Mycorrhizal Fungal Inoculation of Established Street Trees

Street trees are frequently subjected to a variety of stressing environmental factors and cultural practices. Those whose roots are colonized by mycorrhizal fungi are reported to better tolerate adverse conditions. However, 6 months after inoculation of 13 cm diameter pin oak (Quercus palustris) with a commercial mycorrhizal fungal product, there was no significant response in root dry weight or root colonization. After 1 year, the same trees showed no significant inoculation response as measured by leaf chlorophyll content or trunk diameter. Root dry weight was significantly increased by a commercial mycorrhizal fungal inoculant/fertilizer combination application to 61 cm diameter willow oak (Quercus phellos). For 13 cm diameter red maple (Acer rubrum), root colonization increased significantly in response to two commercial mycorrhizal fungal inoculant treatments, and root dry weight increased significantly in response to two fertilizer treatments 6 months after application. Our data indicated no apparent measurable growth benefit, under the terms and conditions of this research, to inoculation with a commercial mycorrhizal fungal product unless combined with fertilizer. Pretreatment evaluation of roots for all tree species revealed some colonization by mycorrhizal fungi; therefore, having tree roots analyzed to determine their existing level of colonization may aid in determining whether any benefits might be derived from applying mycorrhizal fungal inoculants.