Assessment of competition between crops and weeds

A two-species model was used to summarize results of four experiments with two component mixtures of barley (Hordeum vulgare L.), peas (Pisum sativum L.), Chenopodium album L. and Sinapis alba L. grown in pots in greenhouses. The results, covering a wide range of plant densities both in mixtures and pure stands, showed that the model was not always able to describe the relationship between the yield of a species and the densities of itself and its companion species. The deviations from the model were in all instances greatest for the least competitive species. The model that quantifies competition and niche differentiation appeared to be too general because competition between species was not independent of the wide plant density ranges used here. The results are discussed in relation to possible improvements of the model and an analogy between the principles of assessing crop weed competition and the assessment of the relative potency of herbicides in bioassay studies is put forward.     

Apical dominance in the shoot and its possible rôle in the survival of Paspalum distichum L.

Summary. When sequential single-node shoot segments (third to fifteenth node, counting from the apex) of the perennial grass weed Paspalum distichum L. were buried in soil, new shoot growth was significantly correlated with initial segment length. Growth from the youngest segment (third), which was about 2 cm long, was only half as great as that from segments 8 to 15, which were initially 2–3·5 times longer. When 14-node shoot segments were buried in soil, the apical bud exerted a dominating influence on shoot emergence and new shoot growth of axillary buds. The degree of suppression increased gradually up to node 8 and then decreased as the distance from the apex increased. A similar result was obtained in these shoot segments following decapitation. However, the degrees and patterns of apical and bud dominance varied in shoots collected during different seasons and also in shoots with different node numbers, node position, cutting and chilling treatments. A possible role of apical and bud dominance in P. distichum in keeping aerial shocks in reserve under adverse conditions, thus providing a survival mechanism for this weed, is discussed.     

The significance of VA-Mycorrhiza to future arable farming in the Netherlands

Present conditions of high inputs and narrow rotations render the importance of vesicular-arbuscular mycorrhiza (VAM) in arable farming in the Netherlands rather low. Changes expected in the near future under the influence of economic pressures and environmental problems may change this situation. This is discussed in the light of some such expected changes in Dutch arable farming.     

The chemical control of the lesser date moth,Batrachedra amydraula meyr., and residue levels of organophosphate insecticides in dates

Larvae of the lesser date moth (Batrachedra amydraula Meyr.) cause damage to dates in Iraq. A formulation based on a mixture of equal parts of wheat flour and pollen grains and containing either chlorpyrifos, fenitrothion or pirimiphos-methyl (all at 5 mg kg^?l) was dusted (5 g formulation bunch^?1) onto female clusters of dates at the time of pollination. Numbers of larvae, yield of dates and insecticide residues were monitored over a three-month period. Results showed that, by this technique, all three insecticides were effective in controlling the pest but that fenitrothion and chlorpyrifos were better than pirimiphos-methyl.     

Inhibition of germination of sporangia of Peronospora hyoscyami by cation deprivation: the effects of substrate and chelating agents

Germination of sporangia of Peronospora hyoscyami de Bary (tobacco blue mould) was affected by the germination substrate. Washed sporangia germinated freely in water on glass slides, but failed to germinate on agarose. Germination was reduced on detached tobacco leaves, being lower on cv. Hicks Q46 than on cv. ZZ100. Inhibition of germination by agarose was reversed by the addition to the inoculum of an extract obtained by centrifuging a suspension of non-living, powdered torula yeast Candida utilis (Hennenberg) Lodder & van Rij. Yeast extract also improved germination in vivo. The siderophore rhodotorulic acid, the chelating agents ferric and sodium citrate, and riboflavin combined with calcium and magnesium salts, also stimulated germination of sporangia on agarose. The chelating agent ethylenediaminetetraacetic acid (EDTA) inhibited germination on glass slides. Inhibition by EDTA was partially reversed by the addition of ferric salts. The inhibition of germination by agarose and stimulation by additives may have been due to effects on the availability of cations to sporangia of P. hyoscyami. The reduction in germination on tobacco leaves may have involved a similar mechanism.     

Diclofop-methyl tolerance in cultivated oats (Avena sativa L.)

The feasibility of achieving selective wild oat (Avena fatua L.) control in a stand of the Australian oat (Avena sativa L.) cv. Savena I was examined. Diclofop-methyl was applied at rates of 0·4–0·7 kg a.i. ha^?1 at the three- and five-leaf stages of A. fatua (84–132 culms m²). Although all rates of diclofop-methyl caused initial chlorosis and necrosis to the crop, the subsequent control of A. fatua permitted increased crop tillering. Avena fatua control and crop yield response were maximized when diclofop-methyl was applied at the three-leaf stage. Crop grain yield was increased by as much as 32% and 22% during 1984 and 1985, respectively. In the absence of A. fatua competition, crop shoot dry weight at ear emergence was reduced only where diclofop-methyl was applied at 0·6 or 0·7 kg ha^?1. However, final grain yield was not reduced by any treatment. Only 11 of 240 cultivars or lines tolerated diclofop-methyl, and none reached the level attained by Savena I and a closely related Australian line [(Irwin × (West × New Zealand Cape/42)) × West]/24. The nature of the inheritance of diclofop-methyl tolerance was examined (1983–1985) by crossing and backcrossing (BC) Savena I with four diclofop-methyl susceptible, but agronomically superior, lines. Inheritance of diclofop-methyl tolerance appeared to be controlled by two genes, with susceptibility being dominant to tolerance in F₃, BCIF₂ and BCIF₃ lines.     

Competition between Silene vulgaris (Moench) Garcke and alfalfa (Medicago sativa L.)

Growth indices describing the nature of the competitive relationship between Silene vulgaris and alfalfa during crop establishment were determined from experiments conducted outdoors in southem Manitoba in 1986 and 1987. The results of the experiments indicated that alfalfa dry matter yields were influenced more by intra- than interspecific competition. In comparison S. vulgaris was affected more by interspecific competition. Replacement diagrams and aggressivity indices both demonstrated that alfalfa and S. vulgaris were the aggressor and sub-ordinate species, respectively. Relative replacement rates for alfalfa versus S. vulgaris were significantly greater than unity, indicating that alfalfa exerted a greater replacement pressure on S. vulgaris than did S. vulgaris on alfalfa. Over time alfalfa could be expected to replace S. vulgaris as the dominant species within a population. Barley was more competitive than alfalfa with S. vulgaris. Not only was S. vulgaris shoot dry weight reduced more by barley than by alfalfa during the establishment year, but fewer S. vulgaris seedlings emerged the following year in plots that had been seeded to barley than in those that had been seeded to alfalfa.     

Phenological development and biomass allocation in Silene vulgaris (Moench) Garcke

Silene vulgaris (Moench) Garcke, grown from seed or from crown buds, exhibited three dis tinct growth phases; establishment, reproduc tive and post-reproductive. The establishment phase was characterized by rapid accumulation of shoot biomass, high relative growth rates (RGRs) and leaf area ratios (LARs). The sexual reproductive phase was characterized by a rapid increase in floral and seed biomass, minimal root development, and declining RGRs and LARs. In the post-reproductive phase, root and crown biomass increased rapidly concomitant with the development of regenerative buds; low RGR and LAR values occurred during this phase. Although similar growth phases were identified in both the first (grown from seed) and second years (grown from crown buds) of growth, LARs and RGRs were lower in the second than in the first year. In the second year of growth, plants exhibited a continual increase in below-ground biomass throughout growth, including the flowering period, a feature which was absent in the first year of growth.     

Effects of gamma radiation on the immature stages ofLiriomyza Trifolii

The effects of γ-radiation on the immature stages ofLiriomyza trifolii Burgess were studied with doses of 2000-25 Gy. The high doses were lethal to all stages. At doses of 750 Gy or less, larvae survived to pupate, but their number was reduced and the majority were either dead or deformed and did not give rise to adults. Eggs and prepupae were more susceptible to radiation than other stages. Larval radiosensitivity decreased with progressing instar, and pupal sensitivity decreased with increasing age. Contribution from the Agricultural Research Organization. No. 2922-E, 1990 series.