Preliminary investigation of the causal agent(s) of a disease causing leaf curl of tobacco in South Africa

A high incidence (86%) of potyvirus infection was noted in tobacco plants exhibiting a form of leaf curl in South Africa. Despite leaf curl being reported in the literature to be of geminiviral aetiology, no geminiviruses were detected. Furthermore, no other virus particles were detected by virus purification, TEM and serology. Twelve species of dsRNA were consistently isolated from these tobacco plants, but were absent from other forms of leaf curl-affected and healthy tobacco. Aphid and mechanical inoculation demonstrated that the purified potyvirus(es) did not cause leaf curl symptoms, but rather mild mottle and mosaic symptoms in tobacco. Partial characterization of the potyvirus preparation showed a possible relationship to a South African strain of potato virus Y. Because potyvirus-inoculated plants did not manifest leaf curl symptoms, and because leaf curl symptoms were noted in some plants not infected with a potyvirus, it was concluded that the potyvirus is not involved in the leaf curl aetiology, but causes a latent infection, the symptoms of which are masked. The pattern of the dsRNA banding, induction of enations and lack of mechanical and seed transmission are common to plant reoviruses. The possibility of a phytoreovirus involvement in this form of leaf curl is currently being investigated. The results from this study suggest that tobacco leaf curl disease worldwide, with regard to geminiviruses, be re-evaluated.     

The effects of ultraviolet-B (UV-B: 290–320 nm) radiation on blister blight disease of tea (Camellia sinensis)

Sunlight is known to reduce the incidence of blister blight disease of tea, caused by Exobasidium vexans . The effects of the ultraviolet-B (UV-B: 290–320 nm) component of solar radiation on the disease and the development of E. vexans were studied in the field in Sri Lanka using UV-screening filter materials held over a commercial crop. Exclusion of UV-B radiation by Polyester, which reduces fluxes in the UV-B region by approximately 75–85%, increased both the number of translucent spots (immature sites of infection) and number of sporulating blisters (mature sites of infection), but it had little or no effect on the sporulation of the pathogen. When basidiospores were artificially inoculated on leaves, and were exposed to full or filtered solar radiation, their survival and germination increased when UV-B wavelengths were removed; significant differences were found 62 h after inoculation. However, UV-B did not affect the extension of germ tubes. This study indicates that the UV-B component of solar radiation plays an important role in the natural regulation of blister blight disease in the field.     

Modelling crop:weed interactions in wheat with ALMANAC

ALMANAC is a dynamic model for plant growth, water balance and soil nitrogen dynamics that can simulate on a daily basis two or more competing species. The simulation of competition for light is based on Beer's law, allowing a different extinction coefficient ( k ) for each species. Light is partitioned between species based on k -values, leaf area index and plant heights. Total hiomass is simulated with radiation use efficiency and grain yield with a harvest index approach, sensitive to water stress. The model simulates competition for water and nutrients based on each species current rooting zone and demand by each species. The effect of crop management on the competition issue can he simulated. The model was evaluated in Dijon (France) using 4 years of experimental data on wheat:oat mixtures, differing in oat ( Avena sativa L.) densities, the period of oat emergence, the date of weed suppression by herbicides and the wheat ( Triticun aestivum L.) genotype. Additional data on oilseed rape ( Brassica nupus L.) and vetch ( Vilcia sauiva L.) competition in spring wheat were also used. The wheat grain yield was reasonably simulated with a root mean square error (RMSE) of 0.10-0.35 t ha^-1. Corresponding values for oats were 0.10-0.55 t ha^-1. The competitiveness of oats, oilseed rape and vetch was correctly simulated. The model appears as a reasonable tool for estimating damage thresholds in integrated weed control programmes.     

Role of ovary-inhibiting hormone from eyestalks of marine penaeid prawns (Parapenaeopsis hardwickii) during ovarian developmental cycle

The activity of the ovary-inhibiting hormone in the eyestalks of female prawns, Parapenaeopsis hardwickii, was determined during its ovarian developmental cycle. In the inactive (stage I) and post-spawned phases of ovarian development, the ovary-inhibiting hormone activity was found to be very high. During the phase of full vitellogenesis (stage IV), the inhibiting activity is almost negligible. After spawning (stage V) it is present, but at a lower level than during inactive and post-spawned phases. Hence it is inferred that ovary-inhibiting hormone appears to be responsible for the phase of ovarian quiescence and to control the rate of vitellogenesis.     

Proposals for a Sangay National Park in Ecuador

This paper describes the main features of the Sangay wilderness area in the Ecuadorian Andes. It is a region of great diversity with life zones ranging from tundra to subtropical rain forest. The conservation value of the area is assessed and the feasibility of protection determined. Boundaries based mainly on the limits of human interference are put forward with management proposals for its establishment as a National Park.     

Nitrogen fixation by white clover in hill pasture

An experiment is described in which the amount of nitrogen fixed by the white clover component of a surface-sown hill sward was determined using small quantities of ¹⁵N-labelled ammonium sulphate. Between 11 July and 22 August 1974 25 kg ha^?1 N was fixed. No evidence of a transfer of N from clover to the associated perennial ryegrass was detectable over this period although the N content of the grass growing with the clover was higher. The relative advantages of the isotope technique and the acetylene reduction assay are discussed.     

THE ELECTRONIC MEASUREMENT OF SMALL HERBAGE YIELDS USING A HIGH SENSITIVITY CAPACITANCE METER

The capacitance meters available in the past for estimating herbage yields have lacked sensitivity at the extremes of their range of measurement. The preliminary evaluation of a highly sensitive capacitance meter described here indicates that accurate measurement is possible in the range 0.1100 kg DM/ha. The implications of this development for the measurement of herbage of low availability are discussed.