Rhynchosporium leaf scald disease incidence: seed source and spatial pattern

A programme of field trials for the study of the winter barley–Rhynchosporium commune pathosystem is reported. The associated seedborne disease rhynchosporium leaf scald is regarded as having an important impact on barley yields. The analysis in this study relates to the impact of the seed source (commercial or farm-saved seed) on disease incidence and to the spatial pattern of rhynchosporium leaf scald disease incidence. Disease incidence data were calculated from field data recorded as disease severity. Mean disease incidence was higher in the crops grown from farm-saved seed than in those grown from commercial seed, although great agronomic significance cannot be attached to this result. The spatial pattern of rhynchosporium leaf scald disease incidence was characterized in terms of the binary power law (BPL) and was indicative of an aggregated pattern. Programme-wide BPL results were described using a novel phytopathological application of a random coefficients model. These results have application in field sampling for rhynchosporium leaf scald disease.     

A comparison of three atrazine-degrading bacteria for soil bioremediation

The ability of three atrazine-degrading bacteria, Pseudomonas sp. strain ADP, a Pseudaminobacter sp., and a Nocardioides sp., to degrade and mineralize this herbicide in a loam soil was evaluated in laboratory microcosms. These bacteria all hydrolytically dechlorinate atrazine, and degrade atrazine in pure culture with comparable specific activities. The Pseudaminobacter and Nocardioides can utilize atrazine as sole carbon and nitrogen source, whereas the Pseudomonas can utilize the compound only as a nitrogen source. The Pseudomonas and Pseudaminobacter mineralize the compound; the end product of atrazine metabolism by the Nocardioides is N-ethylammelide. At inoculum densities of 10⁵ cells/g soil, only the Pseudaminobacter and Nocardioides accelerated atrazine dissipation. The Pseudaminobacter mineralized atrazine rapidly and without a lag, whereas atrazine was mineralized in the Nocardioides-inoculated soil but only after a lag of several days. The Pseudaminobacter remained viable longer than did the Pseudomonas in soil. PCR analysis of recovered bacteria indicated that the genes atzA (atrazine chlorohydrolase) and atzB (hydroxyatrazine ethylaminohydrolase) were less stable in the Pseudaminobacter than the Pseudomonas. In summary, this study has revealed important differences in the ability of atrazine-hydrolyzing bacteria to degrade this compound in soil, and suggests that the ability to utilize atrazine as a carbon source is important to establish "enhanced degradation" by ecologically meaningful inoculum densities.     

Responses of the parthenogenetic isopod, Trichoniscus pusillus (Isopoda: Oniscidea), to changes in food quality

Long-term observations of Trichoniscus pusillus revealed that chemical composition and microbial activity of the leaf litter serving as food influenced the reproductive success of females and the mortality rates of juveniles as well. Of the food sources investigated, only alder litter, exhibiting high microbial activity, allowed the maintenance of a stable population with frequently reproducing females. This study concludes that the food type is a significant factor for population parameters of this parthenogenetic isopod. Our results on T. pusillus, belonging to the taxon Synocheta, are compared with data on terrestrial isopod species of the taxon Crinocheta.     

Antibiotic resistance in Escherichia coli and Enterococcus spp. isolates from commercial broiler chickens receiving growth-promoting doses of bacitracin or virginiamycin

Antibacterial agents such as zinc bacitracin (ZB) and virginiamycin (VG) are used as growth promoting agents (GP) in broiler chicken production. The objective of this study was to evaluate the effect of the use of ZB and VG on the emergence of antibacterial resistance in a commercial broiler chicken farm. Three trials were conducted using 3 different diets: one without antibacterial agents, one containing VG, and one with ZB. Escherichia coli and Enterococcus spp. strains were isolated and tested for their susceptibility to various antibacterial agents. The occurrence of the resistance genes vatD, ermB, and bcrR in Enterococcus spp. isolates was determined by polymerase chain reaction (PCR). Comparative quantification of vatD and bcrR genes in total deoxyribonucleic acid (DNA) extracts from litter was done by SYBR Green Real-Time PCR (QPCR). Escherichia coli and Enterococcus spp. isolates from diet groups had different levels of resistance to various antibacterial agents over time. These GPs did not select for specific antibacterial agent resistance (AAR) in Enterococcus spp. The use of GPs seemed to lower the percentage of E. coli isolates resistant to some antibacterial agents. The presence of the bcrR gene could not explain all resistant phenotypes to ZB. Genes other than vatD and ermB might be involved in the resistance to VG in Enterococcus spp. Use of GPs was not associated with presence of the bcrR gene in DNA extracts from litter, but use of VG was associated with vatD presence.     

Relationships between woodlice (Isopoda: Oniscidea) and microbial density and activity in the field

 Field studies on microbial populations and their activity in soil and litter layers of different sites revealed statistically significant correlations with climatic factors, soil characteristics and the number of terrestrial isopods (Isopoda: Oniscidea). The number of captured Porcellio scaber was negatively correlated with the density of soil microbiota, but positively correlated with both respiration and cellulolytic activity of litter-colonizing microorganisms. By contrast, the number of Oniscus asellus was positively correlated with soil microbial density and respiration, and cellulolytic and respiratory activity of the litter. These interspecific differences were unexpected between species that frequently occur sympatrically, and, thus, probably have similar ecological requirements. Hence, the results suggested species-specific interactions of these species with soil and litter-colonizing microbiota. However, further field studies are required to prove this. Received: 16 December 1998     

Spatial patterns of litter-dwelling taxa in relation to the amounts of coarse woody debris in European temperate deciduous forests

We studied the leaf litter-dwelling fauna of managed deciduous forests and primeval reference sites in Western and Central Europe and addressed the questions if the higher overall species richness close to downed coarse woody debris (CWD) is related to intra-specific or inter-specific aggregation, if the aggregation pattern changes with the amount of CWD on the forest floor, and how much CWD is needed for different taxa. The analysis is based on shelled Gastropoda, Diplopoda/Isopoda, Chilopoda and Coleoptera. Among-sample heterogeneity was lower close to CWD than distant from CWD. This was most pronounced in Diplopoda/Isopoda and Gastropoda. Diplopoda/Isopoda are comparatively mobile and assemblages were already quite homogenous close to CWD at levels above 5 m³ downed deadwood ha⁻¹. Gastropoda have a low mobility, and more than 20 m³ downed deadwood ha⁻¹ is needed for assemblage homogeneity. We further focused on the Gastropoda as sensitive indicators. Enhanced densities and species richness close to CWD were not a simple function of leaf litter weight, thus effects of densities on heterogeneity are not solely driven by leaf litter accumulation close to CWD. In contrast to euryecious litter-dwellers such as the Punctidae, stenecious slow active dispersers such as the Clausiliidae clearly require more than 20 m³ CWD ha⁻¹ for an even distribution. Specialists depending on CWD even seem to have gone extinct in some managed forests. For conserving the litter-dwelling fauna, we propose a target of at least 20 m³ downed CWD ha⁻¹ in already managed forests and rigorous restrictions for deadwood removal from still (almost) pristine systems.     

Influence of deer exclusion on soil nutrients in oak forests of a central European low mountain range

High densities of red deer can have severe impacts on soil nutrient status by removing the ground vegetation and enhancing erosion of the fertile soil layers. We compared four fenced deer exclosures with adjacent unfenced sites to evaluate the effect of deer grazing and trampling on the soil nutrient status (C_org, N_t, PO₄³⁻, Ca²⁺, Mg²⁺, K⁺) in sloping oak forests of a central European low mountain range. The investigation was set up as a three‐factor randomized complete block design (RCBD) with the factors: ‘fencing’ (fenced/unfenced), ‘gradient’ (gentle/steep) and ‘aspect’ (windward/leeward). We hypothesized that exclusion of red deer by fencing would increase soil nutrient contents independent of slope gradient and aspect. Fencing increased the contents of C_org, N_t, Ca²⁺ and PO₄³⁻, but only at the windward sites. This effect was less pronounced at the gently sloping site than at the steep site, which exhibited the lowest contents of C_org, N_t and PO₄³⁻ of all sites. We did not find increased soil nutrients in the fenced exclosures at the leeward sites, neither at the steep nor at the gently sloping site. At both slope aspects nutrient contents (C_org, N_t, PO₄³⁻, K⁺) tended to be higher on the gentle than at the corresponding steep slopes. Our results suggest that red deer trampling and grazing enhance the loss of soil nutrients at predisposed sites such as windward locations with a high slope gradient. Only at such sites did the exclusion of red deer increase several soil nutrients. Copyright © 2005 John Wiley & Sons, Ltd.     

Survival of various ERIC-genotypes of Shiga toxin-producing Escherichia coli in well water

Recently, there has been a surge of interest in understanding the survival of Shiga toxin-producing Escherichia coli (STEC) in aquatic environments. Fifteen strains of STEC were monitored, individually, in untreated well water samples incubated at 10 and 22^{^}C for 56 days. The strains were selected from three serogroups (O26, Ol11 and O157) and represented five distinct ERIC (enterobacterial repetitive intergenic concensus)-genotypes. The microcosms were prepared in triplicate and inoculated at an initial cell density of about 7.0 log CFU/ml well water. At 10^{^}C, the cell density of five STEC strains fell below the detection limit of 0.8 log CFU/ml by day 56. Of the ten persisting strains, four showed superior survival with cell densities decreasing to an average of about 5 log CFU/ml while the remaining six strains showed moderate levels of survival, decreasing to an average cell density of about 3 log CFU/ml. At 22^{^}C, strain H32 (genotype I) and H15 (genotype B) persisted at 1.1 log CFU/ml and 2.2 log CFU/ml in 56 days, respectively. The other 13 STEC strains dropped below the detection limit between weeks 3 to 8. The 15 strains demonstrated highly variable levels of survival with no correlation between ERIC-genotypes or serogroups and the strains' ability to persist in the well water samples. Although strain H32 (O157:H7) persisted significantly longer than strain H22 (O157:H7) in natural well water at both 10 and 22^{^}C, both strains survived equally well in sterile well water, indicating that individual STEC strains vary in their ability to compete with background microbial populations.     

Acquisition of microbial communities and enhanced availability of soil nutrients by the isopod Porcellio scaber (Latr.) (Isopoda: Oniscidea)

 In microcosm experiments Porcellio scaber increased litter disappearance of oak and alder litter. Alder litter disappeared at more than twice the rate of oak litter. Soil texture did not influence the disappearance of oak litter; however, disappearance of alder litter was enhanced on silt rather than on sand. P. scaber enhanced microbial communities (i.e. microbial respiration and microbial biomass) on both silt and sand when feeding on either alder or oak. Overall, microbial respiration increased 10-fold when isopods fed on oak litter on sand and 20-fold when isopods fed on alder litter on sand. On silt, the initially high microbial respiration remained constant when isopods fed on oak and doubled when they fed on alder litter. In all treatments without P. scaber there was a decrease in microbial respiration over the 12-week experimental period. The availability of macronutrients (C_org, N_tot, P, K, Mg, Ca) in the topsoil was increased when P. scaber fed on alder litter but less pronounced when P. scaber fed on oak litter. Using sand as a substrate, there was an apparent increase only for C_org, Mg and Ca; on silt, increases in C_org, N_tot and P were measured. Under field conditions the contribution of P. scaber to nutrient fluxes will be higher on sand than on silt. Received: 1 July 1999