Introduction

Progress in agronomy over the last 30 years has allowed world food production to almost double. In part, this was possible through the control of diseases caused by air-borne pathogens. However, the damage caused by soil-borne pathogens has increased during the same period; this has resulted in important yield reduction in protected and unprotected crops. To date, satisfactory methods of control have not been developed. Few plant varieties are resistant to soil-borne pathogens. Unfortunately, crop rotation is often not possible for economic reasons. Pesticides are seldom effective in soil and growing concern for the environment restricts their use. Biological control of root diseases can be considered as an alternative for the future. The pseudomonads are currently receiving attention world-wide because some strains, added at a few grams or milligrams per hectare, effectively increase crop yield and improve plant health. These beneficial effects are well documented, however the mechanisms involved are only partially understood. A better understanding of these mechanisms is crucial to safety if pseudomonads are to be released on a large scale. Data on safety are scarce because the investigation of potential risks associated with the release of pseudomonads has just begun. This review will mainly focus on the results obtained with strain CHAO of Pseudomonas fluorescens.

Beneficial effect of resident populations of pseudomonads

Resident populations of pseudomonads are part of the natural disease suppressiveness of soils and of suppressiveness induced by monoculture or solarization (Smiley, 1979; Scher and Baker, 1980; Rovira and Wildermuth, 1981; Cook and Weller, 1987; Defago and Haas, 1990; Gamliel and Katan, 1991). It is important to realize that natural suppressiveness occurs only in distinct areas.