Maximizing rhizosphere populations of fluorescent pseudomonads on potatoes and their effects onErwinia carotovora

Field-grown potato plants were treated withPseudomonas putida andP. fluorescens strains in order to maximize the rhizosphere populations of the pseudomonads and determine their effects on indigenous populations ofErwinia carotovora. Single applications of the pseudomonads to seed pieces at planting resulted in peak populations of approximately 10⁵ colony-forming units (CFU) per gram (fresh weight) of root in July, whereas a second application in June boosted rhizosphere populations 10- to 100-fold higher throughout the remainder of the growing season; they composed nearly 20% of the average total population of rhizosphere bacteria in early July. Although rhizosphere populations of the introduced pseudomonads were high,E. carotovora populations sharply increased from very low numbers to reach populations ranging between 10⁵ and 10⁷ CFU/g by early July. Furthermore, coapplications of pseudomonad strains failed to suppress native populations ofE. carotovora despite the growth and maintenance of high rhizosphere populations of both pseudomonad strains.