Potential of plant growth-promoting rhizobacteria (PGPR) as a biological control agent against warm-season turfgrass pests

Abstract

Plant growth-promoting rhizobacteria (PGPR) are non-pathogenic, beneficial bacteria that colonize seeds and roots of plants to enhance plant growth. Despite work in agronomic crops, there has been little emphasis on development of PGPR for grasses. Accordingly, experiments were designed to evaluate novel bacterial inoculants for growth promotion in hybrid bermudagrass, Cynodon dactylon (L). Pers. x Cynodon transvaalensis Burtt-Davy (Tifway). Replicated laboratory and greenhouse experiments compared various blends of bacteria genera and species applied as weekly root inoculations. Growth promotion was assessed by measuring foliar growth from 3–8 wk and root growth at 8 wk after the first treatment. In all experiments, at least one bacterial treatment resulted in significantly increased top growth and greater root growth (length, surface area, volume, or dry weight). Blends 20 and MC3 caused the greatest growth promotion of roots and shoots. These results suggest that the bacterial strains could be used in strategies to help reduce nitrogen or water inputs to turf. Non-pathogenic, soil microbes can induce changes associated with phytohormones that may influence plant-insect interactions. Much of this work has focused on mycorrhizal fungi or certain rhizobacteria. Soil microbes may deter herbivore oviposition, influence performance of above ground herbivores, while attracting natural enemies. Only a few studies have explored these interactions. Blends previously studied were screened for their ability to deter oviposition in no-choice greenhouse assays, negatively impact larval development in growth chamber conditions, and recruit natural enemies and parasitoids of the fall armyworm (FAW) in field. FAWs deposited most of their eggs on the grass in the control plants and ≤ 29% on the inoculated treated grass, suggesting that microbes can mediate interactions between females and oviposition hosts. Three blends negatively impacted larval weights and two of these blends negatively impacted pupal weight and eclosion. Inoculants have shown to increase the attraction of natural enemies in laboratory settings, however the results failed to demonstrate this under field conditions. These experiments were some of the first to examine parasitoid recruitment to plants treated with bacterial inoculants under field conditions and the first attempt to use microbial inoculants to manipulate natural enemies in turfgrass. Induced resistance in plants from microbe inoculation to plant pathogens is well documented in the literature, but whether these interactions extend to herbivores, like insects, remains inconclusive. Nitrogen (N) is the most important macronutrient for sustaining plant growth in turfgrass, and is abundantly applied to amenity grass. Plant N use efficiency is estimated to be only 50%. Avenues of loss are leaching, immobilization, and denitrification which releases nitrous oxide, a greenhouse gas. Use of PGPR and other microbial inoculants could allow for a reduction of N rates if they can alter the soil microbe community by improving nutrient uptake and efficiency while reducing greenhouse gas emissions. Replicated field study experiments evaluated varying N rates with and without a bacterial inoculant were conducted on a golf course. Parameters evaluated were foliar N content, visual turfgrass ratings, chlorophyll content, and tensile strength. We failed to demonstrate that the addition of PGPR to an N fertility management practice increased bermudagrass quality based on the parameters evaluated. Factors like site selection, weather, and N rates evaluated may have negatively impacted this study, and may explain why differences were not observed. Differences may have been observed if the study had been conducted in a less managed turfgrass setting, like a home lawn or pasture.