List of Abstracts:

• Tropical Spiderwort: Perspectives from a County Agent (Flanders)
• Demography of Commelina benghalensis in the Southern US (Burton)
• After Five Years of On-Farm Testing, Have We Learned How to Manage Tropical Spiderwort? (Prostko)
• Tropical Spiderwort: Winning the Battle in Georgia Cotton (Culpepper)
• Impact of Tillage and Herbicide on Tropical Spiderwort (Brecke)
• Natural Variation in Commelina benghalensis (Commelinaceae) (Faden)
• The Effect of Environment on Invasibility in the Commelinaceae (Burns)
• The Ecology of Tropical Spiderwort in Agroecosytems of the Southeast US (Webster)
• Kill Tropical Spiderwort and Starve a Nematode (Davis)
• Effect of Moisture Stress on Tropical Spiderwort Response to Herbicides (Vencill)
• Herbicide Absorption and Translocation in Commelina benghalensis (Grey)
• Effects of Elevated Atmospheric CO2 on Tropical Spiderwort
• Invasive.Org: The Source for Information and Images of Invasive and Exotic Species

Tropical Spiderwort: Perspectives from a County Agent

J. Timothy Flanders
Grady County Extension Coordinator
University of Georgia, Cairo

In a 1994 weed survey of Grady County, several weed species were noted that were not major weeds in Georgia, including: wild poinsettia, ground cherry, redweed and tropical spiderwort. Although present in several locations in 1994, tropical spiderwort was not considered a troublesome weed. However, in 1998 this weed suddenly became a problem for growers of Roundup Ready cotton. In 1999 the first cotton research trials were conducted and a by 2000 tropical spiderwort had become the most troublesome weed in Grady County cotton. In 2001 the first peanut research trials were conducted and tropical spiderwort became the most troublesome weed in Grady County peanuts that same year. Today tropical spiderwort is present in 60 to 70% of the county’s cropland, and in 80% of those fields it is the most predominate weed. In 1999, tropical spiderwort was known to exist in 5 Georgia counties based on inquiries made by county agents to extension weed scientist. By 2005 the Georgia Department of Agriculture had confirmed the existence of tropical spiderwort in 33 Georgia counties. The rapid spread of tropical Spiderwort can be attributed to several factors. The introduction of glyphosate resistant cotton has revolutionized weed management and allowed broad-scale adoption of conservation tillage. Florida research shows increased incidence of tropical spiderwort in conservation tillage systems. Contributing to the rapid adoption of the most troublesome weed status of tropical spiderwort is its natural tolerance to glyphosate. With the introduction of Roundup Ready technology, many older cotton herbicides, some which have activity on tropical spiderwort, were replaced with glyphosate-based herbicide programs. The growth and reproductive characteristics of tropical spiderwort has also promoted its rapid spread. Tropical spiderwort has tremendous reproductive characteristics with the ability to produce seed under field conditions in 40 to 45 days and the ability to produce multiple generations in a year. Tropical spiderwort also has the ability to germinate throughout the growing season putting extraordinary pressure on any weed management program. In the past 3 to 4 years, the largest contributor to the spread of tropical spiderwort in Grady County is related to the county’s corn acreage. Grady County farmers’ grow corn on 11,000 to 12,000 acres a year (20% of counties row crop land). This acreage is generally beginning to dry down by mid- to late-July, with most of the harvest taking place in August. Once corn starts to dry down, sunlight penetrates the canopy allowing tropical spiderwort to emerge and grow uncontrolled until frost. The lack of any widespread control strategies following corn harvest allows fields to become a monoculture of tropical spiderwort and allows a large soil seedbank to build for future crops.

Demography of Commelina benghalensis in the Southern US

Michael G. Burton
Weed Biology and Ecology Research
North Carolina State University, Raleigh

Why do we have so much of it? (Propagation)

•  Vegetative reproduction – Simulated fall disking experiments indicate that a small to large fraction of stem fragments remain turgid after two weeks burial. Weather is believed to be a large factor in the variability observed over the two years of study. Less than 5% of stem fragments entered a reproductive phase by producing flowers or rhizomes bearing flowers. Seeds can be produced by these flowers.

•  Sexual reproduction
In controlled environment studies (NCSU Phytotron), C. benghalensis flowered about 30 days after emergence (male imperfect and bisexual perfect flowers are present). Seeds dehisced from fruit 14 days after flowering. Some of the seeds produced germinated in the same environment 14 days after the fruit dehisced.
Subterranean flowers can also be produced on rhizomes. These flowers/fruit appear to mature more slowly, but spathe tissue decays about 30 days after subterranean spathe initiation.

•  Seedbank Longevity – Field observations indicate a small fraction survives at least three years, but 2004 emergence was about 25% of that observed in 2003.

How is it getting around? (Dispersal)

•  Moved with equipment (tillage, custom cotton/peanut/potato harvesters, mowers, etc.).
•  Moved with seed/plant material/soil.
•  Field disposal of gin trash (NC test: ~57 seeds per kg of seed cotton)
•  Nursery and livestock industries
•  Blown by wind or water erosion, floods, hurricanes.
•  Animal movement [deer browse, doves (?), rodents].

Where is it now? (Distribution)

•  Affected southern states: GA, FL, NC, SC, LA, AL
•  Where is it going? Difficult to predict, however, at a minimum cotton, peanut and rice producing regions of the USA are at risk.

After Five Years of On-Farm Testing, Have We Learned How to Manage Tropical Spiderwort?

Eric P. Prostko
Peanut and Corn Extension Weed Science
University of Georgia, Tifton

Over the past several years, tropical spiderwort has become one of the most troublesome weeds in row crop agriculture in southern Georgia. In response to this threat, University of Georgia and USDA/ARS weed scientists have conducted more than 85 laboratory, greenhouse, and field trials to address the biology and control of this weed.

Generally, the control of tropical spiderwort in row crops will force producers to increase production inputs which will ultimately decrease economic returns. Several herbicides have been identified for use in potential management strategies including 2,4-D, s-metolachlor, carfentrazone, and paraquat. The discovery of adequate control programs in field corn have been elusive because the majority of tropical spiderwort emergence occurs after the critical period of weed control (i.e. after the time that corn growth is far enough along that new tropical spiderwort emergence will not affect corn yield) and/or when the field corn it is too tall to apply herbicide treatments. Consequently, production inputs for the management of tropical spiderwort in field corn might be better utilized post-harvest. However, growers are reluctant to implement post- harvest control tactics because of time constraints, current diesel fuel prices, and limited immediate, economic benefit from their use.

Despite significant research and extension efforts, tropical spiderwort continues to spread at an alarming rate in Georgia.

Tropical Spiderwort: Winning the Battle in Georgia Cotton

A. Stanley Culpepper
Cotton and Vegetable Extension Weed Science
University of Georgia, Tifton

Southeastern cotton weed control has changed drastically since the commercialization of Roundup Ready cotton in 1997. This technology was rapidly adopted by cotton farmers and occupies at least 94% of the acreage in the Southeast. Several weed control changes and challenges have occurred, at least in part, as a result of Roundup Ready technology, including the following: 1) greater adoption of conservation tillage (44% in Georgia); 2) reduction in mechanical weed control after crop emergence, with many growers relying completely on herbicides to manage weeds; and 3) reduction in the use of residual herbicides like fluometuron and norflurazon while relying very heavily on postemergence control from glyphosate. These factors have lead in part to weed shifts and resistance, which have become problematic in several areas throughout the southeast.

Tropical spiderwort was not a pest of cotton in 1999, but because of its ability to adopt to conservation tillage and Roundup Ready cotton programs, the weed quickly emerged into being quite troublesome. It was ranked as the 9th most troublesome weed in 2001 and became the most troublesome weed for Georgia cotton growers by 2002. It has maintained its dominance as the most troublesome weed facing Georgia cotton producers from 2002 through 2005.

Several scientists suggest that tropical spiderwort has simply become more troublesome because of its tolerance to glyphosate, as most cotton growers rely heavily on glyphosate for weed management. Although tropical spiderwort is tolerant to glyphosate, the weed can be controlled by glyphosate if applications are made to small (< 2 inch) plants growing in a favorable environment. Moreover, producers who have been challenged with this pest for several years now no longer rely simply on glyphosate to manage the weed. Growers quickly adopted other more effective herbicides such as s-metolachlor, carfentrazone, and diuron. In spite of the adoption of these more effective herbicide programs, tropical spiderwort continues to spread throughout Georgia, now infesting 33 Georgia counties.

Continued spread of this weed is likely in response to: 1) the ability of the weed to avoid weed control tactics by often emerging after the weed management program has been completed, 2) the ability of this weed to proliferate in conservation tillage, 3) the lack of physical weed control (i.e. in-crop tillage) as growers now rely on herbicides for in crop weed control, and 4) the lack of use of effective residual herbicides applied throughout the season, a standard practice before Roundup Ready technology was so widely adopted.

Impact of Tillage and Herbicide on Tropical Spiderwort

Barry J. Brecke, Daniel O. Stephenson, IV, and Kendal Hutto
University of Florida, West Florida Research and Education Center, Jay, FL

Studies were conducted at the University of Florida, West Florida Research and Education Center, Jay, FL in an area naturally infested with tropical spiderwort to determine the effect of tillage and herbicides on management of tropical spiderwort.

In the first study, peanut and cotton were grown under two tillage regimes: 1) conventional tillage which included use of a moldboard plow, disk and field cultivator prior to planting and 2) and reduced tillage which included use of a strip-till implement fitted with an in-row subsoil shank, closing discs and rolling baskets. The strip-tillage operation left at least 50% of the soil surface undisturbed. Cotton and peanut were planted following the tillage operations. Weed counts indicated a lower tropical spiderwort density (3 weed/m2) in the conventional tillage area compared with the strip-tillage area (60 weeds/m2) in both cotton and peanut.

In a second study (with only cotton) three levels of tillage were evaluated for effect on tropical spiderwort. Conventional tillage and strip-tillage were employed as in the first study. The third tillage system involved the use of a para-till implement which resulted in a level of soil disturbance greater than strip-tillage but less than conventional tillage. Weed counts indicated that tropical spiderwort density was highest in strip-tillage (8 plants/m2), next highest in para-till (4 plants/m2) and lowest in conventional tillage (2 plants/m2). Herbicide treatments were more effective in the conventional tillage area probably due to the reduced tropical spiderwort density. Two application of glyphosate in glyphosate tolerant cotton failed to provide adequate control of tropical spiderwort. Adding metolachlor to the first glyphosate application improved control to 96% in conventional tillage, 80% in para-till and 75% in strip-till cotton.

Natural Variation in Commelina benghalensis (Commelinaceae)

Robert B. Faden
Department of Botany, MRC 166, Smithsonian Institution
PO Box 37012, Washington, DC 20013-7012, U.S.A.

Commelina benghalensis L., a paleotropical native, is one of six naturalized species of Commelina, out of a total of 9 species in the United States. The typical plant is an annual with broad leaves, red hairs at the summit of the leaf sheath, blue, chasmogamous, aboveground flowers, and cleistogamous underground flowers. The spathes are funnel-shaped, nearly sessile, and may be solitary or clustered. The inflorescence consists of 2 two cymes, the upper cyme producing a single male flower and the lower cyme several bisexual flowers. The anthers of the lateral fertile stamens have white pollen, whereas that of the medial stamen has yellow pollen. The aboveground capsules have up to 5 seeds, whereas the belowground capsules have up to 3 seeds. The seeds in both capsules are dimorphic for a total of 4 seed types. In Africa, two varieties of C. benghalensis are recognized, var. benghalensis, a diploid annual with cleistogamous flowers, and var. hirsuta, a polyploid perennial that lacks cleistogamous flowers. Other morphological variants occur in East Africa, especially Kenya, that do not belong to either of these taxa. Some features that occur among the variants are: very narrow leaves, purple- marked spathes, some flowers in the lower cyme cleistogamous, blue-lavender or white flowers, and yellow pollen in the lateral anthers. C. benghalensis has the unusual basic chromosome number x = 11. Diploids are known throughout the range of the species, but tetraploids and hexaploids are known from wild collections only from Africa. Within the U.S. C. benghalensis, arrived in Hawaii by 1909, in the southeast by at least the 1930s and in southern California by 1980. Southeastern plants are diploids (2n = 22), Hawaiian plants are probably diploids, and Californian plants probably hexaploids.

The Effect of Environment on Invasibility in the Commelinaceae

Jean H. Burns, Alice A. Winn, Stacey L. Halpern, and Thomas E. Miller
Department of Biological Science
Florida State University, Tallahassee, FL 32306-1100

Studies of the traits of invasive species have generally either confounded species characteristics with phylogenetic relationships or have compared a single invader with a related native. This series of studies examined traits associated with multiple invasive species in the Commelinaceae while controlling for shared evolutionary history. We compared invasive species to non-invasive species, rather than to native species, whose invasive potential is unknown. Comparisons between 5 pairs of invasive and non-invasive congeners were conducted in a greenhouse across a factorial water and nutrient experiment. Invasive Commelinaceae species had greater relative growth rate, fecundity, and vegetative reproduction than their noninvasive congeners. Other experiments found that thin, less tough leaves are also associated with invasiveness in these species. These traits may prove useful in predicting invasive ability in the Commelinaceae. Also, taking relatedness into account in comparative studies improves our ability to detect trait associations. However, trait differences between invasive and non-invasive species were environment-dependent, suggesting that care should be taken in generalizing about the traits of invasive species without taking into account the environmental conditions under which those traits were measured.

The Ecology of Tropical Spiderwort in Agroecosytems of the Southeast US

Theodore M. Webster
Research Agronomist, Crop Protection and Management Research Unit
USDA-Agricultural Research Service, Tifton, GA

There are numerous factors that have allowed tropical spiderwort to become a weed in our agroecosystems. Some factors are related to the biology of the plant, ecology of the tropical spiderwort-crop interactions, and management practices that do not deter tropical spiderwort growth. I will outline five of these factors. Tropical spiderwort is a persistent weed in Southeast agroecosystems due to: 1. its amazing growth habit, 2. unique emergence characteristics, 3. the ability to tolerate drought stress, 4. the slow growth habit of cotton, and 5. its ability to capitalize on unused resources following crop harvest.

Amazing growth habit. Greenhouse studies were conducted to evaluate tropical spiderwort growth. Five-leaf tropical spiderwort plants were transplanted into 30-cm diameter pots and growth evaluated over 11 weeks. There were five plants evaluated and the study was repeated over time. Plant growth was nearly linear between one and six weeks, with plants with 50 leaves, 10 shoots, and 10 aerial spathes (leafy bract that encloses the flowers and fruit). However, tropical spiderwort growth was geometric between six and 11 weeks after planting, with weekly additions of 70 leaves, 10 shoots, and 26 aerial spathes.

Emergence characteristics. The ability to predict tropical spiderwort emergence is critical for optimizing timing of control tactics. The lack of soil residual activity from glyphosate coupled with the plant size-linked tolerance of tropical spiderwort to glyphosate underscores the importance of understanding tropical spiderwort germination and emergence dynamics. The bulk of tropical spiderwort emergence (50 to 70%) in cotton fields in 2004 and 2005 occurred in July, which is at least a month later in the growing season than peak emergence for most other agronomic summer annual weeds. While up to 36% of the tropical spiderwort population emerged prior to July 1 (which will need to be addressed with some type of weed control tactic), the relatively late emergence characteristics of tropical spiderwort can be exploited to the benefit of the crop.

Based on observations of tropical spiderwort emergence patterns, it was hypothesized that early planted cotton (i.e. April or May) would be more competitive than late planted cotton (i.e. June) as the crop would have more time to establish prior to peak tropical spiderwort emergence and would form a crop canopy faster. A light-limiting crop canopy has been observed to curtail tropical spiderwort emergence. Studies were conducted to evaluate the interval that cotton must be kept free of tropical spiderwort in order to avoid a yield loss of greater than 5%. There is a time at the beginning of the season that cotton can tolerate the presence of tropical spiderwort (or any weed) as resources (i.e. water, nutrients, and especially light) are not limited. Likewise, there is also a point at which cotton has established itself and newly emerged tropical spiderwort populations will not influence cotton yield. The interval between these two times is the critical period of weed control (CPWC) during which all tropical spiderwort needs to be controlled.

May-planted cotton had narrow CPWC intervals between 475 and 525 growing degree days (GDD; calculated with a base temperature of 10 C) in 2004 and approximately 300 to 500 GDD in 2005. In contrast, June-planted cotton had wide CPWC intervals between 200 and 750 GDD in 2004 and 200 and 900 GDD in 2005. These data indicate that cotton was more competitive and required less aggressive management tactics when cotton was planted in May relative to June. Also supporting this contention is the maximum yield loss in the weedy controls; when tropical spiderwort competed with May-planted cotton for the entire season, yield loss was 20%. However, yield loss was at least double in the weedy control in the June-planted cotton (40 to 45%).

Drought stress. Preliminary studies indicated that tropical spiderwort is affected by drought stress, but maintained green leaves and produced spathes under extreme drought. Single plants were grown in the greenhouse for eight weeks in replicated trials. Treatments included four weekly watering regimes: field capacity (1X), half of field capacity (1/2X), one-fourth of field capacity (1/4X), and one-eighth of field capacity (1/8X). Tropical spiderwort width was a more robust measurement of growth than was plant height, as tropical spiderwort is a low-growing, sprawling plant. Plant width was reduced greater than 50% by watering at ˝X relative to 1X. Plants from all watering regimes produced aerial and subterranean spathes and numbers increased in a linear manner with amount of water.

Tropical spiderwort in cotton. Field studies were conducted in Grady County, Georgia in 2004 and 2005 to evaluate the effect of crop type on tropical spiderwort emergence and growth. Corn, cotton, peanut, and soybean were planted the final week of April in replicated plots with a naturalized tropical spiderwort population. Tropical spiderwort emergence was similar among crops early in the season, with divergence among crop types occurring around 450 GDD in 2004 and 300 GDD in 2005 (Tb=10C). Total season emergence was greatest in cotton in both seasons. Peanut and soybean had 30 and 40% less emergence than cotton, respectively. Cotton is slow to form a light-limiting canopy relative to soybean and peanut; low light levels tended to suppress tropical spiderwort emergence. Emergence in corn was variable between seasons, but 8 to 22% less than cotton. Tropical spiderwort biomass in the non-cropped (fallow) plots were greater than in any of the crop treatments. However, only soybean had less tropical spiderwort biomass than peanut, which had the most tropical spiderwort biomass per plant in the four crops. Therefore, while cotton allowed the most new tropical spiderwort seedlings to emerge throughout the season, once established tropical spiderwort plants growing in competition with peanut attained the greatest growth.

Corn is often planted prior to the last week in April in Georgia, therefore the comparisons of growth between the crops in the above study may not reflect the differences in actual planting dates that occur in south Georgia. Another study was conducted in 2005 where corn was planted April 14; cotton, peanut, and soybean planted May 16; and 1-leaf tropical spiderwort transplanted June 16. These dates were selected to simulate the differences in crop planting dates as well as the late emergence characteristics of tropical spiderwort. At 12 weeks after tropical spiderwort transplanting (WATr), tropical spiderwort plants in corn and soybean were less than one-third the plant width of those in cotton and peanut. Similarly, there were less than 5 aerial spathes per plant in corn and soybean treatments, while peanut and cotton had 40 and 55 aerial spathes per plant, respectively. Leaf area, leaf biomass, and total plant biomass revealed similar trends (data not shown).

Post-crop harvest reproduction. Corn is a silent accomplice to tropical spiderwort, allowing populations in the soil seedbank to increase. Research indicated that tropical spiderwort growth in corn is less than in cotton and peanut during the growing season. However, tropical spiderwort capitalizes on the corn growth habit. Corn is planted and completes much of its lifecycle prior to tropical spiderwort emergence. By the time peak tropical spiderwort emergence occurs, corn foliage is beginning to desiccate, which allows more light through the crop canopy and tropical spiderwort plant establishment. Following corn harvest, most fields are left undisturbed, allowing tropical spiderwort populations to grow until frost, all the while increasing propagules in the soil seedbank.

Research Needs: It is vital that cropping systems are developed that possess low susceptibilities to tropical spiderwort invasion (preventing new tropical spiderwort establishment) and high tolerance to tropical spiderwort presence (suppressing impact of an existing tropical spiderwort population). These cropping systems will be characterized by: 1. elimination of tropical spiderwort safe-sites (conditions that allow for tropical spiderwort germination, emergence, and establishment), 2. optimized benefits of cultural practices (i.e. early planting dates, aggressive crop cultivars, inclusion of some type of tillage), 3. utilization of aggressive control tactics, including the use of s-metolachlor in cotton and effective herbicides rotation crops, and 4. elimination of the opportunity for tropical spiderwort reproduction, especially following crop harvest.

Studies are also needed to evaluate: 1. tropical spiderwort seedbank longevity, 2. primary dispersal mechanisms, and 3. post-crop harvest management. A critical need for the southern region is the development of a model that characterizes the environmental limits of tropical spiderwort in the US. A proactive approach to minimize the spread of tropical spiderwort to susceptible habitats is key for minimizing the regional impact of this weed in our agroecosystems.

Kill Tropical Spiderwort and Starve a Nematode

Richard F. Davis1, Timothy B. Brenneman2, and Theodore M. Webster1
1 USDA-ARS, Crop Protection and Management Research Unit, Tifton, GA 31793
2 University of Georgia, Department of Plant Pathology, Tifton, GA 31793

The major plant-parasitic nematodes in the southeastern US are soil-borne microscopic worms that feed on plant roots. They are obligate parasites which can only feed on plants. Most nematodes have a wide range of plants on which they can feed (their host range), but nematode host ranges do differ, and those differences are the reason that crop rotation can be used to reduce nematode population levels. Nematodes are the most damaging pathogens of cotton, and one of the most important pathogens of peanut. The southern root-knot nematode (Meloidogyne incognita) reproduces well on cotton and corn, but not on peanut; the peanut root-knot nematode (Meloidogyne arenaria) reproduces well on peanut, but not on cotton or corn; and the reniform nematode (Rotylenchulus reniformis) reproduces well on cotton, but not on peanut or corn. Therefore, crop rotation sequences utilizing cotton, peanut, and corn can be selected to manage these nematodes. However, weeds also can support nematode reproduction, and the amount of reproduction on some weeds can be enough to reduce the effectiveness of crop rotation as a nematode management tool. In addition to the host status of the weed, the amount of nematode reproduction will be affected by the weed population density (plants/m2) and root mass: a weed that produces a lot of root mass, has a high plant density, and is a good host may support as much nematode reproduction as would a susceptible crop thereby negating the nematode-suppressive benefit of a crop rotation. Tropical spiderwort (Commelina benghalensis) typically has high plant population density with a lot of root mass, but its host status for nematodes was not known. We initiated a study to document the relative host status of tropical spiderwort for M. incognita, M. arenaria, and R. reniformis, each of which was tested in two separate experiments. We also evaluated the host status of tropical spiderwort for fungal pathogens Sclerotium rolfsii, which causes southern stem rot or white mold of peanut, and Cylindrocladium parasiticum, which causes Cylindrocladium black rot (CBR) of peanut: these diseases are the primary reason that peanuts in the southern US usually are grown in a given field only once every three years. Tomato was grown as a susceptible standard for M. incognita and M. arenaria, and cotton was grown as a susceptible standard for R. reniformis. Peanut was used as a susceptible standard for Sclerotium rolfsii and Cylindrocladium parasiticum. Tomato, cotton, and peanut were started from seed, and C. benghalensis was started from vegetative cuttings. Each pot contained one plant in 1,100 cm3 of soil and was inoculated with 8,000 nematode eggs approximately 3 weeks after planting. A reproductive factor (RF) was calculated for each nematode and host combination as the final population level divided by the initial population level (Pf/Pi). Only egg counts were used to calculate RF for M. incognita and M. arenaria, but egg and vermiform counts were used to for R. reniformis. Galling was estimated on a 0 to 10 scale for the Meloidogyne species. Data from the two trials with M. incognita were statistically similar, so the data were combined into a single analysis. Data from the trials with M. arenaria also were combined, but data from the trials with R. reniformis could not be combined. Meloidogyne incognita reproduced well on C. benghalensis, leading to a mean gall rating of 3.1 and a mean RF of 15.5 on C. benghalensi. Meloidogyne incognita caused a mean gall rating of 6.4 and a mean RF of 41.3 on tomato. Meloidogyne arenaria also reproduced well on C. benghalensis, with a mean gall rating of 2.1 and a mean RF of 7.2. Meloidogyne arenaria caused a mean gall rating of 6.3 and a mean RF of 12.4 on tomato. In the first trial with R. reniformis, the RF was 2.4 on C. benghalensis and 1.4 on cotton. In the second trial, the RF was 3.6 on C. benghalensis and 13.5 on cotton. In soils more conducive to reproduction by R. reniformis, the RF on cotton can be much higher than observed in this study, and it is likely that the RF also would be higher on C. benghalensis in such soils. The severity of symptoms caused by Sclerotium rolfsii was estimated on a 0 to 10 scale. In the first trial, peanut had a mean disease severity rating of 4.0 and C. benghalensis had a mean rating of 1.4, and the fungus could be seen growing on 40% of the C. benghalensis plants. In the second trial, peanut had a mean disease severity rating of 10.0 and C. benghalensis had a mean rating of 5.0, but the fungus could be seen growing on all of the C. benghalensis plants. Sclorotium rolfsii is a virulent pathogen that will kill a wide range of hosts. It is noteworthy that C. benghalensis was able to survive complete girdling by S. rolfsii lesions by forming new roots from nodes above the lesions. Such plants exhibited growth similar to the non-inoculated controls. The trials with Cylindrocladium parasiticum were inconclusive due to low infection rates, but the fungus appears to be weakly pathogenic to C. benghalensis. In conclusion, it appears that C. benghalensis is a sufficiently good host for some of the primary nematode and fungal pathogens of major crops in the southeastern US that its presence at high plant population densities can greatly reduce the pathogen-suppressive effects of crop rotation.

Effect of Moisture Stress on Tropical Spiderwort Response to Herbicides

William K. Vencill
University of Georgia, Athens, Department of Crop and Soils Sciences

The objective of this research was to observe the effects moisture stress has on greenhouse grown tropical spiderwort plants and also to observe the effects of herbicidal control, uptake and translocation. Leaf characterization was observed under a scanning electron microscope to determine differences caused by moisture stress. Treatments were subjected to three different moisture stress levels (25, 50 and 100% of field capacity), and sprayed post emergence with selected herbicides. Translocation of diclosulam, imazapic and s-metolachlor into the roots and shoots was observed. Foliar uptake of diclosulam and imazapic were not affected by changes in moisture stress. Studies were conducted to observe the effects of varying rates of different herbicides on aerial and underground seed. s-metolachlor had the greatest control of aerial and underground seed.

Results of these studies reveal that soil moisture has an impact on the morphology and can impact herbicidal control of tropical spiderwort. As the soil moisture content decreased from 100% to 25% of field capacity, the thickness in cuticle increased approximately 27%, trichome frequency increased approximately 44%, and wax content increased almost 26%. This change in morphology most likely affects the uptake of foliar applied herbicides.

Herbicides affected by soil moisture content were atrazine, flumioxazin and imazapic. The ED50 values decreased with increasing moisture. The foliar uptake of these herbicides increased when moisture content increased. Diclosulam and sulfentrazone foliar uptake was not affected by moisture content. Although glyphosate alone did not provide effective control, the foliar uptake was increased at the highest (100% field capacity) moisture level. This is one reason why tropical spiderwort is such a problem in glyphosate-resistant crops, since growers in these cropping situations use glyphosate as the main form of herbicidal control. S-metolachlor, was translocated from the shoots to the belowground parts. The translocation of S-metolachlor may affect the germination of underground seed.

Application of S-metolachlor to aerial and underground seed resulted in the lowest germination percentage when compared to technical diclosulam and imazapic. Diclosulam and imazapic did not effectively inhibit aerial or underground tropical spiderwort seed germination, regardless of rate. Further studies are needed on the aerial and underground seed of tropical spiderwort to learn how to effectively control this weed.

Herbicide Absorption and Translocation in Commelina benghalensis

Timothy L. Grey1, Paula Steptoe2, and William Vencill2
1 University of Georgia, Tifton, Department of Crop and Soils Sciences
2 University of Georgia, Athens, Department of Crop and Soils Sciences

The translocation of diclosulam, imazapic, and S-metolachlor were investigated for C. benghalenis using 14C-labeled isotopes for each herbicide. Live cuttings were placed in Styrofoam cups filled with soil. The cups were split vertically so they could be opened up to reveal the root and underground floral structures. Plants were allowed to grow for three weeks prior to treatment. The cups were opened up to reveal the roots and underground flower. Plants were then treated with the respective 14C-herbicide either on the shoot, root, or underground flower with a spot application. After 48 hrs, plant parts were divided into above and below ground sections, dried, ground into a powder, and then oxidized, and 14C quantified by liquid scintillation spectrometry.

For 14C-diclosulam applied to shoots, 74% remained in the shoot with less than 26% moving into the root. When the root was treated with diclosulam, 63% remained in the root while 37% translocated to the shoot. The underground flower treated with diclosulam resulted in equal distribution with 46% moving into the root and 53% to the shoot. Diclosulam distribution was dependent on site of uptake, and thus could provide activity via soil or foliar uptake.

C. benghalenis shoots treated with 14C-imazapic retained 63% of the herbicide while 37% translocated to the roots. When the root was treated, the inverse occurred with 80% remaining in the root and only 20% moving to the shoot. In contrast, when the underground flower was treated, 24% translocated to the root while 76% went to the shoots. As with diclosulam, these data indicate that imazapic absorption can occur via root, shoot, or underground flower with some translocation occurring through out the plant.

Eighty two percent of shoot applied 14C-S-metolachlor was translocated to the root with only 18% remaining in the shoot. In contrast, 56% of the root applied herbicide remained in the root while 44% translocated to the shoot. When the underground flower was treated, similar results occurred with 60% going to the roots and 40% to the shoots. Thus, S-metolachlor soil applied would translocate to the entire plant but as a foliar application it would move to the roots. Previous research has indicated that S-metolachlor does not provide foliar control of C. benghalenis and this is probably due either lack of absorption or to translocation from the shoot to the roots.

Similar trends for translocation of 14C-diclosulam and 14C-imazapic occurred for the specific treated plant part (i.e. root, shoot, or underground flower). The speculation for this similarity could be because both are ALS herbicides that have pre- and post-emergence activity. In contrast, 14C-S-metolachlor is a residual chloracetamide herbicide that has only pre-emergence activity. Translocation of 14C-S-metolachlor did occur, but it was uniquely different from 14C-diclosulam and 14C-imazapic.

Effects of Elevated Atmospheric CO2 on Tropical Spiderwort

A.J. Price1, S.A. Prior1, G.B. Runion1, E. van Santen2, H.H. Rogers1, D.H. Gjerstad3, and H.A. Torbert1
1 USDA-ARS National Soil Dynamics Laboratory, Auburn, AL
2 Department of Agronomy & Soils, Auburn University, AL
3 School of Forestry & Wildlife Sciences, Auburn University, AL.

Invasive weeds are estimated to cost U.S. agricultural and forest producers $34 billion each year from decreased productivity and increased weed control costs. One neglected aspect of global change is how invasive plants might react to the increasing atmospheric CO2 concentration. Since elevated CO2 stimulates photosynthesis, resource use efficiency, and carbon allocation to belowground plant structures, it may impact the competitiveness of invasive plants. Tropical spiderwort (Commelina benghalensis L.) is considered an invasive noxious weed and is becoming more of a problem in agricultural settings of the southeastern US. This recently funded National Institute for Global Environmental Change (Southeast Regional Center) research project evaluates tropical spiderwort responses to CO2 enrichment. Tropical spiderwort was grown under ambient and elevated levels of CO2. Under elevated CO2 conditions, plant organ parts exhibited significant increases in dry weight (leaf, +36%; flower, +30%; stem, +48%) and the overall increase in total aboveground biomass was 44%. Total stem length was unaffected by CO2 level while total leaf number and total flower number showed trends for increase (~20%) due to additional CO2. The strong growth responses of tropical spiderwort suggest that its competitive ability with native plants will be enhanced in a future high CO2 environment.

Invasive.Org: The Source for Information and Images of Invasive and Exotic

Chris W. Evans, G. Keith Douce, David J. Moorhead, and Charles T. Bargeron
The Bugwood Network, P.O. Box 748, 4601 Research Way Admin. Bldg.
The University of Georgia, Tifton, GA 31793, Phone: (229) 386-3298

The University of Georgia's Bugwood Network ( developed as a tool for collecting, providing and maintaining information and images related to invasive and exotic species for North America. This includes plants, insects, pathogens, nematodes, mollusks and vertebrates, as well as many biological control agents. Identification, ecology and management information is easily accessible for many of these species. currently provides information and/or images for over 600 different species. The project has been funded in part by the USDA Forest Service and USDA APHIS PPQ. The overall goal of the project is to cross agency and organizational barriers to provide the most useful information to the largest audience. In order to accomplish this goal cooperation is required at a regional, national and even international level.

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