82 research outputs found
Efficacy of fungicides, plant resistance activators and biological control agents against guava wilt disease caused by Nalanthamala psidii
Guava wilt disease (GWD) caused by the fungus Nalanthamala psidii remains a major
constraint to guava production in South Africa and South East Asia. In the current study,
chemical and biological products as well as plant resistance activators were evaluated for
control of GWD in shadehouse and glasshouse trials. In all trials, one-year-old âTS-G2â
guava plants were used. Plants were inoculated with a macerated culture suspension of a
mixture of three isolates of N. psidii after artificial wounding of the roots. Products were
applied as a soil drench or as a full cover spray. In trial 1 plants were evaluated according
to a disease severity scale. In trial 2 and 3 data were recorded as number of dead plants at
the termination of the trial. None of the chemical treatments caused a significant
suppression of the disease. The best control was achieved with the combination of
rhizobacterial strains Bacillus cereus S7 and Paenibacillus alvei T29 resulting in 53.4%
and 50% disease control in trials 2 and 3 respectively. This treatment also seems to have a
plant growth enhancing effect apart from disease suppression. This is to our knowledge
the first report of control of GWD by means of bacterial antagonists.http://www.tandfonline.com/loi/tjps202017-11-30hb2017Microbiology and Plant Patholog
Diversity and biogeographical patterns of legumes (Leguminosae) indigenous to southern Africa
The principal aim of this study was to establish biogeographical patterns in the legume flora of southern
Africa so as to facilitate the selection of species with agricultural potential. Plant collection data from the
National Herbarium, South Africa, were analysed to establish the diversity and areas covered by legumes
(Leguminosae/Fabaceae) indigenous to South Africa, Lesotho and Swaziland. A total of 27,322 records
from 1,619 quarter degree grid cells, representing 1,580 species, 122 genera and 24 tribes were included
in the analyses. Agglomerative hierarchical clustering was applied to the presence or absence of legume
species in quarter degree grid cells, the resultant natural biogeographical regions (choria) being referred
to as leguminochoria. The description of the 16 uniquely formed leguminochoria focuses on defining
the associated bioregions and biomes, as well as on the key climate and soil properties. Legume species
with a high occurrence in a leguminochorion are listed as key species. The dominant growth form of key
species, species richness and range within each leguminochorion is discussed. Floristic links between the
leguminochoria are established, by examining and comparing key species common to clusters, using a
vegetation classification program. Soil pH and mean annual minimum temperature were found to be the main drivers for distinguishing among legume assemblages. This is the first time that distribution data for
legumes has been used to identify biogeographical areas covered by leguminochoria on the subcontinent.
One potential application of the results of this study is to assist in the selection of legumes for pasture
breeding and soil conservation programs, especially in arid and semi-arid environments.This paper is dedicated to the memory of Robert Howard (Bobby) Westfall
(17 December 1944â21 January 2016), vegetation ecologist and friend
whose sudden death during the preparation of this manuscript deprived
us of an invaluable collaborator.Financial assistance from the University of Pretoria is acknowledged
with thanks.http://phytokeys.pensoft.nethttp://opendatacommons.org/licenses/odbl/1.0/am2016Plant Scienc
Diversity and biogeographical patterns of legumes (Leguminosae) indigenous to southern Africa
This paper is dedicated to the memory of Robert Howard (Bobby) Westfall
(17 December 1944â21 January 2016), vegetation ecologist and friend
whose sudden death during the preparation of this manuscript deprived
us of an invaluable collaborator.The principal aim of this study was to establish biogeographical patterns in the legume flora of southern
Africa so as to facilitate the selection of species with agricultural potential. Plant collection data from the
National Herbarium, South Africa, were analysed to establish the diversity and areas covered by legumes
(Leguminosae/Fabaceae) indigenous to South Africa, Lesotho and Swaziland. A total of 27,322 records
from 1,619 quarter degree grid cells, representing 1,580 species, 122 genera and 24 tribes were included
in the analyses. Agglomerative hierarchical clustering was applied to the presence or absence of legume
species in quarter degree grid cells, the resultant natural biogeographical regions (choria) being referred
to as leguminochoria. The description of the 16 uniquely formed leguminochoria focuses on defining
the associated bioregions and biomes, as well as on the key climate and soil properties. Legume species
with a high occurrence in a leguminochorion are listed as key species. The dominant growth form of key
species, species richness and range within each leguminochorion is discussed. Floristic links between the
leguminochoria are established, by examining and comparing key species common to clusters, using a
vegetation classification program. Soil pH and mean annual minimum temperature were found to be the main drivers for distinguishing among legume assemblages. This is the first time that distribution data for
legumes has been used to identify biogeographical areas covered by leguminochoria on the subcontinent.
One potential application of the results of this study is to assist in the selection of legumes for pasture
breeding and soil conservation programs, especially in arid and semi-arid environments.Financial assistance from the University of Pretoria is acknowledged
with thanks.http://phytokeys.pensoft.nethttp://opendatacommons.org/licenses/odbl/1.0/am2016Plant Scienc
Probing the atmosphere of a solar-like star by galactic microlensing at high magnification
We report a measurement of limb darkening of a solar-like star in the very
high magnification microlensing event MOA 2002-BLG-33. A 15 hour deviation from
the light curve profile expected for a single lens was monitored intensively in
V and I passbands by five telescopes spanning the globe. Our modelling of the
light curve showed the lens to be a close binary system whose centre-of-mass
passed almost directly in front of the source star. The source star was
identified as an F8-G2 main sequence turn-off star. The measured stellar
profiles agree with current stellar atmosphere theory to within ~4% in two
passbands. The effective angular resolution of the measurements is <1
micro-arcsec. These are the first limb darkening measurements obtained by
microlensing for a Solar-like star.Comment: Accepted for publication in A&A Letters. 5 pages, 2 embedded colour
ps figures plus 1 jpg figure. Version with all figures embedded available
from: http://www.roe.ac.uk/~iab/moa33paper
Limits on additional planetary companions to OGLE-2005-BLG-390L
We investigate constraints on additional planets orbiting the distant M-dwarf
star OGLE-2005-BLG-390L, around which photometric microlensing data has
revealed the existence of the sub-Neptune-mass planet OGLE-2005-BLG-390Lb. We
specifically aim to study potential Jovian companions and compare our findings
with predictions from core-accretion and disc-instability models of planet
formation. We also obtain an estimate of the detection probability for
sub-Neptune mass planets similar to OGLE-2005-BLG-390Lb using a simplified
simulation of a microlensing experiment. We compute the efficiency of our
photometric data for detecting additional planets around OGLE-2005-BLG-390L, as
a function of the microlensing model parameters and convert it into a function
of the orbital axis and planet mass by means of an adopted model of the Milky
Way. We find that more than 50 % of potential planets with a mass in excess of
1 M_J between 1.1 and 2.3 AU around OGLE-2005-BLG-390L would have revealed
their existence, whereas for gas giants above 3 M_J in orbits between 1.5 and
2.2 AU, the detection efficiency reaches 70 %; however, no such companion was
observed. Our photometric microlensing data therefore do not contradict the
existence of gas giant planets at any separation orbiting OGLE-2005-BLG-390L.
Furthermore we find a detection probability for an OGLE-2005-BLG-390Lb-like
planet of around 2-5 %. In agreement with current planet formation theories,
this quantitatively supports the prediction that sub-Neptune mass planets are
common around low-mass stars.Comment: 10 pages, 4 figures, accepted by A&
The unusual optical afterglow of the gamma-ray burst GRB 021004: Color changes and short-time-scale variability
We report UBVRI observations of the optical afterglow of the gamma-ray burst
GRB 021004. We observed significant (10-20%) deviations from a power law decay
on several time scales, ranging from a few hours down to 20-30 minutes. We also
observed a significant color change starting ~1.5 days after the burst,
confirming the spectroscopic results already reported by Matheson et al.
(2002). We discuss these results in the context of several models that have
recently been proposed to account for the anomalous photometric behavior of
this event.Comment: Submitted to ApJL, 14 pages, 4 figures. Data available on anonymous
ftp at ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB02100
OGLE-2005-BLG-018: Characterization of Full Physical and Orbital Parameters of a Gravitational Binary Lens
We present the analysis result of a gravitational binary-lensing event
OGLE-2005-BLG-018. The light curve of the event is characterized by 2 adjacent
strong features and a single weak feature separated from the strong features.
The light curve exhibits noticeable deviations from the best-fit model based on
standard binary parameters. To explain the deviation, we test models including
various higher-order effects of the motions of the observer, source, and lens.
From this, we find that it is necessary to account for the orbital motion of
the lens in describing the light curve. From modeling of the light curve
considering the parallax effect and Keplerian orbital motion, we are able to
measure not only the physical parameters but also a complete orbital solution
of the lens system. It is found that the event was produced by a binary lens
located in the Galactic bulge with a distance kpc from the Earth.
The individual lens components with masses and are separated with a semi-major axis of AU and
orbiting each other with a period yr. The event demonstrates
that it is possible to extract detailed information about binary lens systems
from well-resolved lensing light curves.Comment: 19 pages, 6 figure
OGLE-2005-BLG-153: Microlensing Discovery and Characterization of A Very Low Mass Binary
The mass function and statistics of binaries provide important diagnostics of
the star formation process. Despite this importance, the mass function at low
masses remains poorly known due to observational difficulties caused by the
faintness of the objects. Here we report the microlensing discovery and
characterization of a binary lens composed of very low-mass stars just above
the hydrogen-burning limit. From the combined measurements of the Einstein
radius and microlens parallax, we measure the masses of the binary components
of and . This discovery
demonstrates that microlensing will provide a method to measure the mass
function of all Galactic populations of very low mass binaries that is
independent of the biases caused by the luminosity of the population.Comment: 6 pages, 3 figures, 1 tabl
The Extreme Microlensing Event OGLE-2007-BLG-224: Terrestrial Parallax Observation of a Thick-Disk Brown Dwarf
Parallax is the most fundamental technique to measure distances to
astronomical objects. Although terrestrial parallax was pioneered over 2000
years ago by Hipparchus (ca. 140 BCE) to measure the distance to the Moon, the
baseline of the Earth is so small that terrestrial parallax can generally only
be applied to objects in the Solar System. However, there exists a class of
extreme gravitational microlensing events in which the effects of terrestrial
parallax can be readily detected and so permit the measurement of the distance,
mass, and transverse velocity of the lens. Here we report observations of the
first such extreme microlensing event OGLE-2007-BLG-224, from which we infer
that the lens is a brown dwarf of mass M=0.056 +- 0.004 Msun, with a distance
of 525 +- 40 pc and a transverse velocity of 113 +- 21 km/s. The velocity
places the lens in the thick disk, making this the lowest-mass thick-disk brown
dwarf detected so far. Follow-up observations may allow one to observe the
light from the brown dwarf itself, thus serving as an important constraint for
evolutionary models of these objects and potentially opening a new window on
sub-stellar objects. The low a priori probability of detecting a thick-disk
brown dwarf in this event, when combined with additional evidence from other
observations, suggests that old substellar objects may be more common than
previously assumed.Comment: ApJ Letters, in press, 15 pages including 2 figure
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