Effect of Organic Management of Soils on Suppressiveness to Gaeumannomyces graminis var. tritici and its Antagonist, Pseudomonas fluorescens

Abstract

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.

Stenotrophomonas maltophilia: a new potential biocontrol agent of Ralstonia solanacearum, causal agent of potato brown rot

“…The percentages of clay and organic matter were also higher in the Egyptian soils, especially the organically managed Egyptian soil, than in the Dutch soils, and there was a positive correlation between the organic matter content and survival of S. maltophilia. Also in this case, there might have been an indirect effect, as the availability of organic matter can increase the activity of biocontrol agents (Hiddink et al 2005;Hoitink and Boehm 1999), which may explain the better survival of the antagonist and the suppression of the pathogen by the antagonist in the Egyptian organic clay soil. The bacterial diversity in the Egyptian organic clay soil was less than in the conventional soil, which may explain the higher efficiency of the biocontrol agent in the Egyptian organic versus conventional soil (Hiddink et al 2005).…”

“…Also in this case, there might have been an indirect effect, as the availability of organic matter can increase the activity of biocontrol agents (Hiddink et al 2005;Hoitink and Boehm 1999), which may explain the better survival of the antagonist and the suppression of the pathogen by the antagonist in the Egyptian organic clay soil. The bacterial diversity in the Egyptian organic clay soil was less than in the conventional soil, which may explain the higher efficiency of the biocontrol agent in the Egyptian organic versus conventional soil (Hiddink et al 2005). On the other hand, the antagonist enhanced the survival of the pathogen in the Dutch organic soil by delaying its decline phase.…”

“…For DGGE analysis of the eubacterial soil population, the V6 to V8 region of the 16S rRNA gene was amplified from total soil DNA with the primers 968 f-GC and 1401 r (Heuer and Smalla 1997). Two nanogram of DNA was added to 50 ml PCR reactions and amplified using a touchdown scheme (Hiddink et al 2005).…”

“…DGGE was performed using the Dcode system (BIOrad Laboratories, Hercules, CA, USA) according to Hiddink et al (2005). All samples were analyzed at least in duplicate.…”

Biotic interactions, ecological knowledge and agriculture

“…Further, take-all disease (Gaeumannomyces graminis var. tritici ) was suppressed in organic relative to conventional soils in wheat and barley production, due to both general and specific suppression (Hiddink et al 2005). Others have examined the role of soil fauna in the general suppression of fungal pathogens and conclude that facultative saprophytes may be most affected by mycelial-grazing soil animals, while obligate parasites may be more influenced by animals that ingest spores and other types of propagules (Friberg et al 2005).…”