C. F. Reinhardt

Does allelopathy explain the invasiveness of Campuloclinium macrocephalum (pompom weed) in the South African grassland biome

Campuloclinium macrocephalum is an Asteraceous alien weed that invades roadside vegetation and grassland in South Africa. The role of allelopathy and competition in its invasiveness was investigated using Eragrostis curvula (weeping lovegrass, an indigenous grass), E. tef and Lactuca sativa (lettuce) as test species. Trials were conducted in Petri-dishes, pots and in the field. Root and shoot extracts of adult C. macrocephalum plants did not inhibit seed germination in any test species. The greatest effect was radicle stunting produced by leaf extracts at 10 and 25% w/v. Eragrostis curvula was less tolerant of the extracts than E. tef. Allelopathic effects could however not be confirmed in pot trials evaluating the interference potential of the weed or weed residue effects against E. curvula. E. curvula growth and biomass was not affected by plant densities of one or five C. macrocephalum per pot, whereas C. macrocephalum suffered a 17% mortality and density-dependant trade-offs of size and biomass for survival. Under field conditions C. macrocephalum had a broader ecological niche than E. curvula, invading hygrophilous and undisturbed grasslands not amenable for E. curvula establishment, this included well drained disturbed soils on which the latter proliferated. Evidence of competitive exclusion of E. curvula by C. macrocephalum or vice versa was not detected. The coexistence of both species irrespective of relative density suggested these species have different resource requirements. Allelopathy was not an adequate causal mechanism to explain invasiveness in Campuloclinium macrocephalum. A more traditional hypothesis such as the absence of natural enemies, at this stage, better justifies the weed’s invasion success.

Persistence of atrazine as affected by selected soil properties

The relationships between selected soil properties and the bioactivity of atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine) were investigated in field trials. Bioactivity was determined periodically for up to twelve months on a variety of soil types using oats (Avena sativa L.) as the test plant. The initial bioactivity of atrazine which was assessed 35 days after application was best correlated with the organic matter content (%C) and P reversion characteristics of the soils. Clay content and CEC were also important but at lower levels of significance. Six months after herbicide application, % C, soil pH and P reversion were important predictors of atrazine persistence, whilst both CEC and clay content were poor criteria. Persistence was negatively correlated with % C, but positively with both soil pH and P reversion. Since the dissipation of atrazine in most of the soils was virtually complete after twelve months, the prediction of persistence by the soil properties was poor at that stage...

The role of temperature in the germination of subterranean and aerial seeds of Commelina benghalensis L.

Information on the temperature requirements for germination of weed seeds can be used to predict the soil temperature required for weed emergence. The germination behaviour of the two types of mature C. benghalensis seeds, ie. subterranean and aerial, were studied to establish whether differences in seed germination provide these plants with an apparent competitive advantage over other weed species. Seeds of aerial and subterranean types were placed separately on germination towels and put inside plastic bags. The subterranean seeds germinated significantly faster than the aerial seeds, indicating greater thermal sensitivity of the former type of seed. The dormancy displayed by aerial seeds could inter alia be attributed to delayed germination and not to loss of viability. The optimum germination temperature for subterranean seeds was 21/28°C, and for aerial seeds 18/25°C. This broad adaptability may require continuous C. benghalensis management efforts and supports the need for an effective residual herb...

Factors affecting the activity of atrazine in plants and soil

Available research findings concerning (he activity of atrazine in the plant and soil are presented in a brief review. The role of the environment is emphasized. This review is supplemented by recent research results of the University of Pretoria concerning evaluation of soil characteristics as a norm for the prediction of atrazine activity in turf soils, the comparison of a few maize hybrids with respect to their resistance towards atrazine and the influence of moisture and temperature on atrazine activity in light-textured soils. Guidelines for future research and for the use of atrazine are given. S. Afr. J. Plant Soil 1984, 1: 67–72

Allelopathic interaction between Chenopodium album L. and certain crop species.

Laboratory and glasshouse studies were conducted to assess the allelopathic effect of Chenopodium album L. on the germination and early growth of certain crop species. Test species were grown in soils containing 1% (m/m) freeze-dried shoot material of the weed. Aqueous extracts of soils containing weed residues and organic solvent extracts of shoots of the weed were tested for effects on the germination of certain crop species. Radish (Raphanus sativus L.) was the only crop not affected by the weed residue in the soil. Growth inhibition was 68% for cucumber (Cucumis sativus L.), 85% for onion (Allium cepa L.), 47% for tomato (Lycopersicon esculentum L.) and 51% for sunflower (Helianthus annuus L.). Except for radish and sunflower, stand loss contributed to reductions in growth. Germination of crop species was not affected by aqueous extracts of soil portions (100g) from soil either grown with the weed (residues later removed by sieving) or containing shoot material of the weed. In fact, the growth of radi...

Residual effect of atrazine on field-grown dry beans and sunflower

The applicability of fixed recropping intervals for crops that are sensitive to atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-striazine) was assessed. Atrazine carry-over was monitored 12 and 24 months after its application at six different rates to maize (Zea mays L.) grown in eight trials at six sites. The rotational test species were dry beans (Phaseolus vulgaris L. cv. Teebus) and sunflower (Helianthus annuus L. cv. SO 222). Yield reductions indicated the carry-over of phytotoxic atrazine residues. At 12 months after treatment dry bean yield was significantly reduced at one site and sunflower yield at six, on plots previously treated with the atrazine rates recommended for maize. During the next season significant yield reductions (dry beans 38%; sunflower 29%) occurred only on the single montmorillonite soil (pH 7.8). The relatively high tolerance of the dry bean cultivar in most soils at the first assessment is not reflected in the single recropping period of 18 months that is currently rec...

Importance of selected soil properties on the bioactivity of acetochlor and metazachlor

Bioassays with grain sorghum (Sorghum bicolor L. Moench) cultivar NK222 were conducted in a glasshouse. Thirty-four natural soils were used in each of two experiments with alachlor [2-chloro-2′,6′-diethyl-N- (methoxymethyl) acetanilide] and metolachlor [2-chloro-6′-ethyl-N-(2-methoxy-1-methylethyl)acet-otoluidide] respectively. Linear regression analyses were performed to determine the importance of certain soil properties in the prediction of herbicide bioactivity. The order of importance for alachlor bioactivity was as follows: % C > clay % ≥ CEC P reversion ≥ soil pH; and for metolachlor: % C > clay % 3 CEC > soil pH > P reversion. Inclusion of either clay content or P reversion with % C in multiple regression equations significantly increased the predictability of alachlor and metolachlor bioactivity. Organic matter content affected alachlor and metolachlor phytotoxicity, despite the relatively low organic matter levels generally found in South African soils.

Morphological identification of the ryegrass hybrid Lolium multiflorum x Lolium perenne and isolation of the pathogen Fusarium pseudograminearum in the Western Cape

Weed resistance to herbicides present one of the greatest current economic challenges to agriculture. Herbicide resistant ryegrass (Lolium spp.) is a serious problem in Western Cape grain-producing areas. Morphological and pathogenic analyses were performed on ryegrass samples. Morphologically, 50% of specimens were classified as rigid ryegrass, 48% as a hybrid, namely L. multiflorum × L. perenne and 2% as perennial ryegrass. Fusarium pseudograminearum (cause of Fusarium crown rot) was isolated from six localities. Pathogencity tests confirmed that F. pseudograminearum isolates obtained from ryegrass and wheat are pathogenic on both crops, indicating that ryegrass can act as an alternative host and a source of inoculum of this important soilborne pathogen. Grass weed infestation can favour the disease, and grass weed control is therefore recommended as part of an integrated strategy to manage crown rot. Knowledge on morphological differences among ryegrass may be important to guide differential weed management of ryegrass. Smother cropping, as part of conservation farming, should receive more prominence to suppress herbicide-resistant ryegrass and simultaneously reduce the occurrence of crown rot.

Allelopathic effects of Chenopodium album L. and Chenopodium polyspermum L. on another weed and two crop species

Chenopodium album L. and Chenopodium polyspermum L. are cosmopolitan weeds that often cause serious crop yield losses. Although it is well known that both species compete strongly with crops, nothing has been published on their relative allelopathic potentials. Experiments for assessing the allelopathic effects of C. album and C. polyspermum on three indicator species were conducted under controlled conditions. The indicator species were sunflower (Helianthus annuus L. cv. SO 222), tomato (Lycopersicon esculentum Mill cv. Heinz 1370) and the cosmopolitan weed Indian goosegrass (Eleusine indica subsp. indica (L.) Gaertn.). Top growth of mature C. album and C. polyspermum plants was freeze-dried, milled to a coarse powder and gamma-irradiated. Steam-sterilized soil was mixed with the powdered material to reach a concentration of 1% for each donor species. The control treatments consisted of an untreated soil and the same soil containing 1% extracted material of the donor species. Only sunflower and tomato s...

High availability of alachlor affects the roots and general growth of sunflower seedlings

The tolerance to alachlor [α-chloro-2′,6′?diethyl-N-(metoxy- methyl) acetanilide] of three sunflower (Helianthus annuus L.) cultivars was evaluated in an aqueous medium in a glass- house. Alachlor (0.79 - 6.32 mg L−1) caused significant reductions in the root and shoot growth of sunflower seedlings exposed to the herbicide in nutrient solution. The roots of seed- lings exposed to alachlor became progressively shorter, thicker and darker in colour as the herbicide concentration increased. Differential cultivar tolerance was demonstrated.