The relationship between superoxide radical generation, the accumulation of the phytoalexin capsidiol and hypersensitive cell death has been examined in
Nicotiana tabacum following challenges by ...compatible and incompatible races of
Phytophthora nicotianae, and the non-host species
Phytophthora palmivora. Challenging suspension cell cultures of
N. tabacum with zoospores of incompatible isolates of
P. nicotianae elicits a biphasic burst of superoxide release. The maximum rate of capsidiol accumulation between 9 and 12 h after challenge coincides with the second oxidative burst and the onset of hypersensitive cell death. Addition of superoxide dismutase or Mn (III) desferal, which scavenge superoxide anions and quench the superoxide burst, suppresses both phytoalexin accumulation and hypersensitive cell death. Mevastatin, an inhibitor of the sesquiterpenoid biosynthesis enzyme HMG-CoA reductase, has no effect on the oxidative burst or hypersensitive cell death, but abolishes capsidiol accumulation. Zoospores of the non-host pathogen
P. palmivora also elicit superoxide release, but in a single, broad burst between 3 and 12 h after challenge. Capsidiol accumulates to levels similar to those seen in incompatible host reactions, although the onset of capsidiol accumulation is more rapid in the non-host interaction. As in the incompatible interaction, phytoalexin accumulation and hypersensitive cell death are both inhibited by superoxide scavengers, although scavenging does not render host cells susceptible to infection by non-host zoospores. Our findings indicate that phytoalexin accumulation and hypersensitive cell death in both incompatible and non-host interactions are regulated by pathways that diverge downstream of superoxide release. While hypersensitive cell death and phytoalexin accumulation appear to be necessary for gene-for-gene resistance, other factors cause non-host infection to fail.
The saprophytic survival of Phytophthora palmivora in infected cocoa residues over the dry season provides primary inoculum for the development of rainy season epidemics of black pod and stem canker ...diseases of cocoa. In field trials conducted in Papua New Guinea, we examined pathogen survival in various types of colonised cocoa tissues under two different ground covers. Pathogen survival was longest in lignified host organs such as pod cases and twigs, and lowest in infected leaf tissue. Survival in each tissue was shorter under leaf litter mulch than under grass cover. The viability of mycelium discs exposed to soil collected under leaf litter declined more rapidly than in soil collected under grass cover. These results suggest that leaf litter mulches reduce the survival of P. palmivora by accelerating substrate decomposition and by stimulating the activity of antagonistic and hyperparasitic microbes. When used in an integrated disease management program, leaf litter mulches may reduce disease by lowering pathogen populations, particularly at the start of the rainy season.PUBLICATION ABSTRACT
The present study shows that Potassium phosphonate has been proven to slow down the growth rate of P. cinnamomi in in vitro. Phosphonate drench as low as 1 g L super(-1) was effective in protecting ...Thryptomene calycina, Banksia grandis and B. spinulosa in pot and field trials. In glass house trials, concentrations as low as 1 g L super(-1) (drench) significantly suppressed the P. cinnamomi population. Concentrations over 2 1/2 g L super(-1) were phytotoxic to all plant species tested. The most sensitive species was B. spinulosa. Phosphonate (5 g L super(-1)) killed all B. spinulosa plants in seven weeks, therefore it must be used with a great care. Phosphonate treatment alone was effective protecting plants from disease in the field, but did not result in high plant health. Despite new root growth in pot trials after seven weeks, poor growth was commonly observed on T. calycina after 14 months in field trials. This suggests that phosphonate is not suitable as sole application particularly for the long term. A combination of phosphonate with compost as well as antagonist as an integrated management will be a good alternative for P. cinnamomi management in the future.
The relative spatiotemporal correspondence between sensory events affects multisensory integration across a variety of species; integration is maximal when stimuli in different sensory modalities are ...presented from approximately the same position at about the same time. In the present study, we investigated the influence of spatial and temporal factors on audio-visual simultaneity perception in humans. Participants made unspeeded simultaneous versus successive discrimination responses to pairs of auditory and visual stimuli presented at varying stimulus onset asynchronies from either the same or different spatial positions using either the method of constant stimuli (Experiments 1 and 2) or psychophysical staircases (Experiment 3). The participants in all three experiments were more likely to report the stimuli as being simultaneous when they originated from the same spatial position than when they came from different positions, demonstrating that the apparent perception of multisensory simultaneity is dependent on the relative spatial position from which stimuli are presented.
We studied the effect of temporary hypoxia on responses associated with disease resistance in cell suspensions of tobacco and soybean challenged with zoospores of compatible and incompatible races of ...Phytophthora nicotianae and Phytophthora sojae, respectively. Under normal atmospheric conditions both hosts respond to incompatible challenge with a burst of superoxide (O2−) release beginning 6h after inoculation, followed within 2h by the onset of cell death. NBT staining reveals that O2− is released around the point of contact between the pathogen and cell in both hosts. Hypoxia, imposed by incubating challenged cells under an atmosphere of nitrogen between 4 and 9h after inoculation, abolishes the O2− burst and cell death in both tobacco and soybean cells. Under these conditions normally incompatible pathogen races infect and colonize host cells, indicating a failure of resistance expression. Compatible interactions, and the viability of uninoculated cells of tobacco and soybean, are not affected by temporary hypoxia. These results strongly implicate a requirement for oxygen, O2− release and hypersensitive cell death in the resistance of tobacco and soybean to incompatible pathogen races.
Seedlings of Pinus radiata transplanted into washed river sand or sand/peat potting mix pre-inoculated with Phytophthora cinnamomi developed severe wilt and root rot symptoms within 7 weeks, and died ...within 12 weeks. By contrast, direct inoculation of pathogen mycelium into stem wounds produced only restricted lesions. Soil incorporation of inoculum was subsequently used to assess the ability of potassium phosphonate to protect pine seedlings against wilt and root rot caused by P cinnamomi. Three concentrations of potassium phosphonate (0.1, 1.0 and 5.0 g/L), applied as either a single foliar spray or as a single root drench at transplanting into pots containing pathogen-infested sand or sand/peat, significantly reduced the incidence of root rot symptoms. At each concentration, soil drenches were more effective than foliar sprays in delaying and suppressing dieback symptoms in container-grown plants. Phosphonate applications were more effective in sand/peat potting mix than in washed river sand. Recovery of the pathogen from diseased plant roots and potting mix was lower in pots drenched with phosphonate than from pots treated by foliar sprays. Potassium phosphonate foliar sprays at a rate of 5.0 g/L were phytotoxic, causing burning of needles and temporary branch dieback.PUBLICATION ABSTRACT
Twenty-six fungi associated with leaf spots of palms in north
Queensland have been identified, including 12 ascomycetes, one
basidiomycete and 13 deuteromycetes. Eight were identified as species new
...to
science. A new species of Pseudospiropes is described
and descriptions of two unnamed species of Glomerella are also
given.
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