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Organic management of soils is generally considered to reduce the incidence and severity of plant diseases caused by soil-borne pathogens. In this study, take-all severity on roots of barley and wheat, caused by Gaeumannomyces graminis var. tritici, was significantly lower in organically-managed than in conventionally-managed soils. This effect was more pronounced on roots of barley and wheat plants grown in a sandy soil compared to a loamy organically-managed soil. Fluorescent Pseudomonas spp. and in particular phlD + pseudomonads, key factors in the take-all decline phenomenon, were represented at lower population densities in organically-managed soils compared to conventionally-managed soils. Furthermore, organic management adversely affected the initial establishment of introduced phlD + P. fluorescens strain Pf32-gfp, but not its survival. In spite of its equal survival rate in organically-and conventionally-managed soils, the efficacy of biocontrol of take-all disease by introduced strain Pf32-gfp was significantly stronger in conventionally-managed soils than in organically-managed soils. Collectively, these results suggest that phlD + Pseudomonas spp. do not play a critical role in the take-all suppressiveness of the soils included in this study. Consequently, the role of more general mechanisms involved in take-all suppressiveness in the organically-managed soils was investigated. The higher microbial activity found in the organically-managed sandy soil combined with the significantly lower take-all severity suggest that microbial activity plays, at least in part, a role in the take-all suppressiveness in the organically-managed sandy soil. The significantly different bacterial composition, determined by DGGE analysis, in organically-managed sandy soils compared to the conventionally-managed sandy soils, point to a possible additional role of specific bacterial genera that limit the growth or activity of the take-all pathogen.

The effect of mixed cropping on disease suppressiveness of soils was tested for two cropping systems, Brussels sprouts-barley and triticale-white clover. Disease suppressiveness of field soils was evaluated in bioassays for the soilborne pathogens Rhizoctonia solani, Fusarium oxysporum f. sp. lini, and Gaeumannomyces graminis var. tritici. For both cropping systems, mixed cropping did not enhance disease suppressiveness of the soils. In some cases, soil cropped to barley alone was significantly more suppressive to F. oxysporum f. sp. lini than soils cropped to Brussels sprouts or the mixture of Brussels sprouts and barley. Analyses of the diversity of the indigenous bacterial and fungal microflora by denaturing gradient gel electrophoresis of amplified 16S- and 18S-rDNA fragments, respectively, revealed, in most cases, no significant differences between mixed and mono-cropped soils. In conclusion, in this study, mixed cropping of soils with Brussels sprouts and barley or with triticale and white clover did not enhance microbial diversity or disease suppressiveness of soils to three different soilborne plant pathogens.

In 2008, a bacterial blight was observed on Raphanus sativus in the Pfalz region in Germany. Disease was sporadic but severe when present within R. sativus fields, which resulted in unmarketable crops. Symptoms consisted of small, angular, water-soaked flecks that often were surrounded by chlorotic haloes. Lesions were visible from adaxial and abaxial leaf surfaces and generally retained chlorotic borders. A gram-negative, bluefluorescing bacterium was isolated from surface-disinfested leaf tissue on King's medium B agar. The radish isolate was levan positive, oxidase negative, and arginine dihydrolase negative. The isolate did not rot potato slices but induced a hypersensitive reaction in tobacco. These reactions corresponded to Lelliot's LOPAT group 1 (2). Repetitive extragenic palindromic sequence (rep)-PCR assays using the BOXA1R primer resulted in different DNA fragment banding patterns between the radish isolate and the pathotype strain of Pseudomonas syringae pv. maculicola (CFBP 1657), but identical DNA fragment banding patterns between the radish isolate and the pathotype strain of P. cannabina pv. alisalensis (CFBP 6866). Unlike P. syringae pv. maculicola, P. cannabina pv. alisalensis and the radish isolate were lysed by bacteriophage PBS1 (1). Pathogenicity was evaluated on two hosts, radish (R. sativus cv. Comet) and broccoli raab (Brassica rapa cv. Sorrento). In each of two independent experiments, 3-week-old radish and broccoli raab plants were inoculated with either the radish isolate, P. cannabina pv. alisalensis, or P. syringae pv. maculicola. Inoculum was prepared by growing the bacteria on nutrient agar for 48 h at 27°C, suspending the bacteria in 0.01 M phosphate buffer (pH 7.0), and adjusting each suspension to 0.6 OD at 600 nm (approximately 1 × 108 CFU/ml). All plants were inoculated by spraying until runoff, incubated in a humidity chamber for 48 h, then placed in a greenhouse at 20 to 25°C for symptom development. Plants inoculated with P. cannabina pv. alisalensis or sprayed with buffer served as positive and negative control treatments, respectively. Seven to ten days postinoculation, the development of symptoms similar to those originally observed in the field were observed on plants inoculated with the radish isolate. In addition, symptoms on radish and broccoli raab plants caused by the radish isolate were similar to symptoms caused by P. cannabina pv. alisalensis in contrast to the lack of symptoms on plants inoculated with P. syringae pv. maculicola. Bacteria isolated from symptomatic tissue and surface-disinfested with sodium hypochlorite (0.525%) had identical characteristics to the radish isolate used to inoculate plants and to the P. cannabina pv. alisalensis pathotype for LOPAT reactions, rep-PCR DNA fragment banding pattern analysis, and sensitivity to phage PBS1, thus fulfilling Koch's postulates. To our knowledge, this is the first report of P. cannabina pv. alisalensis isolated from diseased crucifers in Germany. Verification of P. cannabina pv. alisalensis in Germany indicates that German crucifer growers should differentiate between outbreaks caused by P. cannabina pv. alisalensis and P. syringae pv. maculicola and apply appropriate, specific management strategies. References: (1) C. T. Bull et al. Syst. Appl. Microbiol. 33:105, 2010. (2) R. A. Lelliott. J. Appl. Bacteriol. 29:470, 1966.