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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Root rot pathogens were found through diagnostic surveys in all departments (regions) of Bénin, West Africa, to affect 86 to 100% and 96 to 100% of cassava fields during the dry and rainy seasons, respectively. Disease incidence in individual fields ranged between 0 and 53%, and averaged 16 to 27% per department. Nattrassia mangiferae was consistently the most frequently isolated root rot pathogen (56% in the dry season and 22 to 52% in the rainy season). Pathogenicity of N. mangiferae was confirmed on four cultivars of cassava using stem cuttings and storage roots. For all four cultivars, N. mangiferae significantly reduced the number of roots. Lesions (3 to 15 cm long) formed on the lower stem portion of all inoculated plants, whereas control plants remained symptom free. On storage roots, the disease profile was similar to that formed on stem cuttings. Other root rot pathogens detected during the dry season were Macrophomina phaseolina (14.2%), Fusarium spp. (11.8%), Botryodiplodia theobromae (7.7%), and Pythium spp. (2.9%). During the rainy season, Fusarium spp. were the second most commonly isolated root rot pathogens in three departments (Atlantique, Borgou, and Mono). In Oueme and Zou, B. theobromae was the second most isolated root rot pathogen (ranging between 24 and 28%) during the rainy season. During the same season, Pythium spp. were pronounced in Borgou (18%), followed by Mono (11%), Atlantique (9%), Atacora (8%), Oueme (5%), and Zou (6%). Results of the study are discussed with a view to creating awareness of the destructive power of N. mangiferae, a hitherto poorly recognized root rot pathogen of cassava in Bénin and West Africa in general.

Germination synchrony may facilitate damping-off epidemics by creating a high density of uniformly susceptible individuals. We tested the hypothesis that synchronous germination causes increased seed and seedling mortality from damping-off in two legume species attacked by the fungal pathogen, Pythium aphanidermatum. Glycine max exhibited rapid, synchronous germination compared to its progenitor, G. soja, and suffered greater mortality from both pre-and postemergent damping-off in controlled environment experiments. However, when mixed-aged populations of G. max were created experimentally by staggering planting times, a significant increase in damping-off mortality occurred. In G. soja, which typically has mixed-aged populations due to asynchronous germination, experimental populations with an even-aged distribution also suffered increased damping-off mortality. Hence, the relationship between population age structure and damping-off mortality was species-specific. We propose that species differences in the duration of individual seedling susceptibility to disbase interact with population age structure to control the cutcome of damping-off epidemics.

Phytophthora megasperma zoospores and cysts and Serratia marcescens cells were infiltered during the establishment of a gradient of matric potential in horizontally positioned columns of soil. Tensiometers placed in the soil columns were used to record matric potentials (in situ). In columns wetted a distance of 65 cm, zoospores moved 35 cm behind the wetting front in sand, 44 cm in sandy clay loam, 48 cm in loam, and failed to move in silt loam soil. In the sand, sandy clay loam, and loam soils, matric potentials of −14.5, −18.4, and −16.2 mbars, respectively, were calculated for soil at the boundary between infested and noninfested soils. Zoospore cysts moved one‐half the distance zoospores did in each soil, respectively. The matric potentials associated with their boundary were −13.2, −13.4, and −12.0 mbars. Serratia marcescens cells were recovered close behind the wetting front in the first three soils but 48 cm behind in the silt loam. The matric potential 48 cm behind the wetting front was −33.5 mbars. Attempts were made to develop a predictive model for infiltration of biological propagules based on propagule size and soil moisture characteristic curves. Results indicated that infiltration of propagules occurred after soil pores with radii considerably larger than a priori estimates of the limiting pore radii were filled with water.

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