62 Rao et al.: Response of Oaks and Elm to Mycorrhizal Fungi and Rhizobacteria Response of Oaks and Elm to Soil Inoculations With Mycorrhizal Fungi and Rhizobacteria in a Nursery Balakrishna Rao, Donald H. Marx, and Brian Jeffers Abstract. Live oak (Quercus virginiana), laurel oak (Q. laurifolia), and Drake elm (Ulmus parvifolia) seedlings were grown for 1 year in4L(1 gal) containers and then transplanted on 3 m (10 ft) centers at a nursery in Florida, U.S. Two years later, in April 2002, ten seedlings per tree species were treated by (1) soil injection with mycorrhizal fungi and rhizobacteria; (2) drenching with rhizobacteria and soil fungi applied monthly for 5 months; (3) a combination of (1) and (2); (4) drenching with Subdue® fungicide; or (5) nontreated controls. Root growth and mycorrhizal development were measured with root ingrowth cores. After 1 year (1 April, 2003), mycorrhizal development and root growth as well as stem calipers were greater in treatments containing the mycorrhizal fungi for all three species. The rhizobacteria treatment also increased root and stem growth on Drake elm. The fungicide, Subdue, did not significantly affect mycorrhizal development or root or stem growth. There were few naturally occurring mycorrhizae on roots of trees in this nursery. Key Words. Ectomycorrhizae; vesicular-arbuscular mycorrhizae (VAM). Mycorrhizal fungi and rhizobacteria inhabit the fine roots of plants and impart a variety of benefits to the plant host: increased root surface area, greater absorption of water and mineral elements, increased solubility of phosphorus and other minerals, fixing of atmospheric nitrogen, reduced trans- plant shock, increased resistance to environmental stress, and decreased incidence of certain diseases (Waisel et al. 1996; Smith and Read 1997; Quarles 1999a,b). In exchange for these benefits the plant furnishes the mycorrhizal fungi and rhizobacteria with organic carbon from photosynthesis. Products containing mycorrhizal fungi and rhizobacteria are commercially available (Marx et al. 2002). Increased root and ectomycorrhizal development following root inoculation with these products, with and without fertilizer, has been reported for mature northern red oak (Quercus rubra), south- ern live oak (Q. virginiana), willow oak (Q. phellos), and pecan (Carya illinoensis) growing in urban settings (Marx et al. 1995, 1997; Smiley et al. 1997; Appleton et al. 2003). Most of these studies were not designed to measure above- ground tree response because each tree received all treat- ments in a grid pattern over the root zone. Others reported increases in root growth and mycorrhizal development and canopy growth of northern red oak in an Ohio, U.S., nursery (Rao et al. 2000) and basswood (Tilia tomentosa) in an urban setting in France (Garbaye and Curin 1996). Increased root and development of vesicular-arbuscular mycorrhizae (VAM) and improved canopy growth were also reported after VAM fungal and rhizobacterial inoculations of 3 to 4 cm (1.2 to 1.6 in) diameter transplanted maple (A. freemanni), ash (Fraxinus americana), crabapple (Malus spp.), and western hackberry (Celtis occidentalis) in urban ©2006 International Society of Arboriculture landscapes (Geist 1998) and transplanted sugar maple (Acer saccharum) (Rao et al. 2000) in a nursery. Appleton et al. (2003) reported increased root and VAM development after inoculation of 12.5 cm (5 in) diameter red maple (A. rubrum). In contrast, the aboveground response of live oak (Gilman 2001) and pin oak (Q. palustris) (Appleton et al. 2003) was not affected by application of these commercial products. There appears to be a deficiency of mycorrhizae on roots of trees in urban landscapes and in many nurseries. The low incidence of naturally occurring mycorrhizae on the nonin- oculated trees in the aforementioned studies and in others (Morrison et al. 1993; Martin and Stutz 1994; Alves and Schmitz-Zeitz 1996; Stabler et al. 2001; Wiseman and Wells, 2005), including an intensely managed tree nursery (Sylvia et al. 1998), indicates that soil and root conditions in these landscapes can suppress mycorrhizal development. Root zone inoculations with commercial inoculants in the above inocu- lation studies routinely increased root and mycorrhizal devel- opment significantly over that found on noninoculated trees. The purpose of this study was to determine the above- ground and belowground response of recently transplanted live (Q. virginiana) and laurel (Q. laurifolia) oaks and Drake elm (Ulmus parvifolia) to soil treatment with products con- taining mycorrhizal fungi and rhizobacteria in a commercial tree nursery in Florida, U.S. Live and laurel oaks form ecto- mycorrhizae, and elms form VAM (Marx et al. 2002). MATERIALS AND METHODS Study Installation One-year-old, gallon-size container-grown seedlings of live oak, laurel oak, and Drake elm were planted in blocks by tree
March 2006
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