Ectomycorrhizal Fungus Inoculations of Established Residential Trees

Methods

Plots consisted of three mature willow oaks (Quercusphelloś), one northern red oak (Q. rubra), and one pecan (Carya illinoinensis) in an abandoned but maintained neighborhood near the Charlotte Douglas Airport, Charlotte, North Carolina. All trees were surrounded by turf. Trunk diameter at 1.3 m (4.5 ft.) above the ground line ranged from 56 to 71 cm (22 in. to 28 in.). Soil was a clay loam with pH ranging from 5.1 to 7.1.

Treatments consisted of the following soil injections:

1. MycorTree™ Pt Injectable at a rate of 114 g per 378 I (1/4 lb. per 100 gal.) of water

2. MycorTree™ Pt Injectable at the same rate plus Bartlett Boost® (28-9-9) at a rate of 18 kg per 378 I (40 lbs. per 100 gal.) of water

3. Bartlett Boost® (28-9-9) slow release fertilizer at a rate of 18 kg per 378 I (40 lbs. per 100 gallons) of water

4. Water control

Quadrants beneath the dripline of each tree were randomly assigned to each treatment. Injections were made to a depth of 20 cm (8 in.) on a 0.9 m by 0.9 m (3 ft. by 3 ft.) spacing. Injection pressure was 10 bars (150 psi) and 1.91 (0.5 gal.) of the treatment solution was injected into each hole. Application rates were 2.7 kg (6 lbs.) of nitrogen, 0.9 kg (2 lbs.) of P₂O₅, 0.9 kg (2 lbs.) of K₂O, and 2.7 billion spores of P. tinctorius per 90 sq. m (1000 sq. ft.). Injection holes were marked with color coded flags to indicate location of treatment. Treatments were made March 13,1995.

Root Ingrowth Cores (RICs) were installed March 16,1995 to monitor new root development (4). RICs are 7.6 cm (3 in.) diameter by 20 cm (8 in.) deep plastic screen cages which allow ingrowth of roots. They were filled with root-free, treated soil, and were located within 15 cm (6 in.) of an injection site.

Roundup® herbicide was applied periodically to the treatment area to eliminate grass root contamination. Five RICs per treatment were removed four months after treatment on July 18 and seven months after treatment on October 23, 1995. Soil was separated from the roots via dry screening and washing. The percentages of each root sample colonized by P. tinctorius and naturally-occurring ectomycorrhizal fungi was visually estimated on October 24, 1995. P. tinctorius is morphologically distinct with a mustard yellow color, pitted mantle and many yellow hyphal strands. Roots were dried for two days at 50°C and weighed to determine the amount of fine root growth into each RIC.

Results and Discussion

Fertilizer, MycorTree™ Pt Injectable and the combination of the two, all produced a significant and rapid increase in fine root growth and ectomycorrhizal development as compared to the control treatment (Table 1).

Both treatments that contained Pt did produce a slight but consistent suppression in the development of naturally-occurring ectomycorrhizae (Table 2). However, the total amount of ectomycorrhizae was greatly increased following application of Pt. Ectomycorrhizae, whether they be natural or from inoculation, increased on two of the three tree species when fertilizer was applied. The control treatment had the lowest levels of mycorrhizae development and the lowest amount of new fine root growth.

The combined increase in fine root density and ectomycorrhizae colonization rates should improve water and nutrient absorption. This in turn should result in healthier trees, more capable of tolerating the effects of drought, root disease and low soil fertility.

Although the scope of this study was limited, in field operations, application of mycorrhizae should be considered as a potential alternative to fertilizer to stimulate fine root and ectomycorrhizae development, where natural mycorrhizae are suppressed due to poor soil conditions. Inoculations can also be considered in sensitive areas such as near surface water, on young trees where rapid top growth should not be encouraged, on trees in confined growing space, in “all natural” programs, and on trees with root disease, especially those caused by Phytophthora. Additional research is required to confirm these recommendations.