Growth Promotion of Apple Seedlings and Rootstocks by Specific Strains of Bacteria

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Abstract
Treatment of apple seedlings and rootstocks with strains of bacteria isolated from plant roots resulted in significant growth increases.sbd.up to 65% in seedlings and up to 179% in rootstocks. Strains were tested for growth-promoting activity on apple seedlings and rootstocks M.7 and M.26 in greenhouse and field trials. Strains were also tested for broad-spectrum in vitro antibiosis against six to 12 apple root-infecting fungi, including Fusarium oxysporum, Alternaria alternata, Pythium spp., and Rhizoctonia solani. Nineteen strains that were isolated from roots and showed in vitro antibiosis against other microorganisms on dilution plates were also tested for growth promotion. Growth responses were correlated with the detection of fewer root-associated fungi on rootstocks and with a lower frequency of root colonization by Cylindrocarpon destructans. Rootstocks treated with the plant growth-promoting rhizobacteria (PGPR) had up to 102% more active lateral root nodes. The most effective PGPR strains were fluorescent Pseudomonas spp. and enteric bacteria.