European Journal of Plant Pathology 108: 821–829, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. Reduction of bacterial speck (Pseudomonas syringae pv. tomato) of tomato by combined treatments of plant growth-promoting bacterium, Azospirillum brasilense, streptomycin sulfate, and chemo-thermal seed treatment Yoav Bashan 1 and Luz E. de-Bashan 1,2 1 Environmental Microbiology, The Center for Biological Research of the Northwest (CIB), POB 128, La Paz, BCS 23000, Mexico (Fax: +52 (612) 1254710; E-mail: bashan@cibnor.mx); 2 Department of Biology, Pontificia Universidad Javeriana, Santafe de Bogota, Colombia Accepted 21 June 2002 Key words: Azospirillum brasilense, bacterial leaf diseases, biological control, disease control, induced systemic resistance, plant growth-promoting bacteria, Pseudomonas syringae pv. tomato, seed treatment Abstract Inoculation of tomato seeds with the plant growth-promoting bacterium Azospirillum brasilense, or spraying tomato foliage with A. brasilense, streptomycin sulfate, or commercial copper bactericides, separately, before or after inoculation with Pseudomonas syringae pv. tomato, the casual agent of bacterial speck of tomato, had no lasting effect on disease severity or on plant height and dry weight. Seed inoculation with A. brasilense combined with a single streptomycin foliar treatment and two foliar bactericide applications at 5-day intervals (a third or less of the recommended commercial dose) reduced disease severity in tomato seedlings by over 90% after 4 weeks, and significantly slowed disease development under mist conditions. A. brasilense did not induce significant systemic resistance against the pathogen although the level of salicylic acid increased in inoculated plants. Treatment of tomato seeds that were artificially inoculated with P. syringae pv. tomato, with a combination of mild chemo- thermal treatment, A. brasilense seed inoculation, and later, a single foliar application of a copper bactericide, nearly eliminated bacterial leaf speck even when the plants were grown under mist for 6 weeks. This study shows that a combination of otherwise ineffective disease management tactics, when applied in concert, can reduce bacterial speck intensity in tomatoes under mist conditions. Introduction Although many studies have demonstrated the efficacy of copper compounds and streptomycin sprays against bacterial speck of tomato caused by Pseudomonas syringae pv. tomato (Conlin and McCarter, 1983; Cooksey, 1988; Jardine and Stephens, 1987), contem- porary control methods are inefficient (Bashan, 1997). This is mainly because the pathogen has acquired resis- tance to copper compounds (Cooksey, 1990; Cooksey and Azad, 1992; Pernezny et al., 1995), which were the most common antibacterial agents used in disease prevention programs (Yunis et al., 1980b). Antibiotics are usually effective, however, large-scale applica- tion of antibiotics in tomato cultivation is restricted. Although bacterial speck of tomato is not consid- ered to have a major economic impact (Bashan et al., 1978; Colin and Chafic, 1986), outbreaks can inflict severe damage to tomato plants and can reduce crop yields and fruit quality when the plants are grown under plastic covers or in greenhouses (Yunis et al., 1980a). Alternatives to chemical control include keep- ing the foliage as dry as possible by increasing ventilation in greenhouses and insertion of pathogen- resistance genes into tomato cultivars (Bashan et al., 1981; Gu and Martin, 1998; Sotirova et al., 1994;