Tobacco growth enhancement and blue mold disease protection by rhizobacteria: Relationship between plant growth promotion and systemic disease protection by PGPR strain 90-166

The effect of plant growth-promoting rhizobacteria (PGPR) on plant growth and systemic protection against blue mold disease of tobacco (Nicotiana tabacum L.), caused by Peronospora tabacina, was investigated in the greenhouse. Five PGPR strains with known plant growth promotion and induced resistance activities in other crops were used in these studies. PGPR strains were applied as seed treatments alone at planting and in combination with root drenches after planting. When PGPR were applied as seed treatments, PGPR strains 90-166, SE34 and C-9 at 10(9) CFU mL(-1) increased all or most parameters of plant growth 7 weeks after planting (WAP), while 89B-61 and T4 did not enhance any or few parameters. Seed treatments with PGPR strains 90-166 and C-9 at 10(9) CFU mL(-1) at 13 WAP resulted in significant disease reduction in blue mold severity compared to the nontreated control. When PGPR were applied as seed treatments and root drenches, all PGPR strains at 10(9) CFU mL(-1) enhanced tobacco growth compared to the nontreated control at 7 WAR The time interval between the last PGPR treatment and challenge with P. tabacina affected systemic disease protection elicited by some PGPR strains. When the time interval was 8 weeks, 3 PGPR strains 90-166, SE34 and T4 at 10(9) CFU mL(-1) reduced disease severity, while treatments with all tested PGPR strains resulted in significantly lower disease compared to the nontreated control when it was reduced to 6 weeks. Regression analysis demonstrated a significant relationship between plant growth promotion and systemic protection against blue mold elicited by PGPR strain 90-166. Tobacco growth promotion (X) was calculated by percentage of increase in total fresh plant weight relative to the nontreated control. Systemic protection (Y) against blue mold disease was represented by percentage of decrease in disease severity over the nontreated control. This relationship was best described by the model Y = -4.48 + 0.37 X (r(2) = 0.86, P = 0.0001) when strain 90-166 was applied as seed treatments. In the experiment in which strain 90-166 was applied as seed treatments and root drenches, Y = 6.60 + 0.14 X (r(2) = 0.88, P < 0.0001) defined this relationship when the time interval was 8 weeks. When the time interval was reduced to 6 weeks, Y = 12.30 + 0.28 X (r(2) = 0.80, P = 0.0005) defined the relationship.