Efficacy of selected rhizobacterial isolates for biocontrol of Rhizoctonia solani and growth promotion of maize in South Africa

Abstract

Plant growth promoting rhizobacteria (PGPR) are bacteria that colonize the volume of soil surrounding plant roots (known as the rhizosphere) which, when in association with a suitable host plant, promote plant growth. The growing demand for sustainable crop production strategies has placed increasing emphasis on PGPR and their potential use in agriculture. The current study therefore focused on the beneficial effects of PGPR isolates (selected from the PGPR culture collection at the University of Pretoria) on maize (Zea mays L.) and the ability of these isolates to control Rhizoctonia solani Kühn. The seed quality and vigour of two maize cultivars (PAN 6236B and PAN 6Q-308B) was determined using the standard germination, moisture and various vigour tests. PAN 6Q-308B was found to have lower vigour and was selected for use in the remainder of the tests performed in the study. In a modified seed germination bioassay the percentage germination, seedling mass, shoot and root length of PAN 6Q-308B was increased by selected PGPR isolates. The vigour index (VI) of PAN 6Q-308B was also increased by five isolates (S6, S7, T19, T29 and A32) which were then selected for evaluation in the greenhouse. In the greenhouse pot trial isolates T19, S7 and T29 gave the most promising results in terms of increasing the dry mass of maize seedlings. These isolates were evaluated in a formulation efficacy trial where they were applied individually and in combination as a soil drench, seed treatment and liquid suspension or powder formulation applied to a pelletised compost carrier. The four most effective treatments at increasing the dry root mass of maize included a soil drench application of either isolate T29 or T19, a soil drench application of the consortium of isolates T19, T29 and S7, and a powder formulation of isolate T29 applied onto the compost pellet carrier. Isolates A08, A07 and T19 were the most effective at reducing the disease symptoms of R. solani Kühn and increasing dry root matter above that of the R. solani inoculated control in a seedling tray trial. Pot trials were conducted to assess the efficacy of isolates A08, A07 and T19 for control of R. solani, when applied as a seed treatment or soil drench. All three isolates failed to control the pathogen when applied as a seed treatment but a soil drench of isolate T19 was found to be very effective. A six month shelf-life study was conducted to determine the survival of the PGPR isolates T19, S7 and T29 applied to a novel carrier comprising pelletised composted chicken manure. The isolates were individually applied to the pellets in a liquid or powder form. The latter was found to be the most suitable for supporting the survival of the PGPR isolates up to six months at 25˚C. The study revealed that the viability of the isolates decreased more rapidly at a temperature of 35˚C as opposed to 25˚C, and that the cell numbers of isolate T19 remained the most stable throughout the six months incubation period in comparison with the other isolates.