The term plant pathology applies to the use of microbial antagonists to suppress diseases as well as the use of host-specific pathogens to control weed population. The organism that suppresses the pest or pathogen is referred to as biological control agent. In the present study, Pseudomonas fluorescens was isolated from rhizosphere soil and it was checked for the ability to produce antagonistic substances like hydrogen cyanide (HCN), salicylic acid and siderophore. The antifungal potential of the Pseudomonas fluorescens and its metabolites against the phytopathogen Alternaria alternata was detected in vitro. The bacteria Pseudomonas fluorescens and its secondary metabolites were found to be efficient biocontrol agents against Alternaria alternata, a common phytopathogen which causes leaf spot. There is a relationship between the antifungal activity of Pseudomonas fluorescens and secondary metabolites produced, especially HCN. Several desirable traits of Pseudomonas fluorescens and its metabolites against the phytopathogenic fungi will help us to produce formulations of biofungicide based on these secondary metabolites which will substitute the chemical fungicides.

Plant diseases need to be controlled to maintain the quality of food, feed and fiber produced by growers around the world. Chemical pesticides and fungicides are important tools to maximize yield in the modern agriculture, but the application of high amounts of pesticides will lead to an additional strain on the environment. These alternatives based on microorganisms are referred to as biological control agents. Biological control agents are an alternative to the use of fungicides for suppression of fungal pathogens in agricultural production (Weisbeck and Gerrits, 1998).

Among them, fluorescent Pseudomonas have received particular attention due to their ability to elicit a phenomenon known as Induced Systemic Resistance (ISR) in plants (Loon et al., 1998). This dual ability of fluorescent pseudomonads, that is, direct antagonism of phytopathogens and induction of host resistance in the host plant, further highlights their potential as Plant Protection Products (PPPs) (Baker and Soyder, 1965). Fluorescent Pseudomonas are able to improve plant health. Some pseudomonads have been recognized as antagonist of plant pathogens and antibiotic producers (Sullivan and Gara, 1998). Adding to the above, it possesses a broad spectrum of antagonistic activities against plant pathogen such as antibiosis, siderophore production and nutrition and site competition. Some species of Pseudomonas can also produce hydrogen cyanide (HCN) that is toxic to certain pathogenic fungi (Bull et al., 1991).

Siderophore are iron-chelating compounds secreted under low iron stress. Fluorescent Pseudomonas are characterized by the production of yellow-green pigments termed pyoverdines which fluoresce under UV light. Pyoverdines chelate iron in the rhizosphere and deprive pathogen of iron which is required for their growth and pathogenesis (Leong, 1986). Induced systemic resistance may be produced by PGPR strains upon inoculation, including salicylic acid, siderophore, lipopolysaccharide and 2, 3-butanediol and other volatile substances (Loon et al., 1998). Salicylic acid produced by Pseudomonas fluorescens helps to induce Systemic Acquired Resistance (SAR) of the infected plants and it will result in the formation of Pathogenesis-Related (PR) protein. These PR proteins act directly to lyse invading cells and reinforce cell wall boundaries to resist infections.

Alternaria alternata is a phytopathogenic fungus. It has a wide host range causing leaf spots and blights on many plant parts (Nishijima, 1993). They affect primarily the leaves, stems, flowers and fruits of annual plants, especially vegetables and ornamentals (Agrios, 1997). Alternaria leaf spots are characterized by depressed, circle to oval lesions. Lesions are restricted to the surface of the fruit and do not cause extensive rotting of the flesh (Nishijima, 1993). Alternaria stem canker caused by Alternaria alternata tomato pathotype (AAL) was first reported in San Diego, USA, in 1960, and was later found worldwide. Symptoms with dark brown to black cankers occur on foliage, including leaves, petioles and stems. Alternaria alternata can also produce endo-polygalacturonase and pectatelyase activities. These enzymes are responsible for the hydrolysis of pectin components of the plant cell wall.

Biotechnology Journal, Stress Adaptation of Bacteria, Extremophiles, Bacterial Adaptation, Cytosolic Components, Denature Cellular Proteins, Reactive Oxygen Species, Monounsaturated Fatty Acids, Ultraviolet Radiation, Environmental Stress, Stress Management, Biological Systems, Cellular Economy.