Induction of systemic resistance in rice against sheath blight disease by Pseudomonas fluorescens

Two Pseudomonas fluorescens strains viz., PF1 and FP7 which inhibited the mycelial growth of sheath blight fungus Rhizoctonia solani and increased the seedling vigour of rice plants in vitro were selected for assessing induced systemic resistance (ISR) against R. solani in rice. The Pseudomonas application as a bacterial suspension or a talc-based formulation through seed, root, soil and foliar application either alone or in combination (seed+root+soil+foliar) effectively reduced sheath blight disease incidence, promoted plant growth and ultimately increased yields under glasshouse or field conditions. Efficacy of Pseudomonas strains against R. solani was comparable to that of the fungicide carbendazim, which is normally used in the field to manage the disease. Pseudomonas treatment of rice cv IR50 led to induction of systemic resistance against R. solani, as a result of increase in chitinase and peroxidase activity. However, the extent of increase varied between treatments, Pseudomonas strains used and their duration. Though two chitinase isoforms (35 and 28 kDa) and five peroxidase isozymes (PO1–PO5) were found to be associated with the ISR, 35 kDa chitinase and three peroxidase isozymes (PO3–PO5) were established as the major determinants of ISR. Although a single application of a Pseudomonas strain resulted in ISR, the combined application through all of the four (seed, root, soil and foliar) methods increased the durability of ISR in rice plants. In addition, the Pseudomonas strains produced chitinase in the culture medium. It is presumed that the induced chitinase, peroxidase and bacterial chitinase may be either directly or indirectly involved in the reduction of sheath blight disease development in rice.