Linear scaling between microbial predator and prey densities in the global ocean.

It has been proposed that microbial predator and prey densities are related through sublinear power laws. We revisited previously published biomass and abundance data and fitted Power-law Biomass Scaling Relationships (PBSRs) between marine microzooplankton predators (Z) and phytoplankton prey (P), and marine viral predators (V) and bacterial prey (B). We analysed them assuming an error structure given by Type II regression models which, in contrast to the conventional Type I regression model, accounts for errors in both the independent and the dependent variables. We found that the data support linear relationships, in contrast to the sublinear relationships reported by previous authors. The scaling exponent yields an expected value of 1 with some spread in different datasets that was well-described with a Gaussian distribution. Our results suggest that the ratios Z/P, and V/B are on average invariant, in contrast to the hypothesis that they systematically decrease with increasing P and B, respectively, as previously thought.

Disease-Suppressive Vermicompost Induces a Shift in Germination Mode of Pythium aphanidermatum Zoosporangia.

Compost amendments to soils can minimize losses from soilborne plant pathogens, yet the mechanisms by which this occurs have not been well elucidated. In the present study, developmental responses of Pythium aphanidermatum zoosporangia to vermicomposts were observed to better understand how suppression of Pythium seedling disease is expressed. Mature zoosporangia were exposed to vermicompost extracts (VCEs) and monitored using time-lapse photomicroscopy. Sterile and nonsterile VCEs inhibited indirect germination and viable zoospore production whereas zoosporangia germinated directly in VCE to produce germ tubes. Additional treatments were tested to determine factors that promote direct over indirect germination. The pH (5 to 9 at 0.001 M) and ionic strength (0.1 to 0.0001 at pH 6) of potassium phosphate buffer did not alter zoosporogenesis compared with sterile water. Decreasing osmotic potentials in glucose and sucrose from -248 to -2,712 kPa or in polyethylene glycol 8000 from -0.335 to -105 kPa led to a decrease in indirect germination with a corresponding increase in direct germination. Significant levels of seed infection were observed within 1 h of exposure to zoospores (produced in sterile water) or to germ tubes (produced in sucrose solution). Our data demonstrate that VCEs suppress zoosporogenesis and stimulate direct germination; however, this did not result in the suppression of germ tube growth and seed infection.

Infection of Onion Leaves by Pantoea ananatis Leads to Bulb Infection.

Pantoea ananatis has been identified as a cause of center rot of onion. In the field, onion leaves can become infected with P. ananatis and lead to leaf blight. Infected bulbs often are detected only after harvest; however, it has not been demonstrated experimentally that leaf infection by P. ananatis can lead to bulb infection. In this study, onion leaf infection by P. ananatis leading to bulb infection was investigated. Of 18 strains of P. ananatis isolated from symptomatic onion bulbs grown in New York, 14 were pathogenic in bulb and leaf tissue. Pathogenic strains of P. ananatis caused nonmacerated, yellow-brown coloration in fleshy bulb scales following inoculation of bulbs and incubation for 2 days at 28°C. Subepidermal inoculation of onion leaves with pathogenic strains of P. ananatis resulted in gray-white foliar lesions that extended acropetally and basipetally from the points of inoculation. In all, 16% of leaf lesions extended to the onion neck and 11% continued into the bulbs, which developed nonmacerated, yellow-brown scales. Bacteria recovered from the leading edges of lesions had microbiological and molecular characteristics of P. ananatis. This is the first experimental evidence that infection of onion leaves by P. ananatis can lead to bulb infection.