A model of plant virus disease epidemics in asynchronously-planted cropping systems

A mathematical model was developed of the dynamics of a plant virus disease within a spatially-referenced lattice of fields of a host crop. The model can be applied to crops in continuous, contiguous cultivation such as tropical irrigated rice. Disease progress in each field of the host crop was assumed to be logistic and determined by incidence within the field itself as well as incidence in neighbouring fields, depending on the gradient of disease spread. The frequency distribution of planting dates (represented by the proportion of the total number of fields planted in successive months) was assumed to follow a normal distribution and the variance of planting date was used as a measure of cropping asynchrony. Analysis of the model revealed that disease incidence within the lattice (i.e. mean incidence over all fields) depended upon the infection efficiency, the slope of the dispersal gradient, and the variance in planting date. Disease endemicity depended mainly on planting date variance and disease persisted in the lattice if this variance exceeded a certain threshold. Above the threshold for persistence, the response of mean disease incidence to planting date variance was non-linear and the region of greatest sensitivity was closest to the threshold. Thus, disease systems that show moderate rather than high cropping asynchrony are more likely to be influenced by changes in the variance of planting date. Implications for the area-wide management of rice tungro virus disease are discussed.     

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.     

Metabolic Profiling and In Vitro Assessment of the Biological Activities of the Ethyl Acetate Extract of Penicillium chrysogenum “Endozoic of Cliona sp. Marine Sponge” from the Red Sea (Egypt)

Marine sponge-derived endozoic fungi have been gaining increasing importance as promising sources of numerous and unique bioactive compounds. This study investigates the phytochemical profile and biological activities of the ethyl acetate extract of Penicillium chrysogenum derived from Cliona sp. sponge. Thirty-six compounds were tentatively identified from P. chrysogenum ethyl acetate extract along with the kojic acid (KA) isolation. The UPLC-ESI-MS/MS positive ionization mode was used to analyze and identify the extract constituents while 1D and 2D NMR spectroscopy were used for kojic acid (KA) structure confirmation. The antimicrobial, antioxidant, and cytotoxic activities were assessed in vitro. Both the extract and kojic acid showed potent antibacterial activity against Staphylococcus aureus and Pseudomonas aeruginosa with MIC 250 ± 0.82 µg/mL. Interestingly, the extract showed strong antifungal activity against Candida albicans and Cryptococcus neoformans with MIC 93.75 ± 0.55 and 19.53 ± 0.48 µg/mL, respectively. Furthermore, KA showed the same potency against Fusarium oxysporum and Cryptococcus neoformans with MIC 39.06 ± 0.85 and 39.06 ± 0.98 µg/mL, respectively. Ultimately, KA showed strong antioxidant activity with IC₅₀ 33.7 ± 0.8 µg/mL. Moreover, the extract and KA showed strong cytotoxic activity against colon carcinoma (with IC₅₀ 22.6 ± 0.8 and 23.4 ± 1.4 µg/mL, respectively) and human larynx carcinoma (with equal IC₅₀ 30.8 ± 1.3 and ± 2.1 µg/mL, respectively), respectively. The current study represents the first insights into the phytochemical profile and biological properties of P. chrysoenum ethyl acetate extract, which could be a promising source of valuable secondary metabolites with potent biological potentials.     

A bibliography of shrimp and prawn nutrition

This paper contains a bibliography of papers published on dietary and nutritional work with shrimps and prawns. In addition, selected papers on related topics, such as the nutrition of other crustacea, the natural food of shrimps and prawns, and physiology, are included. The bibliography is intended to be an aid to existing workers in the field of shrimp and prawn nutrition, to be a launch pad for scientists entering this field for the first time, and to assist those wishing to review specific aspects of this subject.     

Biodegradation of litter in Eucalyptus pauciflora communities—II. Fungal succession in fungicide- and insecticide-treated leaves

Fungal succession on untreated and fungicide- and insecticide-treated leaves of Eucalyptus pauciflora was compared. Natural succession on untreated leaves fits the general scheme for succession on deciduous tree leaves. Initial colonizers are species of Coelomycetes and Moniliales. When the numbers of Coelomycetes decrease in the late stages of decomposition, the frequency of Penicillium species and species of Zygomycetes increases. The succession is affected only marginally by DDT and dieldrin treatment while benomyl severely restricts the occurrence of most fungal species. The effect of benomyl persists even after exposure for 48 weeks in the field.     

Relations between stratigraphy,geomorphology and soils in Coastal Plain areas of Southeastern U.S.A.

Small infiltrometers were used to measure infiltration rates into strongly and maderately developed compound prisms, sampled individually from surface layers of two dry clay soils. Initially, both rates were equally high but after three hours a higher rate was measured for the moderately developed compound prisms. These data were used to interpret adsorption and outflow phenomena of water in undisturbed, large and initially dry soil cores (diameter 20 cm) sampled from the upper 20 cm of the two soils, following application rates of about 8 mm/h and 28 mm/h. The two soils adsorbed an average of 6.4 mm water before the start of discharge, independent of the applied intensity. They discharged water at a constant and high rate for several hours, allowing adsorption of about 10% of the applied water. This represents a high degree of “short-circuiting” for all treatments, except one: adsorption was 40% at the 28 mm/h rate for the soil with the moderately developed prisms. The moisture content in the transmission zone was not constant as it would have been in homogeneous soil but increased regularly as a function of time. Slow wetting of both soils through a crust, followed by rain with an intensity of 30 mm/h, resulted in the adsorption of only about 3% of the applied rain in both soils.In general, more effective wetting of a dry clay surface soil at a given rainfall intensity is initially associated with less “short-circuiting” to deeper horizons. However, the resulting higher moisture content allows less adsorption in the surface layer and is therefore associated with increased “short-circuiting” thereafter, as long as large vertical pores have not been closed by swelling.