|Non-pathogenic, soil microbes that occupy the rhizosphere can influence plant growth and induce changes in the plant’s physiological, chemical, metabolic, molecular activities; influencing plant-microbe interactions with abiotic and biotic stressors. Plants colonized by these microbes express unique plant phenotypes that show increased root and shoot mass, enhanced nutrient uptake, and stress mitigation. Additionally, the microbes may fix nitrogen and phosphate or produce siderophores for plant use. Among the plant-associated microbes, plant growth-promoting rhizobacteria (PGPR) are among the most commonly used as inoculants for biofertilization. Plant growth-promoting rhizobacteria are non-pathogenic, free-living soil and root-inhabiting bacteria that colonize seeds, root tissue (endophytic/epiphytic), or the production of root exudates
A review of the existing literature related to turfgrasses is provided in Chapter 1. This review provides background information and introduces major concepts that will be referenced throughout the dissertation. The review provides information on turfgrass, turfgrass economics, turfgrass stress related to drought and insect pests.
In Chapter 2, research was designed to track the colonization of rhizobacterial strains as well as determination of beneficial characteristics that may explain the observed growth promotion in bermudagrass from inoculation. Rhizobacterial inoculants have been previously shown to demonstrate growth promotion in bermudagrass, yet mechanisms for growth promotion and colonization of bermudagrass are unknown. Using rifampicin resistant strains of Bacillus
spp., colonization and persistence of bacteria under field condition in the rhizoplane, rhizosphere, endorhiza, and endophytic phyllosphere were determined in a loamy sand soil. Strains of Bacillus pumilus and B. sphaericus were determined to have nitrogenase and phosphate solubilization activity as well as metabolites that resulted in the production of siderophores. These results showed differences between strains of the same species, and phosphate solubilization was greatest under alkaline conditions. The characteristics of the rhizobacterial strains provides greater insights into the growth promotion demonstrated in bermudagrass. All bacterial strains tested were detectable in plant and soil within 24 h after inoculation and persistent through 12 wk post inoculation. Colonization occurred on both external and internal plant structures, but was typically higher in rhizoplane and rhizosphere samples. Populations remained stable for 2 wk after inoculation with drastic declines occurring after 6 wk. Bacillus sphaericus was the most prolific colonizer, having the greatest population density per sample and least drastic population decline 12 wk after inoculation. These results provide better understanding of plant-microbe-interactions in amenity grasses and can aid in determining application frequencies and intervals of biostimulants for turfgrass management.
In Chapter 3, I tested the hypothesis that PGPR treatment of bermudagrass would increase the tolerance of bermudagrass to tawny mole crickets. Inoculation of bermudagrass with rhizobacterial biostimulants can increase plant growth and influence relationships with above-ground herbivores. Tunneling and root-feeding behaviors of tawny mole crickets cause severe damage to grass in pastures, golf courses, and lawns. Since bacterial inoculants enhance root growth, the goal of this study was to determine if inoculation of bermudagrass by PGPR can increase the tolerance of hybrid bermudagrass to tawny mole crickets, and if PGPR are compatible with current commonly used insecticides for mole cricket control. In large arenas, bacteria-treated grass infested with mole crickets produced more shoot and root mass and 128-200% greater root lengths compared to fertilized, infested, and non-infested bermudagrass. Field plots with mole cricket activity were established and treated with PGPR only, a PGPR-bifenthrin insecticide mixture, the insecticide alone, and compared to non-treated control plots. Plots were rated post-treatment for damage. Damage ratings after 3 and 8 weeks were lowest in plots treated with a bacteria-insecticide mixture, with controls having the highest damage. Lab experiments further confirmed that the PGPR used in the field study were compatible with neonicotinoid, phenylpyrazole, and pyrethroid insecticides when mixed in solution for up to 2 wk. Bacterial mediated interactions increase tolerance of bermudagrass applied before, or in response to, damage. Application of PGPR to field plots reduced tunneling relative to control plots and provided comparable reductions to a short residual, synthetic pyrethroid insecticide. Rhizobacterial products or products contained PGPR and certain insecticides may have utility for IPM of root herbivores.
In Chapter 4, I tested the hypothesis that PGPR treatment of grasses would increase tolerance to root-feeding white grubs (Coleoptera: Scarabaeidae). Inoculation of hybrid bermudagrass with PGPR can increase plant growth and influence relationships with above-ground herbivores like Fall armyworms and mole crickets (Chapter 3), however, few experiments have evaluated PGPR applications to tall fescue. Root-feeding white grubs cause severe damage to grasses, especially tall fescue, in pastures, golf courses, and lawns. Since bacterial inoculants enhance root growth, the goal of this study was to determine if inoculation of hybrid bermudagrass by root-colonizing bacteria (PGPR) can increase the tolerance of tall fescue and hybrid bermudagrass to white grubs, and if PGPR are compatible with neonicotinoid insecticides commonly used for white grub control. In trials with tall fescue and hybrid bermudagrass, grasses were treated with Blend 20 or nitrogen or left non-treated, then infested with Japanese beetle grubs. PGPR and nitrogen fertilized grasses produced significantly more top growth than the non-treated infested controls. Bacteria treated roots tall fescue roots produced greater fresh and dry mass than non-treated and fertilized grasses. Bacterial treated hybrid bermudagrass roots produced greater root mass than non-treated and fertilized roots. No treatment negatively impact grub survival, and weight gains were similar for all treatments. Bacterial mediated interactions increase tolerance of tall fescue and hybrid bermudagrass applied in response to white grub infestation. Application of PGPR to increased root biomass over non-treated and fertilized grasses. Rhizobacterial products have utility for IPM of root herbivores.
Chapter 5 was focused on experimental verification of drought observations made with bermudagrass and PGPR. Drought and water scarcity due to unavailable irrigation are major limiting factors in the productivity of grasses. Rhizobacterial inoculants have been previously shown to mitigate drought stress in crops and grasses. Experiments were designed to determine if a blend of three Bacillus strains (Blend 20) could enhance drought stress responses in hybrid bermudagrass varieties with differing drought tolerances compared to fertilized and non-treated controls. Experiments were designed to examine tolerant (Tifway), moderately tolerant (LaPaloma), and susceptible (Yukon) grown pots with sand under greenhouse conditions and treated for 5 wk before being subjected to 3 wk of drought stress, and a recovery period. Drought stress response variables measured RWC, chlorophyll content, EL, and root length and weights. Bacterial inoculated grasses maintained lower RWC during drought periods, but maintained higher content than non-treated grass during recovery. Depending on the variety, bacterial inoculation may enhance chlorophyll content during and post-drought. The most pronounced benefits of bacterial inoculation were on EL and root growth. Bacterial treatment of bermudagrass could alleviate varietal EL differences between LaPaloma and Yukon varieties. Roots of bacteria-treated grasses often had increased root fresh and dry weight and length over non-treated and fertilized grasses. The results of these experiments confirm the observations that PGPR can mediate or alter abiotic stress responses in hybrid bermudagrass. Furthermore, it provides a better understanding of plant-microbe-interactions in amenity grasses which can aid in incorporation of biostimulants for turfgrass management in areas with reduced water availability.
Chapter 6 provides a summary of the major findings and results. The summary presents future research avenues for PGPR in turfgrass and with insects and drought stress experiments.