The potential of biological soil disinfestation to manage Fusarium foot and root rot in Asparagus

In a field experiment on an abandoned asparagus field we studied the effect of Biological Soil Disinfestation (BSD) on survival of buried inoculum samples of three test pathogens (Fusarium redolens f.sp. asparagi (FRA), Rhizoctonia tuliparum (RT) and Verticillium dahliae (VD)) and on the Fusarium infestation level. The BSD treatments involved incorporation of grass into moist soil and covering the soil with airtight plastic. The amount of grass incorporated was varied (42, 62 or 102 tons of grass/ha) as well as the depth of incorporation (40 or 80 cm). It was found that BSD greatly reduced all three pathogens in buried soil samples and that incorporation of 62 or 102 tons of grass per ha to 80 cm soil depth resulted in a significant decrease in soil infestation in the upper 40 cm; in the deeper layer the decrease was lower. Asparagus plants grown from seed in the field for one year showed a strong decrease in Fusarium root rot severity with all BSD treatments. The results clearly show the potential of BSD to decrease soil infestation levels of Fusarium pathogens and to contribute to an enhanced life span of replanted asparagus crop

Asymmetric output profile of Xe Laser

A new set of asymmetric modes was recently revealed in a Xe slab laser with pronounced lens effects originating from gas heating in the discharge. The appearance of these modes is a threshold effect. Their domain of existence in the Xe laser is discussed. It is shown that mode competition can result in output radiation asymmetry. Theoretical results are compared with experimental data

The physics and technology of CW RF-excited Ar-He-Xe laser with output power density of 0.5 W/cm2

Research interest in the Ar-Xe laser has been strengthened considerably because this laser now produces output powers at the level of Watts and is becoming the most promising light source in the wavelength region of a few microns. The lasing occurs on the IR transitions (1.73-3.51 ¿m) between the 5d-6d manifolds of atomic xenon. Our breakthrough in this type of laser came when our results obtained with the electron beam sustained technology showed the beneficial effect of operating at high gas densities. From these observations and the accompanying modeling, the idea was supported to consider the dissociative recombination of ArXe+ or Xe2 + molecular ions with electrons as the primary channel for filling the upper laser level. Apart from this recombination process by three body collisions the increased density leads to homogeneous line broadening which has also a favourable effect on the power and efficiency. However, a homogeneous inversion density requires a homogeneous discharge in a high-density gas. This is from a technical point of view a great challenge. For continuous operation the waveguide structure with RF excitation has shown so far the best prospects for maintaining a homogeneous discharge at high densities, say above 100 Torr. The RF discharges are characterized by thin positively charged layers (sheaths) near the electrodes. They play an important role in the stabilisation of the discharge. At increasing current density the discharge switches from ¿- to ¿-mod

Early decline of asparagus in the Netherlands : etiology, epidemiology and management

Asparagus plants on fields cropped with asparagus before establish well but economic life of the crop is only half of that on fresh land. Fusarium oxysporum f.sp. asparagi was identified as the main cause of this early decline. Autotoxic compounds were detected in residues of asparagus roots even 11 years after the crop was finished but evidence for a role of these compounds in the etiology of the disease was not obtained. The pathogen was found on asparagus seed, on one-year-old crowns used as planting material and was isolated from soil, frequently even from fields without an asparagus history. In inoculation experiments, the pathogen caused severe root rot in asparagus, mild root rot occasionally in pea and lupin, and it colonized the roots of many plant species that did not show symptoms. Twenty-four isolates of the pathogen were assigned to 18 vegetative compatibility groups indicating the large genetical diversity of the population in the Netherlands. The potential of nonpathogenic isolates of F. oxysporum to reduce severity of Fusarium root rot was shown in growth chamber and greenhouse tests but not when plants were grown in the field for one year. A new method for reducing soil infestation with Fusarium oxysporum f.sp. asparagi and other soilborne pathogens was developed. It is based on induction of fermentative soil conditions. When fresh broccoli or grass was incorporated into soil that was subsequently irrigated and covered with plastic mulch, oxygen in soil was rapidly depleted and redox potential (Eh) reached values as low as -200 mV. After 15 weeks, populations of F. oxysporum f.sp. asparagi , Rhizoctonia solani , Verticillium dahliae , and Globodera pallida were strongly reduced in inoculum samples buried in soil demonstrating the potential of this approach to control a range of soilborne pathogens

The effect of the discharge frequency on the performance of RF excited CO2 waveguide lasers

Research interest in the Ar-Xe laser has been strengthened considerably because this laser now produces output powers at the level of Watts and is becoming the most promising light source in the wavelength region of a few microns. The lasing occurs on the IR transitions (1.73-3.51 ¿m) between the 5d-6d manifolds of atomic xenon. Our breakthrough in this type of laser came when our results obtained with the electron beam sustained technology showed the beneficial effect of operating at high gas densities. From these observations and the accompanying modeling, the idea was supported to consider the dissociative recombination of ArXe+ or Xe2 + molecular ions with electrons as the primary channel for filling the upper laser level. Apart from this recombination process by three body collisions the increased density leads to homogeneous line broadening which has also a favourable effect on the power and efficiency. However, a homogeneous inversion density requires a homogeneous discharge in a high-density gas. This is from a technical point of view a great challenge. For continuous operation the waveguide structure with RF excitation has shown so far the best prospects for maintaining a homogeneous discharge at high densities, say above 100 Torr. The RF discharges are characterized by thin positively charged layers (sheaths) near the electrodes. They play an important role in the stabilisation of the discharge. At increasing current density the discharge switches from ¿- to ¿-mod

Transverse Optical Mode Patterns for an RF Excited Ar-He-Xe Laser

Transverse optical modes for an RF excited Ar-He-Xe laser are studied both experimentally and theoretically. A diffraction model for a waveguide with a nonsaturable gain and refractive index gradients placed between two plane mirrors is formulated. The effects of gain and diffraction index gradients and of diffraction in free space are evaluated for typical experimental conditions. A direct comparison between theoretical mode patterns and experimentally measured ones at distances of 17 and 114 cm from the output mirror demonstrated a satisfactory agreement for various laser wavelengths and gas mixture composition

Peripheral fillings of relatively hyperbolic groups

A group theoretic version of Dehn surgery is studied. Starting with an arbitrary relatively hyperbolic group $G$ we define a peripheral filling procedure, which produces quotients of $G$ by imitating the effect of the Dehn filling of a complete finite volume hyperbolic 3--manifold $M$ on the fundamental group $\pi_1(M)$. The main result of the paper is an algebraic counterpart of Thurston's hyperbolic Dehn surgery theorem. We also show that peripheral subgroups of $G$ 'almost' have the Congruence Extension Property and the group $G$ is approximated (in an algebraic sense) by its quotients obtained by peripheral fillings. Various applications of these results are discussed.Comment: The difference with the previous version is that Proposition 3.2 is proved for quasi--geodesics instead of geodesics. This allows to simplify the exposition in the last section. To appear in Invent. Mat