Gregory E. MacDonald

Cogongrass (Imperata cylindrica) - biology, ecology, and management.

Cogongrass is considered to be one of the ten most troublesome and problematic weedy species in the world. This species is found throughout tropical and subtropical regions, generally in areas disturbed by human activities. Over 100 common names have been associated with cogongrass, including japgrass, speargrass, alang-alang, and bladygrass. Although this species has several commercial uses, the problems associated with its weediness far outweigh most positive benefits. Cogongrass is a major impediment to reforestation efforts in southeast Asia, the number one weed in agronomic and vegetable production in many parts of Africa, and is responsible for thousands of hectares of lost native habitat in the southeastern U.S. Biologically, cogongrass possesses several features that foster its spread and persistence. Management efforts for cogongrass consist of an integrated approach with several control strategies. In agronomic production, the use of cover crops is widely successful, but incorporation into the overall production scheme is challenging. Success has been achieved with continuous deep tillage or chemical applications, but long-term eradication/suppression must employ sustainable revegetation strategies.

A Survey of Weeds in Various Crops in Georgia1

A survey of county extension agents was conducted in 1998 to determine the most troublesome weeds in corn, cotton, forages and pastures, peanut, small grains, soybean, tobacco, and vegetables in Georgia. The most troublesome weed statewide averaged over all crops was sicklepod. It was the most troublesome weed in cotton and soybean and among the four most troublesome weeds in corn, peanut, tobacco, and vegetables. Sicklepod was found in each of the nine climatological districts and in all the crops surveyed. Perennial nutsedge species were the second most troublesome weeds in Georgia. They ranked as the most troublesome weeds in tobacco and vegetables and were among the top five most troublesome weeds in corn, cotton, peanut, and soybean. Pigweed species were ranked third averaged over all the crops surveyed and were the second most troublesome weeds in cotton and vegetables and among the top five most troublesome species in corn, soybean, and tobacco. Morningglory species were listed as troublesome in six of the eight crops surveyed and ranked fourth overall. Similarly, Texas panicum was found in all districts and was the fifth most troublesome weed species. Texas panicum was the most troublesome weed in corn and among the top five most troublesome weeds in peanut, soybean, and tobacco. Florida beggarweed was the most troublesome weed in peanut, the second most troublesome weed in tobacco, and a top-10 weed species in corn, cotton, soybean, and vegetables, resulting in a ranking of sixth overall. Wild radish, large crabgrass, and tropic croton were the seventh through the ninth most troublesome weeds. Wild radish was the most troublesome weed of small grains and the sixth most troublesome weed of vegetables. Large crabgrass was the second most troublesome weed of forages and pastures and was reported in six other crops. Tropic croton was a troublesome weed in seven of the eight crops surveyed and was among the top five most troublesome weeds of cotton and peanut. The 10th most troublesome weed overall was bahiagrass, the most troublesome weed of forages and pastures. Nomenclature: Bahiagrass, Paspalum notatum Fluegge #3 PASNO; Florida beggarweed, Desmodium tortuosum (Sw.) DC. # DEDTO; large crabgrass, Digitaria sanguinalis L. # DIGSA; morningglory species, Ipomoea spp.; nutsedge species, Cyperus spp.; pigweed species, Amaranthus spp.; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby # CASOB; Texas panicum, Panicum texanum Buckl. # PANTE; tropic croton, Croton glandulosus var. septentrionalis Muell.-Arg. # CVNGS; wild radish, Raphanus raphanistrum L. # RAPSN; corn, Zea mays L.; cotton, Gossypium hirsutum L.; peanut, Arachis hypogaea L.; soybean, Glycine max (L.) Merr.; tobacco, Nicotiana tabacum L. Additional index words: Economically important weeds, weed distributions, weed population shifts, weed survey. Abbreviations: C, Central district; EC, East-Central district; NC, North-Central district; NE, Northeastern district; NW, Northwestern district; SC, South-Central district; SE, Southeastern district; SW, Southwestern district; SWSS, Southern Weed Science Society; WC, West-Central district.

Factors Affecting Seed Germination of Cadillo (Urena lobata)

Abstract Cadillo is an invasive species commonly found in pastures, rangelands, and disturbed areas. It is becoming a significant problem weed in Florida pastures and natural areas. The objectives of this research were to determine effective techniques to break seed dormancy and the effect of light, temperature, pH, water stress, and depth of seed burial on Cadillo germination. Cadillo seeds had significant levels of innate dormancy imposed by a hard seed coat; chemical scarification was the most effective technique for removing dormancy. Seeds germinated from 15 to 40 C, with an optimal temperature of 28 C. Germination was unaffected by pH levels. Water stress below −0.2 MPa reduced seed germination. Cadillo germination was not light-dependent and seeds emerged from depths up to 9 cm, with the greatest occurring emergence near the soil surface. Considering that Cadillo seed can germinate under a wide range of environmental conditions, it is not surprising that it has become a serious invasive weed in Florida. Nomenclature: Cadillo, Urena lobata L. URLO

Response of Three Runner Market-Type Peanut Cultivars to Diclosulam1

Field studies were conducted in 1999 and 2000 to evaluate the response of three runner market-type peanut cultivars to diclosulam applied preplant incorporated at 0,18, 27, or 54 g ai/ha in a weed-free environment. Peanut cultivars evaluated included ‘Georgia Green’, ‘C-99R’, and ‘MDR-98’. Peanut injury was not observed with diclosulam at any rate or with any cultivar. Diclosulam did not affect peanut canopy development, percentage extra-large kernels, sound mature kernels, sound splits, total sound mature kernels, other kernels, or yield for any cultivar. Nomenclature: Diclosulam; peanut, Arachis hypogaea L., ‘Georgia Green’, ‘C-99R’, ‘MDR-98’. Additional index words: Herbicide injury, extra large kernels, grade parameters, other kernels, sound mature kernels, sound splits, total kernels. Abbreviations: DAP, days after planting; ELK, extra large kernels; EPOST, early postemergence; OK, other kernels; PPI, preplant incorporated; PRE, premergence; SMK, sound mature kernels; SS, sound splits; TSMK, total sound mature kernels.

Picloram and Aminopyralid Sorption to Soil and Clay Minerals

Abstract Research was conducted to determine picloram and aminopyralid sorption in five soils and three clay minerals and to determine if the potential for off-target movement of aminopyralid in soil is less than that of picloram. Nearly all sorption of picloram and aminopyralid occurred between 0 and 8 h, and the maximum theoretical sorption of picloram and aminopyralid were 10.3 and 15.2%, respectively. Freundlich distribution coefficients (Kf) for picloram ranged from 0.12 in a Cecil sandy loam to 0.81 in an Arredondo fine sand, while Kf values for aminopyralid ranged from 0.35 in a Cecil sandy loam to 0.96 in an Arredondo fine sand. Furthermore, Kf values of aminopyralid were higher than those of picloram in all soils tested. Kf values of picloram in clay minerals were 0.25 (kaolinite), 1.17 (bentonite), and 1,016.4 (montmorillonite), and those of aminopyralid were 5.63 (kaolinite), 2.29 (bentonite), and 608.90 (montmorillonite). It was concluded that soil sorption of aminopyralid was greater than that of picloram and that the potential for off-target movement of aminopyralid is less than that of picloram. Nomenclature: Aminopyralid; picloram.

Dogfennel (Eupatorium capillifolium) Size at Application Affects Herbicide Efficacy

Abstract Dogfennel is one of the most problematic weeds in Florida pasturelands and its control can become inconsistent as the plant matures. A premix of triclopyr + fluroxypyr has been recently introduced for weed control in pastures and rangeland; however, little published information exists concerning the control of dogfennel in pastures with this herbicide combination. Therefore, experiments were initiated to determine the efficacy of triclopyr + fluroxypyr compared with commonly used pasture herbicides on dogfennel at three heights. All herbicides utilized in this study are commonly used for dogfennel control. Dogfennel control was affected by both herbicide treatment and dogfennel height. In general, 0.80 + 0.28 kg ai/ha of 2,4-D amine + dicamba resulted in inconsistent control, especially as dogfennel plants increased in size. Increasing the rate of 2,4-D amine + dicamba to 1.21 + 0.42 kg/ha increased the consistency. Triclopyr + fluroxypyr provided similar levels of control as that of 1.21 + 0.42 kg/ha 2,4-D amine + dicamba. In all locations, control of 154-cm dogfennel was signficanatly lower than that of 38-cm dogfennel. These data indicate that triclopyr + fluroxypyr is an effective option for dogfennel control, but dogfennel height at the time of application is an important factor for optimizing control. Nomenclature: 2,4-D Amine; dicamba; fluroxypyr; triclopyr; Dogfennel, Eupatorium capillifolium L.

Phytoene and β-carotene response of fluridone-susceptible and -resistant hydrilla (Hydrilla verticillata) biotypes to fluridone

Abstract Fluridone has been widely used for control of the submersed aquatic weed hydrilla in Florida for over 25 years. Recently, some hydrilla biotypes were suspected of having developed resistance to fluridone. Laboratory studies were conducted to monitor changes in phytoene and β-carotene contents as a function of suspected susceptible and resistant hydrilla biotypes to fluridone treatment. Hydrilla shoot tips from each biotype were exposed to 5, 10, 15, 20, 30, and 50 μg L−1 fluridone. Higher β-carotene and lower phytoene content was observed in all resistant hydrilla biotypes compared with the susceptible. The susceptible biotype showed an increase in phytoene or a decrease in β-carotene content when treated with as little as 5 μg L−1 of fluridone, whereas much higher doses were needed for the resistant biotypes. EC50 β-carotene values of 9 and 63 μg L−1 fluridone were found in the susceptible and the most resistant biotype, respectively. Consistent levels of hydrilla injury occurred at phytoene/β carotene index values of 5.5–7 and occurred at exposure to 5–10 μg L−1 fluridone in susceptible and 50 μg L−1 in the most resistant biotype. A resistance factor (R/S) was calculated for each hydrilla biotype which ranged from as low as 2X (R1 and R2) to 7X (R5). Aggressive spread of fluridone resistant dioecious hydrilla in aquatic ecosystems can severely impact hydrilla management, and consequently cause substantial and long-lasting ecological and economic problems throughout the southern United States. Nomenclature: Fluridone; hydrilla, Hydrilla verticillata (L.f.) Royle HYLLI.

Effects of Alternaría destruens, Glyphosate, and Ammonium Sulfate Individually and Integrated for Control of Dodder (Cuscuta pentágona)

Abstract Dodder is a serious parasitic weed in the crops in which it is a problem (particularly citrus). Alternaria destruens is the active ingredient in a registered bioherbicide for control of dodder species. In greenhouse studies, the treatments applied to citrus parasitized with field dodder were a nontreated control; oil at 7.5% v/v in water; ammonium sulfate at 0.125% w/v in water; glyphosate at 0.02 kg ae/L; A. destruens at 1.8 × 1010 spores/L; A. destruens (1.8 × 1010 spores/L) + oil at 7.5% v/v in water; and a mixture of A. destruens (1.8 × 1010 spores/L) + oil at 7.5% v/v in water + glyphosate at 0.02 kg ae/L + ammonium sulfate 0.125% w/v (the mixture treatment). The highest disease or damage severity rating out of all treatments, measured as the area under the disease or damage progress curve (AUDPC), was obtained for the mixture treatment. By 35 d after treatment, all field dodder plants that received the mixture treatment were dead but the host plant, citrus, was not. These results indicate the feasibility of integrating glyphosate, ammonium sulfate, and A. destruens to manage dodder. Nomenclature: Glyphosate; ammonium sulfate; field dodder, Cuscuta pentagona Engelm. CVCPE; citrus, Citrus spp. ‘Smooth Flat Seville’; Alternaria destruens L. Simmons, sp. nov.

Influence of Herbicide and Application Timing on Blackberry Control

Abstract Blackberry is a troublesome species across much of the southeastern United States. Control of blackberry with the pyridine herbicides is often variable among different locations. Experiments were conducted to determine whether application timing, either spring or fall, affected efficacy of the pyridine herbicides triclopyr, fluroxypyr and picloram, and metsulfuron. The pyridine herbicides provided greater control when applied in the fall. At 12 mo after treatment, fluroxypyr plus picloram and fluroxypyr plus triclopyr provided 83% control when applied in the fall and 65% when applied in the spring. Conversely, metsulfuron provided 85% control, and application timing was not significant. Although metsulfuron effectively controls blackberry, it is also highly injurious to bahiagrass. Therefore, chlorosulfuron was tested to determine whether it would provide blackberry control while not injuring bahiagrass. Blackberry control with chlorosulfuron was similar to metsulfuron. These data indicated blackberry control in bahiagrass pastures can be effectively accomplished with chlorosulfuron. Nomenclature: Chlorosulfuron; fluroxypyr; metsulfuron; picloram; triclopyr; blackberry, Rubus spp.; bahiagrass, Paspalum notatum Flüggé, ‘Pensacola’.

Mutations in Phytoene Desaturase Gene in Fluridone-Resistant Hydrilla (Hydrilla verticillata) Biotypes in Florida

Abstract Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the only U.S. Environment Protection Agency (USEPA)–approved herbicide that provides relatively long-term systemic control. Recently, hydrilla biotypes with varying levels of fluridone resistance have been documented in Florida. One susceptible and five fluridone-resistant biotypes of hydrilla varying in resistance levels were maintained in 950-L tanks under ambient sunlight and day-length conditions from September 2004 to September 2005 in absence of fluridone. Because fluridone is an inhibitor of the enzyme phytoene desaturase (PDS), the gene for PDS (pds) was cloned from fluridone-susceptible and -resistant hydrilla biotypes. Somatic mutations in amino acid 304 of hydrilla PDS are known to confer herbicide resistance. We determined pds sequence from these hydrilla biotypes at planting and 12-mo after planting. Two independent mutations at the arginine 304 codon of pds were found in the resistant hydrilla plants. The codon usage for arginine 304 is CGT, and a single point mutation yielding either serine (AGT) or histidine (CAT) was identified in different resistant hydrilla biotypes. There were no differences at codon 304 in the PDS protein of any hydrilla biotype 12-mo after planting. Several other mutations were also found in resistant pds alleles, though their possible role in herbicide resistance is unclear. Nomenclature: Fluridone; hydrilla, Hydrilla verticillata (L.f.) Royle HYLLI