Resistance responses of Abutilon theophrasti were investigated to determine defense mechanisms of the weed against Colletotrichum coccodes and to verify if some chemical suppression of the resistance mechanism could be exploited to enhance the virulence. Induced resistance in A. theophrasti has been confirmed in treatments with C. coccodes, benzothiadiazole, bentazon, and acifluorfen. Induction of peroxidase and phenylalanine ammonia lyase (PAL) activities in the leaves that did not contact with the inducing agents was observed after the localized stresses to the first leaf or the root of the plant with those agents. alpha-Amino-oxy acetic acid (AOA), 2-deoxy-D-glucose (DDG), mannose, oxalic acid, and analogues of oxalic acid and mannose were tested to enhance C. coccodes virulence. However, the compounds did not enhance C. coccodes virulence or affect A. theophrasti growth. Strong antifungal effects, poor inhibitory effects on plant defense mechanisms, or minor dependence of A. theophrasti on the defense mechanisms that the chemicals affected could be reasons. The efficacy of C. coccodes increased in the presence of 0.25 kg a.i. ha-1 bentazon more than when C. coccodes was applied alone, while the effect of glyphosate was minimal. Peroxidase activity was strongly induced by the treatment of C. coccodes and increased over time. PAL and activation of peroxidase was inhibited in the presence of bentazon, suggesting the synergy effect by bentazon is probably due to the suppression on the two defense-related enzymes. In conclusion, A. theophrasti exploits various biochemical and morphological types of defense mechanisms against C. coccodes infection. However, the activation of the defense responses can be suppressed or by-passed in an integrated weed management system.

Mechanical weeders were tested on both mineral and organic soils in 1995 and in 1996, to measure their impact on soil physical properties. The weeders were tested in three crops, carrots in mineral and organic soil, beans in mineral soil, and lettuce in organic soil. Soil bulk density and water content were measured at regular intervals during the weeder trial period, and samples for soil structural analysis were collected in the mineral soil at the beginning and at the end of each season. / No significant results were found in differences in mean weight diameters of dry and water-stable aggregates. Soil moisture did not vary significantly due to treatment. There was significant compaction caused by the first pass in the field (i.e., seeder) in all the crops at both sites in the 0--10 cm layer, and in some cases, to a depth of 25 cm. Additional field passes with the weeders caused some compaction in the tractor wheel at the 0--10 cm depth, and sometimes in the 10--25 cm layer, in all trials in mineral soil, yet no significant effects of treatments were detected in the trials at the organic soil site. No conclusions could be drawn as to the benefits and/or negative impacts of mechanical versus chemical weeding in mineral soil, with respect to the relative compaction caused by the mechanical weeders. This is because in this experiment spraying of herbicides did not involve tractor traffic in the field.

No description available.

Field and growth bench experiments were performed to assess the effect of a selective fungal pathogen of Abutilon theophrasti (velvetleaf) on various aspects of intra- and interspecific competition between this vigorous agricultural weed and soybean (Glycine max). In the absence of the foliar pathogen, Colletotrichum coccodes, A. theophrasti and soybean responded differently to the presence of conspecies or to individuals of the other species. In pure stand, the deleterious effects of intraspecific competition on reproductive output were substantially greater for A. theophrasti than for soybean, especially at lower monoculture densities. In mixtures, however, A. theophrasti reproductive performance was markedly higher than at equivalent monoculture densities, particularly at the lower mixture densities. Soybean reproduction at these lower mixture densities (10 to 20 plants m$ sp{-2}$) was severely curtailed compared with reproductive output at equivalent pure stand densities. A. theophrasti reproductive output was limited more by the presence of conspecies than by the presence of soybean, whereas the opposite trend was observed for soybean. In pure stand, application of C. coccodes had limited impact on either A. theophrasti or soybean yield. However, application of the fungal pathogen in A. theophrasti monocultures caused significant (30-44%) aboveground biomass reductions within five weeks of inoculation, in two of the three years in one field study. Eight weeks following C. coccodes inoculation, A. theophrasti biomass within inoculated monoculture plots did not differ significantly from biomass within uninoculated control plots, although height hierarchies were significantly more developed. In mixtures, C. coccodes applications caused reductions in A. theophrasti growth and reproduction when provided with an adequate dew period. Alternatively, soybean yield losses within inoculated mixture plots were generally lower than for uninoculated control plots, althoug

Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.

Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.

Weeds severely limit agricultural production in many Third World tropical countries where productivity is already marginal due to unfavourable environmental, biological, economic and social factors. Lack of both ecological and distributional information at the scale of the weed assemblage, and lack of detailed data on the life histories and responses of the constituent weedy species hinders development of cheap and effective weed management strategies not contingent on intensive use of herbicides. / Plant geographers can contribute to the solution of tropical weed problems by investigating factors which mediate the dynamics of species: regional and local ranges. The research in this thesis departs from other plant geographic studies on tropical week assemblages (e.g. Kellman, 1973) by undertaking experimental field studies which are designed to uncover aspects of the differential abilities of species to make the transition from localized immigrant population to widespread and abundant weed. / The research focusses on six alien species invading agricultural and non-agricultural land in a lowland tropical region: coastal Guyana. Experiments are directed towards propositions concerning the basis for differential colonizing success of two common (Echinochloa colonum (L) Link, Macroptilium lathyroides L.) and four uncommon (Asclepias curassavica L., Emilia fosbergii D. H. Nicholson, Malachra alceifolia Jacq., Paspalum virgatum L.) species of cultivated fields in an area of prime agricultural land. Experimental colonizing trials, which equalize the opportunities of each species to colonize, provide a reference point against which to assess inter- and intra-species differences. / Results of colonizing trials suggest that the species represent four ecological groups. These are Macroptilium and Emilia (Group 1), Echinochloa (Group 2), Malachra and Asclepias (Group 3), and Paspalum (Group 4). / Species of Group 1 have the highest potential for establishment in cultivated fields within a single growing season. Emilia, though currently not a prominent species in the study area, should be controlled early. Heavy insect predation and consequent slow maturity of Asclepias reduce the likelihood of its rapid establishment in the area. Malachra and Paspalum, though not predated, have slow generation times, and hence are also unlikely to rapidly invade frequently cultivated fields. However, the high incidence of interruption of cultivation in the coastal farmlands is likely to promote their range expansion.

An anthracnose-inducing pathogen, Colletotrichum dematium, was studied as a bioherbicide for Epilobium angustifolium. A comparative study involving other C. dematium isolates suggests that the isolate from E. angustifolium is a forma specialis and should be designated as Colletotrichum dematium f.sp. epilobii. The most severe damage was achieved on seedlings using a conidial density of $1 times 10 sp9$ conidia m$ sp{-2}$. Virulence decreased with plant maturity. Satisfactory levels of control were limited to long dew duration ($>$18 h) and high temperature treatments. Of various adjuvants tested, significantly higher levels of control were achieved when inoculum was sprayed in a vegetable oil emulsion (25% v/v). An inoculum buffered to acidic pH levels (pH 3.0) similarly increased level of control compared with an unbuffered conidial suspension and the adjustment to more alkaline pH levels using the citrate-phosphate buffer (pH $>$ 4.0), inhibited disease expression. In host range studies, C. dematium f.sp. epilobii was restricted to the Onagraceae family. Both Camissonia bistorta and Clarkia pulchella were susceptible whereas the fungus was highly virulent on all but one E. angustifolium ecotype. Among nine commercial tree species tested for susceptibility yellow birch (Betula alleghaniensis) was susceptible to the fungus when conidia were applied in an oil emulsion. The application of inoculum in a tank mix combination with the oil emulsion and a low rate of glyphosate provided significant growth control of E. angustifolium seedlings (7-wk-old), whereas the effectiveness of this suspension significantly decreased with plant maturity. Under controlled conditions, post-emergence application of an oil-based formulation including the ground colonized substrate of another bioherbicide candidate of E. angustifolium, Alternaria sp., significantly reduced above-ground biomass when provided a 12-h dew and applied at a rate as low as $5 times 10 sp6$ conidia m$ sp{-2}$.

Field experiments were conducted from 1989 to 1991 to evaluate several sulfonylurea herbicides, glyphosate and clopyralid for the control of bunchberry and other hexazinone tolerant weeds in lowbush blueberry. Broadcast applications of chlorosulfuron, metsulfuron and glyphosate reduced bunchberry densities at all application dates, though crop damage and subsequent yield reductions were unacceptable. Glyphosate was very effective in controlling a large number of plant species when applied as a spot spray treatment. Tribenuron and DPX R9674 were effective in suppressing bunchberry stem densities at all application dates, without major adverse effects on blueberry, and also controlled a large number of hexazinone tolerant weeds when applied as a spot spray treatment. Clopyralid, at rates as low as 100 g a.i. ha$ sp{-1}$, was very effective as a broadcast treatment for the control of tufted vetch, although problems with crop tolerance and yield reductions were evident in some instances. Clopyralid did not control a large number of hexazinone tolerant species when applied as a spot spray treatment.

No description available.