Biological control is an effective and sustainable alternative or complement to conventional pesticides for fungal and bacterial plant disease management. Some of the most intensively studied biological control agents are bacteria that can use multiple mechanisms implicated in the limitation of plant disease development, and several bacterial-based products have been already registered and marketed as biopesticides. However, efforts are still required to increase the commercially available microbial biopesticides. The inconsistency in the performance of bacterial biocontrol agents in the biological control has limited their extensive use in commercial agriculture. Pathosystem factors and environmental conditions have been shown to be key factors involved in the final levels of disease control achieved by bacteria. Several biotic and abiotic factors can influence the performance of the biocontrol agents, affecting their mechanisms of action or the multitrophic interaction between the plant, the pathogen, and the bacteria. This review shows some relevant examples of known bacterial biocontrol agents, with especial emphasis on research carried out by Spanish groups. In addition, the importance of the screening process and of the key steps in the development of bacterial biocontrol agents is highlighted. Besides, some improvement approaches and future trends are considered.

Short peptides of 11 residues were synthesized and tested against the economically important plant pathogenic bacteriaErwinia amylovora,Pseudomonas syringae, andXanthomonas vesicatoriaand compared to the previously described peptide Pep3 (WKLFKKILKVL-NH₂). The antimicrobial activity of Pep3 and 22 analogues was evaluated in terms of the MIC and the 50% effective dose (ED₅₀) for growth. Peptide cytotoxicity against human red blood cells and peptide stability toward protease degradation were also determined. Pep3 and several analogues inhibited growth of the three pathogens and had a bactericidal effect at low micromolar concentrations (ED₅₀of 1.3 to 7.3 μM). One of the analogues consisting of a replacement of both Trp and Val with Lys and Phe, respectively, resulted in a peptide with improved bactericidal activity and minimized cytotoxicity and susceptibility to protease degradation compared to Pep3. The best analogues can be considered as potential lead compounds for the development of new antimicrobial agents for use in plant protection either as components of pesticides or expressed in transgenic plants.

The tree constitutes an ecosystem in which microorganisms play an essential role in its functionality. Interactions that microorganisms establish with plants may be beneficial or detrimental and are of extreme importance in the exploitation of trees in agriculture as crop production systems. Fruit trees, especially pomefruit trees including apple, pear and several ornamentals are of great economic importance but its production is affected by several diseases. Fungal and bacterial fruit tree diseases are mainly controlled with chemical fungicides and bactericides, but health and environmental concerns about the use of chemical pesticides have result in strong regulatory actions and have stimulated the development of beneficial microorganisms as microbial pesticides. Up to now, several microorganisms have been registered in different countries and in the EU as biocontrol agents (BCA) covering mainly fire blight, soil-borne fungal diseases and postharvest fruit fungal rot. The key aspects in the success of this technology for disease control are related to biosafety and environmental impact of biocontrol agents, the traceability and fate in the environment and food chain, the improvement by physiological, genetic engineering or the use of mixtures or formulations as well as the industrial production and development of delivery systems for treatment application to trees.