Groundnut viral diseases in West Africa

This paper describes groundnut viral diseases observed in West Africa. Six viruses are identified and their main properties are reported here: peanut Clump, groundnut rosette, groundnut eyespot, groundnut crinkle, tomato spotted wilt and groundnut chlorotic spotting viruses. Four other diseases are described in part: groundnut streak, groundnut mosaic, groundnut flecking and groundnut golden mosaic diseases. Some of them are economically very important such as the two strains of rosette, peanut clump and tomato spotted wilt diseases. Others are apparently of minor importance though they occur relatively frequently and show a wide distribution, such as groundnut eyespot, groundnut crinkle, groundnut streak and groundnut golden mosaic diseases. The others appear occasionally but are nevertheless described: some which are very infectious, as groundnut chlorotic spotting disease could become very important within a few years. (Résumé d'auteur

Isolation of phytoplasma DNA from the coconut palms (Cocos nucifera L.) collected from Ghana

This study aimed to verify the presence of the causative agent of Lethal Yellowing which is phytoplasma in samples provided from infected coconut trees. Study was carried out by using various samples like zygotic embryo, young leaves and immature & mature inflorescences. These materials were collected from trees at the stage 1 and 2 of the disease development.. Stage 1 of disease development is characterized by leaf yellowing and the start of the falling nuts while at the stage 2 of disease development, the trees has not bear nuts longer. From infected material, DNA was extracted by three different processes and isolated DNA was amplified by PCR. 16S rRNA gene was amplified by two specific primers of phytoplama viz P1/P2 and Ghana 813/AKSR. Among the various tested materials presence of phytoplasma was reported from the mature inflorescences while the presence of the phytoplasma was not reported from the leaves and embryos of the coconut

Foam on troubled water: Capillary induced finite-time arrest of sloshing waves

International audienceInterfacial forces exceed gravitational forces on a scale small relative to the capillary length-two millimeters in the case of an air-water interface-and therefore dominate the physics of sub-millimetric systems. They are of paramount importance for various biological taxa and engineering processes where the motion of a liquid meniscus induces a viscous frictional force that exhibits a sublinear dependence in the meniscus velocity, i.e. a power law with an exponent smaller than one. Interested in the fundamental implications of this dependence, we use a liquid-foam sloshing system as a prototype to exacerbate the effect of sublinear friction on the macroscopic mechanics of multi-phase flows. In contrast to classical theory, we uncover the existence of a finite-time singularity in our system yielding the arrest of the fluid's oscillations. We propose a minimal theoretical framework to capture this effect, thereby amending the paradigmatic damped harmonic oscillator model. Our results suggest that, although often not considered at the macroscale, sublinear capillary forces govern the friction at liquid-solid and liquid-liquid interfaces

Flow of foam through a convergent channel

International audienceWe study experimentally the flow of a foam confined as a bubble monolayer between two plates through a convergent channel. We quantify the velocity, the distribution and orientation of plastic events, and the elastic stress, using image analysis. We use two different soap solutions: a sodium dodecyl sulfate (SDS) solution, with a negligible wall friction between the bubbles and the confining plates, and a mixture containing a fatty acid, giving a large wall friction. We show that for SDS solutions, the velocity profile obeys a self-similar form which results from the superposition of plastic events, and the elastic deformation is uniform. For the other solution, the velocity field differs and the elastic deformation increases towards the exit of the channel. We discuss and quantify the role of wall friction on the velocity profile, the elastic deformation, and the rate of plastic events

Structure-dependent mobility of a dry aqueous foam flowing along two parallel channels

The velocity of a two-dimensional aqueous foam has been measured as it flows through two parallel channels, at a constant overall volumetric flow rate. The flux distribution between the two channels is studied as a function of the ratio of their widths. A peculiar dependence of the velocity ratio on the width ratio is observed when the foam structure in the narrower channel is either single staircase or bamboo. In particular, discontinuities in the velocity ratios are observed at the transitions between double and single staircase and between single staircase and bamboo. A theoretical model accounting for the viscous dissipation at the solid wall and the capillary pressure across a film pinned at the channel outlet predicts the observed non-monotonic evolution of the velocity ratio as a function of the width ratio. It also predicts quantitatively the intermittent temporal evolution of the velocity in the narrower channel when it is so narrow that film pinning at its outlet repeatedly brings the flow to a near stop