Appl Microbiol Biotechnol (2004) 64: 263–269 DOI 10.1007/s00253-003-1439-8 ORIGINAL PAPER D. Romero . A. Pérez-García . M. E. Rivera . F. M. Cazorla . A. de Vicente Isolation and evaluation of antagonistic bacteria towards the cucurbit powdery mildew fungus Podosphaera fusca Received: 14 April 2003 / Revised: 20 July 2003 / Accepted: 1 August 2003 / Published online: 16 September 2003 # Springer-Verlag 2003 Abstract Powdery mildew is one of the most important limiting factors for cucurbits production in Spain, its management being strongly dependent on chemicals. The aim of this work was to evaluate the possibility of exploiting antagonistic bacteria in the biological control of the cucurbit powdery mildew fungus Podosphaera fusca (syn. Sphaerotheca fusca). Among a collection of bacterial strains isolated from distinct cucurbit powdery mildew diseased plants and rhizospheric soils, four isolates were selected, by means of a screening method based on antibiotic production, and identified as Bacillus spp. These isolates proved to be efficacious in the control of cucurbit powdery mildew in in vitro detached leaves and seedling biocontrol assays, where reductions of disease severity of up to 80% were obtained. Furthermore, bacterial popula- tions on melon leaves remained at similar levels (10 5 cfu cm -2 ) over the 16-day period studied and, as observed by scanning electron microscopy analysis, they were able to establish microcolonies associated with an extracellular matrix, which reveals that these isolates efficiently colonize melon phylloplane. These results indicate that the bacterial isolates selected are promising candidates for biological control agents of cucurbit powdery mildew in southern Spain. Introduction Powdery mildew is one of the most important diseases of cucurbits worldwide. As the most characteristic visual symptom the disease induces development of a whitish, talcum-like powdery growth on both leaf surfaces, petioles and stems. Infected leaves usually wither and die, and plants senesce prematurely (Zitter et al. 1996). The disease is a major production problem in many areas of the world, the reduction of fruit quality and crop yield being the most striking aspects of disease loss. On cucurbits the disease can be caused by two fungal species, Golovinomyces cichoracearum or Podosphaera fusca (syn. Sphaerotheca fusca), obligate biotrophic ectoparasites that induce identical symptoms but can be distinguished easily under light microscopy (Braun et al. 2002). In southern Spain, however, P. fusca has been identified as the sole cause of the disease (Del Pino et al. 2002; Torés et al. 1990), and is responsible for significant yield losses in cucurbit crops under both field and greenhouse conditions. Fungicide applications and the use of resistant cultivars are the main means of disease control. All cucurbits are potentially susceptible to P. fusca; however, symptoms are less common in commercial cucumber, because many cultivars are resistant, and in watermelon, which is considered resistant to the fungus (Zitter et al. 1996). Application of fungicides is currently the principal practice in most cucurbit crops for managing powdery mildew. Unfortunately, there is generally a high risk of developing resistance to these compounds because of their very specific modes of action, and powdery mildew fungi have a high potential for resistance development (McGrath 2001). In the development of new, environmentally friendly, control strategies, biological control has emerged as a harmless alternative that can be combined with chemicals and other strategies in what is known as an integrated approach for powdery management in cucurbits (Bélanger et al. 1998). Several biological strategies have been proposed for disease control, such as the use of mycoparasites (Romero et al. 2003, Verhaar et al. 1997), yeasts (Bélanger et al. 1998), or induction of systemic resistance in host plants (Vogt and Buchenauer 1997). Biological control using antagonistic bacteria has been reported as an attractive alternative due to their ability to antagonize the pathogen by different modes of action, and to effectively colonize distinct plant habitats (Raaijmakers et al. 2002). Most attention has been focused on the use of D. Romero . A. Pérez-García (*) . M. E. Rivera . F. M. Cazorla . A. de Vicente Departamento de Microbiología, Facultad de Ciencias, Universidad de Málaga, Campus Universitario de Teatinos s/n, 29071 Málaga, Spain e-mail: aperez@uma.es Tel.: +34-95-2131890 Fax: +34-95-2132000