Salicylic Acid and its Role in Systemic Resistance Induced byPseudomonas fluorescensto Early Blight Disease of Tomato

Abstract

Seventeen isolates of Pseudomonas fluo-rescens (Pf) were examined for growth promotion and the induction of systemic resistance against early blight disease of tomato. A noticeable in-crease in shoot and root length was observed in plants treated with P. fluorescens. However, the root development was reduced by a combination of P. fluorescens and Alternaria alternata (A. alter-nata). P. fluorescens isolates systemically induced resistance against early blight of tomato caused by A. alternata and reduced (approx.18–42%) the dis-ease significantly. All the isolates of P. fluorescens produced salicylic acid (SA) in King's B (KB) broth and induced its accumulation in tomato leaf within 24 h of bacterial inoculation. SA levels were signifi-cantly increased in plants treated with bacteria compared to the split control from one to five days following inoculation. Low SA concentration (0.1–0.5 mg/ml) did not inhibit mycelial growth of A. alternata in vitro, contrary to inhibition at high concentrations. The exogenously applied SA first accumulates in host tissues and induces resistance, however, endogenous SA accumulation in plant tissue is considered to be involved in induced sys-temic resistance to early blight disease of tomato caused by A. alternata.