Mycorrhizal Fungal Inoculants to Soil - No Answers Yet

Debate continues to rage over the validity of applying spores of mycorrhizae-forming fungi to the backfill soil as a way of ensuring landscape plant establishment, improving post-plant performance, and reducing post-plant stress. Mycorrhizal fungi (either naturally occurring or artificially introduced) are able to colonize the roots of woody plants and provide benefits such as, (1) increased water and mineral element uptake, (2) increased resistance to environmental extremes, and (3) increased resistance to disease-causing pathogens. And with the arrival of spring, the fires of this debate have been rekindled as anecdotal reports extolling the benefits of introducing mycorrhizal fungal inoculants to the soil around newly-planted and established trees and shrubs fill the radio waves and pages of the print media. But have you ever wondered if sufficient scientific evidence exists to back up claims that commercially produced inoculants actually increase survival percentages and enhance growth after installation? In an attempt to shed some light on this important question, here in chronological order, is a recap of research published on this subject to date:

• Landscape trees and shrubs growing under high fertility conditions and inoculated with vesicular-arbuscular mycorrhizae showed some positive growth responses, however, because of competition from indigenous mycorrhizal fungi, a direct response to the inoculum used in this study could not be ascertained.

  Morrison, S.J., P.A. Nicholl, and P.R. Hicklenton. 1993. VA mycorrhizal inoculation of landscapetrees and shrubs growing under high fertility conditions. J. Environ. Hort. 11:64-71.

• Pre-transplant inoculation of container-grown Argentine mesquite (Prosopis alba) enhanced mycorrhizal root colonization but suppressed post-transplant growth compared to non-inoculated control plants. This study was conducted under low soil moisture conditions and therefore reduced aboveground growth may have been an adaptive response rather than a response to the mycorrhizal inoculant.

  Martin, C.A. and J.C. Stutz. 1994. Growth of Argentine mesquite inoculated with vesicular-arbuscular mycorrhizal fungi. J. Arboric. 20:134-138.

• The growth of silver linden (Tilia tomentosa) was stimulated and yellowing of leaves in the fall was delayed when trees were inoculated with ectomycorrhizae at time of transplanting, however, the response was first noted 2 years after installation.

  Garbaye, J. and J.L. Churin. 1996. Effect of ectomycorrhizal inoculation at planting on growth and foliage quality of Tilia tomentosa. J. Arboric. 22:29-34.

• Established willow oak (Quercus phellos), northern red oak (Quercus rubra) and pecan (Carya illinoinensis) showed a significant and rapid increase in fine root growth and ectomycorrhizal development 4 to 7 months after being treated with fertilizer, mycorrhizal inoculant, and a fertilizer/mycorrhizal inoculant combination.

  Smiley, E.T., D.H. Marx, and B.R. Fraedrich. 1997. Ectomycorrhizal fungus inoculationsof established residential trees. J. Arboric. 23:113-115.

• Established live oaks (Quercus virginiana) also showed a significant and rapid increase in fine root development 6 months after fertilizer, mycorrhizal inoculant, and fertilizer/mycorrhizal inoculant treatments.

  Marx, D.H., M. Murphy, T. Parrish, S. Marx, D. Haigler, and D. Eckard. 1997. Root response of mature live oaks in coastal South Carolina to root zone inoculations with ectomycorrhizal fungus inoculants. J. Arboric. 23:257-263.

• In another study using willow oak (Quercus phellos), trees inoculated at time of installation showed no survival or growth enhancement 1 year after the inoculation treatment.

  Carlson, J., Powell, M.A., and S.L. Warren. 2000. Can mycorrhizae improve tree establishment in the landscape? Proc. SNA Res. Conf. 45:405-407.

• Spores of mycorrhizae-forming fungi added to the backfill soil around recently transplanted live oak (Quercus virginiana) had no significant effect on post-transplant stress, tree survival, or growth.

  Gilman, E.F. 2001. Effect of nursery production method, irrigation, and inoculation with mycorrhizae-forming fungi on establishment ofQuercus virginiana. J. Arboric. 27:30-38.

• Using established pin oak (Quercus palustris), willow oak (Quercus phellos), and red maple (Acer rubrum), scientists found no apparent measurable growth benefit to inoculation with a commercial mycorrhizal fungal product, unless it was combined with fertilizer.

  Appleton, B., J. Koci, S. French, M. Lestyan, and R. Harris. 2003. Mycorrhizal fungal inoculation of established street trees. J. Arboric. 29:107-110.

And there you have it. Give or take a stray scientific paper or two, this is the sum total of the published research on this topic. Therefore, it seems premature to suggest adding spores of mycorrhizae-forming fungi to the backfill soil should become standard operating procedure when installing woody plants this spring. Of course, in years to come, as we learn more from the scientific community and green industry professionals, we may in fact fully embrace this promising but largely unproven treatment. But in the meantime, let's all agree to continue stressing the proven importance of choosing the right plant for a given site, adherence to proper planting technique, and the need for delivering timely and appropriate amounts of water to newly-planted trees and shrubs.

This article originally appeared in the 4/23/2004 issue.