Seed treatment with selected plant growth-promoting rhizobacteria increases maize yield in the field

Maize, Zea mays is the most important grain crop in South Africa and is a staple food in many African countries. The beneficial effects of plant growth-promoting rhizobacteria on crop growth and yield have been well documented, but obtaining reproducible results under field conditions is often difficult. In the current study, five selected rhizobacterial strains that showed plant growth-promoting activities in pilot studies were evaluated for potential enhancement of maize yield under field conditions. The five strains together with a commercial standard were assessed as seed treatments of maize over three seasons in four different soil types. The strains were identified on the basis of 16S rRNA sequencing as Lysinibacillus sphaericus (T19), Paenibacillus alvei (T29), Bacillus safensis (S7) Bacillus pumilus (A26) and Brevundimonas vesicularis (A40). The best yield increases in maize were obtained during the 2011/2012 and 2012/2013 seasons in the Shortlands ecotope with the rhizobacterial strains T19, T29 and S7, resulting in yield increases ranging from 24% to 34%. Strain T19 rendered the most consistent yield increases during the three successive field trials amounting to 33% and 24% in Shortlands ecotope and 12% in Clovalley ecotope, respectively. During 2013/2014 a consortium of three strains, viz. T19, S7 and A26 gave a 32% yield increase in Clovalley ecotope. All the rhizobacterial strains solubilised phosphate in vitro except T19. Strain T29 showed the best nitrogen-fixing activity in vitro, proliferating on a nitrogen-free substrate and also producing ammonia. All the strains tested positive for indole acetic acid production. The current study demonstrates the ability of rhizobacterial strains T19, T29, S7 and A26 applied as seed treatments to significantly enhance maize yield in the field, making development and commercialisation of these strains a viable option.