Leaf blast (Magnaporthe oryzae) suppression and growth promotion by rhizobacteria on aerobic rice in Brazil

Rice blast caused by Magnaporthe oryzae has the potential to cause 100% grain yield loss. The objective of this investigation was to identify rhizobacteria showing potential for plant growth stimulation and resistance induction under greenhouse conditions. Bacterial isolates were collected from the rhizosphere of rice plants from soils of Amazon, PA. Soil application with rhizobacteria was done by drenching with bacterial cell suspension before inoculating with virulent isolate of M. oryzae. Mass screening of 148 isolates for growth promotion showed that 12.7% stimulated plant height, whereas 52.0% increased root length, total biomass and root biomass. Based on these growth promotion results, 18 isolates were further tested for in vitro inhibition of the pathogen and reduction of leaf blast severity (LBS) in greenhouse test. All isolates inhibited pathogen growth and reduced disease severity from 16% to 95%. The two isolates showing the greatest suppression of leaf blast (Rizo-46 and Rizo-55) were further tested in a subsequent greenhouse trial using three replications and three application methods (drenching the soil, 15 and 2 days before inoculation with rice pathogen, and spraying 2 days before inoculating with virulent isolate). The soil drenching with isolate Rizo-55, 15 days prior to challenging with virulent isolate of M. oryzae reduced LBS by 90%, whereas Rizo-46 applied 2 days before reduced LBS by 95%. The capacity to suppress leaf blast by isolates Rizo-46 and Rizo-55 varied according to the mode of application. Also, the enzymatic tests were conducted to quantitate the presence of proteins related to pathogenesis (PRPs) during induction process of resistance by rhizobacteria. The enzyme activity of peroxidase, beta-1,3-glucanase and chitinase greatly increased, and the results are in accord with greenhouse tests in relation to leaf blast disease suppression. (C) 2011 Elsevier Inc. All rights reserved.