanes,
and 6.) animal wildlife. Tropical spiderwort has been found in Florida (as early as 1934),
Georgia (33 counties), North Carolina (7 confirmed locations, which includes 2 retailers of
nursery stock), South Carolina (1 nursery), Alabama (isolated plants), Louisiana and Missouri
(adjacent to botanical gardens), and California (likely a different biotype from a separate
introduction).
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Dr. Eric Prostko (Associate Professor, University of Georgia, Tifton) presented data on his five
years of experience with on-farm test with tropical spiderwort. In total, the Georgia Weed
Science Group (University of Georgia and USDA-ARS colleagues) have completed 88 research
trials since 2000. Dr. Prostko summarized his research findings: 1.) current crop production
systems are to blame for the tropical spiderwort problem and short-term fixes will suppress
tropical spiderwort, but long-term answers are still required; 2.) there are several good herbicides
for the control of tropical spiderwort (e.g. 2,4-D; Dual Magnum; Aim; and Gramoxone),
however, growers will have to spend significantly more money to control/suppress tropical
spiderwort; 3.) tropical spiderwort control in corn was not profitable unless corn was planted late
in the growing season (i.e. late-May or June) because tropical spiderwort did not affect corn
yield when corn was planted in a timely manner. This was likely due to the emergence of
tropical spiderwort in June and July after early-planted corn had already set ears; 4.) post-harvest
weed control following corn may be critical for long-term management. However, post-harvest
treatments will be difficult to convince growers to adopt unless research can demonstrate that
there is a significant economic impact above and beyond the costs (i.e. costs associated with
tillage, herbicides, and/or fuel prices).
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Dr. Stanley Culpepper (Associate Professor, University of Georgia, Tifton) presented
information on tropical spiderwort in cotton. The cotton industry in Georgia is an $804 million
(2003 farm gate), second only to production of chicken broilers in Georgia, with 1.2 to 1.4
million acres of cotton planted annually. In 1999, tropical spiderwort was not listed among the
top 10 most troublesome weeds in cotton, but it was the #1 most troublesome weed in Georgia
cotton by 2002. All effective tropical spiderwort management programs are built around Dual
Magnum. However, there are several caveats associated with the use of Dual Magnum in
Georgia cotton: 1.) severe cotton injury can occur if Dual Magnum is applied preemergence to
cotton; 2.) Dual Magnum must be applied after cotton emergence, but prior to tropical spiderwort
emergence (i.e. it will not control emerged weeds); 3.) rainfall/irrigation is required for herbicide
activation. Without moisture, Dual Magnum will not work; 4.) too much rainfall will move Dual
Magnum out of the tropical spiderwort germination zone and reduce or eliminate herbicide
efficacy; 5.) weed management costs are increased at least 25% by Dual Magnum, whether
conditions are favorable for tropical spiderwort control or not. Further complicating the issue is
the future adoption of Roundup Ready-Flex technology which allows for topical applications of
Roundup throughout the growing season and at higher rates than are currently allowed (which
will not improve tropical spiderwort control). The current recommendations for Georgia cotton
fields with tropical spiderwort are: 1.) plant early in the season before the bulk of tropical
spiderwort emerge in June or July; 2.) plant aggressive growing cultivars that form a crop canopy
early in the season, shading the ground and suppressing tropical spiderwort emergence and
growth; 3.) reduce seed production after crop harvest; 4.) reduce the alarming rate of spread; 5.)
Cotoran preemergence, if planting cotton near the time of peak tropical spiderwort emergence;
6.) postemergence herbicides need to include Dual Magnum; 7.) layby options include: Aim,
Dual Magnum, Direx, Valor, and MSMA. Dr. Culpepper estimates that conservatively in 2005
100,000 acres in Georgia were treated with 1 pt/acre of Dual Magnum to control tropical
spiderwort, at a cost of $1.2 million.
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Dr. Barry Brecke (Professor, University of Florida, Jay) presented information concerning the
impact of tillage and herbicides on tropical spiderwort. Dr. Brecke observed that: 1.) tropical
spiderwort infestations were worse in reduced tillage, 2.) Roundup was not effective in
controlling tropical spiderwort, 3.) many peanut and cotton herbicides did not provide long-term
control of tropical spiderwort, and 4.) tropical spiderwort had a germination pattern that allowed
for emergence throughout the growing season. In long-term tillage and rotation studies that have
been infested with tropical spiderwort since 2003, tropical spiderwort populations in
conventional tillage were less than 5 plants/m
2, whereas strip tillage systems had population
densities of 60 plants/m
2. In another study, tropical spiderwort population densities were fourtimes
greater and two-times greater in strip-tillage relative to conventional and para-tillage,
respectively. Tropical spiderwort control from Roundup plus Dual Magnum followed by
Roundup was 20 to 35% higher than Roundup followed by Roundup in the three tillage systems.
Conventional tillage that included Dual Magnum controlled tropical spiderwort at least 94%,
while control in para-tillage and strip-tillage ranged from 75 to 92%. Dr. Brecke proposed that
more research is needed to evaluate the influence of tillage treatments: on seed distribution in the
soil profile, depth of tropical spiderwort emergence, seed predation, and seedbank longevity.
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The morning concluded with a presentation by
Dr. Robert Faden (Research Botanist,
Smithsonian Institution, Washington D.C.) on the "Natural Variation in
Commelina
benghalensis". In the plant family Commelinaceae, there are 41 genera with approximately 650
species, most of which are tropical.
Commelina is the largest genus in the Commelinaceae
family, with 170 species, most of which are African. In the US there are nine species of
Commelina, six of which are introduced, and the species are concentrated in the Southeast US,
with no native
Commelina species in the West Coast states.
Commelina species are perennials or
annuals with terminal inflorescences that may become leaf-opposed. The inflorescences are
composed of one or two cymes enclosed in a spathe (leafy bract). The flowers are strongly
zygomorphic; the lower petal is usually greatly reduced. Capsules can be up to five-seeded.
Tropical spiderwort was first collected in Hawaii in 1909 and first reposted in Florida in 1934.
In 1967, tropical spiderwort was found on Sapello Island off the Georgia coast. Tropical
spiderwort was added to the Federal Noxious Weed List in 1983. In 1993, tropical spiderwort
was reported in 13 counties in Florida, three counties in Georgia, and at one location in
Louisiana. North Carolina reported tropical spiderwort in 2002. Tropical spiderwort is typically
an annual plant with broad leaves, red hairs on the leaf sheaths, blue flowers, and underground
cleistogamous flowers. Spathes are funnel-shaped with fused margins containing an upper cyme
with a solitary male flower and lower cyme with perfect flowers. Lateral anthers possess white
pollen, while the medial anther has yellow pollen. Dr. Faden indicated that there are two
recognized varieties of tropical spiderwort.
Commelina benghalensis var.
benghalensis is an
annual with underground cleistogamous flowers, found in mesic to dry open habitats throughout
the range of the species and is diploid.
Commelina benghalensis var.
hirsuta is a perennial that
normally lacks cleistogamous underground flowers, is found in mesic to moist habitats
(including forests) in Africa and is tetraploid or hexaploid. There are four kinds of seed on each
plant (2 each from the cleistogamous and chasmogamous flowers), and Dr. Faden indicated that
there is no obvious means of seed dispersal. Dr. Faden proposed the following future research
directions: 1.) morphological, anatomical, and cytological studies to work out the taxonomic
entities within
Commelina benghalensis; 2.) DNA studies to determine the phylogenetic
relationships among the taxa within
Commelina benghalensis; 3.) reproductive biological studies
in the different taxa within
Commelina benghalensis.
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The afternoon sessions began with
Ms. Jean Burns (Ph.D. Student, Department of Biological
Sciences, Florida State University, Tallahassee) presenting "The Effect of Environment on
Invasibility in the Commelinaceae". Five pairs within the Commelinaceae were selected with
similar morphology, with one classified on an invasive species list and the other not listed as
invasive. Ms. Burns discussed a series of studies involving these five pairs of species in the
Commelinaceae family to evaluate whether there were some specific traits associated with these
invasive species. In the first study, Ms. Burns evaluated the role of environmental quality (high
and low nutrient and moisture regimes) and found the invasive Commelinaceae species to be
more fecund (in terms of more seed biomass produced) and have greater vegetative production.
Greater fecundity and higher vegetative production with invasive Commelinaceae are associated
with higher relative growth rates, higher specific leaf area, and greater plasticity in root to shoot
biomass ratio. In a second study, an invasive Commelinaceae species responded more
opportunistically to increases in resource availability and had higher specific leaf area (i.e.
thinner leaves capable of greater light interception) than non-invasive Commelinaceae. Ms.
Burns concluded from a third study that invasive Commelinaceae species are not less resistant to
herbivory and have less tough leaves than non-invasive Commelinaceae species.
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I presented "The Ecology of Tropical Spiderwort in Agro-Ecosystems of the Southeast US",
which is thoroughly summarized in the abstract.
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Dr. Richard Davis (Research Plant Pathologist, USDA-ARS, Tifton) presented "Kill Tropical
Spiderwort and Starve a Nematode or Tropical Spiderwort as a Host for Plant Pathogenic
Nematodes". He indicated that there are approximately a dozen nematode genera with economic
importance in the Southeast US. Most nematode species have a fairly wide host range, but there
are some differences. These differences are the basis of crop rotation as a means of managing
nematode problems. Important nematodes in cotton include southern root-knot (
Meloidogyne
incognita) and reniform (
Rotylenchulus reniformis). Peanut root-knot (
M. arenaria) is a
significant pest in peanut. Southern root-knot reproduces well in corn. Weeds can serve as hosts
for nematodes, allowing for reproduction even when a suitable crop host is absent from the field.
Dr. Davis concluded that tropical spiderwort is a good host for southern root-knot nematode and
a moderate host for reniform nematode and peanut root-knot nematode. Due to its host status,
tropical spiderwort possesses the potential to significantly reduce the effectiveness of crop
rotation and/or host plant resistance as a nematode management tactic.
Dr. Davis also presented some results from collaborative research with
Dr. Tim Brennemen
(Professor, Plant Pathology Department, University of Georgia, Tifton) on soil-borne fungal
diseases of peanut. Southern stem rot or white mold (
Sclerotium rolfsii) and cylindrocladium
black rot (CBR) (
Cylindricladium parasiticum) are the primary reasons for a three-year rotation
cycle in peanut. Tropical spiderwort plants demonstrated signs of the pathogen (40 to 100% of
the plants). Dr. Davis indicated that the pathogen is normally lethal to susceptible plants under
these conditions, but tropical spiderwort branches with necrotic tissue simply put down roots at
the next node from the infected area and continued growing. Tropical spiderwort will probably
cause some increases in southern stem rot levels, but the fungus will have minimal effects on the
weed. In the CBR trial, there was poor disease development on peanut and tropical spiderwort.
While this test was not definitive of tropical spiderwort susceptibility to CBR, the fungus
appeared to be weakly pathogenic. Tropical spiderwort will likely have little effect on CBR
inoculum density and CBR will likely have minimal effect on tropical spiderwort growth.
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Dr. Bill Vencill (Professor, University of Georgia, Athens) presented information on the "Effect
of Tropical Spiderwort Response to Herbicides". Studies were conducted in which tropical
spiderwort plants were grown in three moisture regimes (25, 50, and 100% of field capacity) for
three weeks. Cuticle thickness and trichome frequency were characterized using a scanning
electron microscope. Wax content for each leaf was also quantified. Dr. Vencill observed that
cuticle thickness, trichome frequency, and wax content per leaf increased with drought stress. A
subsequent study evaluated the influence of drought stress on herbicide response, using the same
three watering regimes. Herbicides were applied postemergence to tropical spiderwort plants
and included 2,4-D; diclosulam; flumioxazin; glufosinate; imazapic; sulfentrazone; atrazine;
glyphosate;
s-metolachlor; and glyphosate plus
s-metolachlor. In most instances, herbicide
efficacy was reduced by drought stress. Tropical spiderwort response to diclosulam, glyphosate,
and
s-metolachlor was not affected by drought stress. Foliar uptake of 2,4-D, flumioxazin,
glyphosate, and
s-metolachlor were reduced by moisture regime, while uptake of diclosulam,
imazapic, sulfentrazone, and atrazine was not affected by moisture.
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The final presentation was by
Dr. Tim Grey (Assistant Professor, University of Georgia, Tifton)
was titled "Herbicide Absorption and Translocation in
Commelina benghalensis". Three
herbicides (diclosulam, imazapic, and
s-metolachlor) were applied to roots, shoots, and
underground flowers of tropical spiderwort plants in greenhouse studies. After 48 hours, the
plants were divided into above- and below-ground parts, dried, and oxidized. Herbicide
movement was quantified using a liquid scintillation spectrometer. Dr. Grey reported that all
three herbicides were mobile in the plant, but diclosulam tended to remain in the shoot, smetolachlor
was primarily recovered in the root tissue, and imazapic was found throughout the
plant. Dr. Grey also presented data from a field trial that evaluated tropical spiderwort
emergence (up until cotton canopy closure) across eight rates of
s-metolachlor. There was a rate
response observed at one month after application that existed until cotton canopy closure.
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