In the Janzen-Connell hypothesis, host-specific natural enemies enhance species diversity and influence the structure of plant communities. This study tests the explicit assumption of host specificity for soil pathogens of the genus Pythium that cause damping-off disease of germinating seeds and seedlings. We isolated Pythium spp. from soil of a tropical forest in Panama. Then, in an inoculation experiment, we determined the pathogenic!ty of 75 tropical isolates of unknown pathogenicity and seven pathogenic temperate isolates of Pythium on seeds and/or seedlings of eight tropical tree species. Only three tropical isolates, one identified as P. ultimum and two as P. dphanidermdtum, were pathogenic. Tropical pathogenic isolates were pathogenic on 4-6 of eight tree species. Temperate isolates were pathogenic on 0-4 of eight species, indicating that some tropical tree species are susceptible to novel isolates of Pythium. No tree species was susceptible to all isolates and two species were not susceptible to any isolate. Collectively, these results indicate that these Pythium isolates vary widely in their pathogenicity, causing differential mortality of potential host species; likewise, the tree species vary in their susceptibility to a given Pythium isolate. These differences in pathogenicity and susceptibility indicate some support for the Janzen-Connell assumption of host specificity. While they are not restricted to a single species, their intermediate level of specificity suggests that Pythium spp. have the potential to have some effect on forest community structure and diversity.Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp.

Octahedral Fe(III) in the crystal structures of three different smectites was reduced to Fe(II) by actively growing microorganisms indigenous to the clay. The smectites were SWa‐1 ferruginous smectite from Grant County, Washington; API 33a, Garfield Nontronite; and API 25, Upton montmorillonite. Bacterial growth was supported by incubating clay suspensions at room temperature in a nutrient broth solution consisting of peptone and beef extract. Some samples were first sterilized (by autoclaving), then seeded with bacteria that had been isolated previously from the SWa‐1 sample. The effect of O2 on microbial reduction of Fe(III) was also tested. Results revealed that, in all three clays, about 0.30 mmol Fe(III)/g clay was reduced to Fe(II) by bacteria in a 28‐day period. The specific organism responsible for Fe reduction has yet to be classified, but it was more efficient in samples that had not been purged of O2, and it appears to be indigenous to the SWa‐1 clay.

Abstract--Structural Ire in ferruginous smectite (sample SWa-1, Source Clays Repository of the Clay Minerals Society) was reduced by a mixture of five Pseudomonas species of bacteria in a defined Fe-free medium to determine the effect of microbial reduction on clay swelling. Iron(II), total Fe, and gravimetric water content (row/too) were determined in clay gels equilibrated at applied pressures of 0.1, 0.3, and 0.5 MPa. The water content of microbially reduced SWa-1 decreased at all three applied pressures as the Fe(II) content approached about 0.8 mmol Fe(II)/g-clay. As Fe(II) increased from 0.8 mmol/g-clay, however, further change in mJmc was negligible. Concurrent with microbial reduction of structural Fe was a significant decrease in the swelling pressure (PI) of SWa-1: for example, when mw/mc = 1.2 (g/g), PI changed from 0.47 MPa at Fe(II) = 0.2, to 0.19 MPa at Fe(II) = 0.9 mmol/g-clay. Both biologically and chemically reduced smectites displayed lower values of mJmc and a concurrent decrease in II as Fe(II) content increased, but the effect of Fe(II) on mJmc was greater for the microbially reduced smectites at all applied pressures.

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