Humboldt University Berlin, Faculty for Agriculture and Horticulture, D-1 0099 Berlin, Germany

Background and objectives
Numerous specific strains of microorganisms capable of suppressing plant diseases have been identified, however, few biocontrol agents have been commercialized, primarily due to their inconsistent performance. The performance of an introduced strain often varies from site to site and year to year. The complexity of the soil ecosystem makes biological control of root pathogens by introduced bacteria particularly challenging [1]. The object of this study was to examine the effects of any formulations of new strains of Bacillus subtilis used alone or in combination, influenced by edaphic factors on potato plants against different soilborne potato diseases. An understanding of these effects would allow for more appropriate formulation and improved use recommendations for B. subtilis, and could lead to more consistent field performance with this beneficial microorganism.

Materials and methods
Various strains of bacteria were isolated from soil- and plant samples and were screened against different bacterial and fungal pathogens. Considering their performance in reducing the pathogens in vitro, the most favourable strains of B. subtilis were selected for field evaluation. The preparation of the water-dispersible granule formulations of B. subtilis was based on the proprietary technology of FZB Biotechnik GmbH, Berlin. These formulations were used as inoculum at the recommended rate of 250 g/ha mixed in 25-250 l water. Treatments with fungicides and an untreated control were used for comparison. The applications of the biological and chemical agents were carried out before planting as a spraying treatment. During and after the growing period, the potato plants were checked for the presence of potato diseases. After disease evaluation the potato tubers were harvested and the yields were determined.

Results and conclusions
Results of the field evaluation from 1994 to 1997 showed that B. subtilis strains significantly decreased both the occurrence of Rhizoctonia disease and the incidence of common scab caused by Streptomyces scabies. Over the 4-year period the relative average disease severity of R. solani was reduced by 42% (ranging from 34 to 50%). The bacterial treatment showed similar control effects compared to chemical products. Both biological and chemical treatments were significantly superior to untreated control. The most effective treatment against R. solani was detected after application of a mixture of chemical and biological agents. In prolonged observation the relative average disease incidence of S. scabies was decreased by 46% (ranging from 33 to 67%). A further interesting effect was observed, that B. subtilis influences the occurrence of black leg of potatoes caused by Erwinia carotovora ssp. atroseptica. As a result, black leg infection was reduced by 53%. Trials revealed that both bacterial diseases were not affected by any chemical treatment. So treatments with bacterial antagonists like B. subtilis can be an effective tool for the management of crop diseases. Furthermore, the biological treatments enhanced yields and the plants appeared more vigorous than untreated plants. These plant growth-promoting effects were an additional interesting aspect in using B. subtilis.

Various mechanisms are involved in the suppression of plant diseases and plant growth promotion. On the one hand, these mechanisms can be led back to direct effects of different metabolites produced by these bacteria. On the other hand, there are also various indirect interactions between plant and B. subtilis. A full use of the potential of B. subtilis will depend on an increase in knowledge about antibacterial and antifungal compounds as well as promoting plant growth and inducing resistance or tolerance produced by B. subtilis metabolites. This report strongly suggests that use of B. subtilis might prove to be highly valuable in future for efficient management of plant pathogens in an ecological friendly agriculture. So in spring 1997, the first bioproduct based on a special strain of B. subtilis was started for commercial use in Germany.

1. Weller DM, 1988. Ann. Rev. Phytopath. 26, 379-407.