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Ecology and Epidemiology

Interactions of Pseudomonas fluorescens strain E6 with Ornamental Plants and Its Effect on the Composition of Root-Colonizing Microflora. G. Y. Yuen, Department of Plant Pathology, University of California, Berkeley 94720, Present address: Department of Plant Pathology, University of Hawaii, Honolulu 96822; M. N. Schroth, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 76:176-180. Accepted for publication 30 August 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-176.

Pseudomonas fluorescens strain E6 increased growth of carnation, stock, sunflower, vinca, and zinnia when inoculated onto seeds or rooted cuttings. Fresh top weights in E6-treated plants 3-4 wk after inoculation were 18-41% greater than those of controls. However, seed inoculation with E6 frequently restricted growth of balsam, marigold, and morning glory by 7-13% and had no affect on cleome, nasturtium, and scarlet runner bean. Enhancement of zinnia growth by inoculation with E6 was not influenced by soil type. The treatment increased top weight in 19 of 23 experiments in four field soils with varying texture and pH. Population sizes of E6 on zinnia roots, however, differed among the soils. Growth promotion in zinnia by strain E6 was related to a change in the composition of root microflora and a reduction in the deleterious effects of minor pathogens. Colonization of zinnia roots by Penicillium spp. was less following inoculation with E6, whereas colonization by Fusarium spp. was greater. The treatment did not change the total number of fungi or bacteria on the roots. When zinnias were planted into soils infested separately with 10⁴ to 10⁵ propagules of Eupenicillium javanicum, Penicillium janthinellum, P. citreonigrum, or P. citrinum (each isolated from zinnia roots) per gram of soil, top weights of plants after 3 wk were reduced by 23-57%. Inoculation of zinnia seed with E6 prior to planting in soils infested with E. javanicum, P. janthinellum, or P. citreonigrum resulted in reduced root colonization by Penicillium spp. and in plant growth similar to that in noninfested soil. Zinnias were not affected when planted into soils infested with other root-colonizing fungi.