Evidence for Induced Systemic Protection to Fusiform Rust in Loblolly Pine by Plant Growth-Promoting Rhizobacteria.

Trials conducted in 1997 and 1998 tested eight strains of plant growth-promoting rhizobacteria (PGPR) for their capacity to induce systemic protection in loblolly pine to the causal agent of fusiform rust. Pine seeds were treated with bacteria at time of sowing, and seedlings were artificially inoculated with basidiospores of Cronartium quercuum f. sp. fusiforme 1 month later. Six months after basidiospore inoculation, seedlings were evaluated for the fusoid swelling or galls characteristic of rust infection. Compared with seedlings from seed not treated with bacteria, two bacterial isolates, Bacillus pumilus (SE34) and Serratia marcescens (90-166), significantly (P = 0.05) reduced the number of galls in 1997 and 1998. Combined data from 1997 and 1998 resulted in two additional isolates, B. pumilus (INR7) and B. pumilus (SE52), significantly (P = 0.05) reducing the number of galls. Averaged over both years, 31% of control seedlings were infected with fusiform rust, while those seedlings treated with bacterial strains SE34, 90-166, INR7, and SE52 had 13, 14, 15, and 16% infection, respectively. These four PGPR strains appear to have induced systemic resistance to fusiform rust in loblolly pine, resulting in less infection over nontreated control seedlings.