Washington State University, Pullman, WA 99164-6430, USA

Background and objectives
The landmark symposium at the University of California, Berkeley, published in 1965 as Ecology of Soilborne Plant Pathogens -- Prelude to Biological Control, launched an era of research and development on biological control of plant pathogens that continues to expand and attract young investigators to this day. This symposium also represents a turning point, when investigations expanded from biological control achieved with resident antagonists, managed by cultural practices, to biological control also achieved with introduced antagonists, managed as formulations. The classic work of Rishbeth with Phlebia gigantea and Kerr with Agrobacterium radiobacter K84 further directed attention to (i) the application of plant-associated microorganisms specifically adapted to the host plant, and (ii) application of the antagonists directly to the infection court where and when needed. Most successes in biological control of soilborne pathogens with introduced antagonists, and all successes in biological control of post-harvest and foliar diseases, including of damage caused by ice-nucleation-active bacteria, satisfy these two criteria. Virtually all of the commercially available products for biological control of soilborne pathogens are for control of damping off diseases, where the antagonist is applied either directly to the seeds where protection is needed or to glasshouse potting mixes in sufficient concentration to ensure direct contact of the antagonists with the infection court. These successes represent important advances in experience with microbial biocontrol of plant pathogens, but fall short of meeting the challenges and expectations for this technology in the 21st century. Antagonists are needed with the ability to spread to distant infection courts from an inoculative release, e.g. to spread deeper into the root zone starting from seed inoculation, and to maintain effective populations for as long as needed.

Results and conclusions
Rhizosphere-competent strains of fluorescent Pseudomonas species with ability to produce the antibiotic 2,4-diacetylphloroglucinol have been identified from wheat-field soils in the USA that show significant biocontrol activity against take-all caused by Gaeumannomyces graminis var. tritici; better, even, than that provided by Pseudomonas strains with ability to produce phenazine antibiotics. The evidence is strong that these strains, represented by P. fluorescens Q8R1-96, account for the widespread take-all decline with long-term wheat monoculture. Q8R1 transformed to also produce phenazine-1-carboxylate is now under testing for ability to suppress rhizoctonia and pythium root rots in addition to take-all. Ability to establish and persist on the host plant or within the cropping system can be disadvantageous if this discourages commercial interest, because of the lack of potential for repeat sales, or results in more stringent requirements for registration, because of the perception that such strains are more likely to have non-target effects. However, ability to establish and persist, when this leads to season-long protection, is one of the major advantages of this technology over chemical control. This is also one of the hallmarks of biological control of insect pests and weeds with natural enemies. Antagonists raise questions of safety, like all other agents or methods intended for control of pests and diseases, but non-target effects, if they occur, are typically so subtle that they cannot be measured except by the most sensitive and sophisticated methods. Microbial biocontrol is environmentally ideal and appears to be socially acceptable, but the need to develop and register a different strain for each pathogen, host, and sometimes each environment has not been affordable. As with the development of pest- or disease-resistant varieties, the challenge for successful microbial biocontrol is in finding or developing the right genotypes of microorganisms, including through RDNA technologies.

Microorganisms represent enormous but still largely untapped genetic resources for use in biological control or as a sources of transgenes for use in crop plants or other antagonists. Some biological control agents may well replace chemical pesticides, as seems to be the expectation of this technology. Others will permit changes in cropping practices, such as the use of less tillage, less open-field burning, or more intensive cropping. Still other microbial biocontrol agents will augment or extend the performance of traditional disease-management tools such as chemicals and host plant resistance.

1. Baker KE, Snyder WC, eds, 1965. University of California Press, Berkeley, CA.