Pecan Mycorrhizae:
A Partnership Between Fine Roots and Fungi
Dr. Donald H. Marx
Chief Scientist, Plant Health Care, Inc.
www.planthealthcare.com

Most pecan growers are aware of the different damaging infections caused by pathogenic, disease-causing fungi that attack various parts of pecan seedlings and trees. Unfortunately, few growers are aware of a more widespread infection phenomenon caused by beneficial root fungi that symbiotically colonize the fine nonwoody roots of pecan. The colonized roots are called mycorrhizae (fungus roots). Mycorrhizae are beneficial to the growth and development of pecan since they increase the surface area of the absorbing (i.e. water and minerals) root system and also reduce root disease.

The pecan is not unique in this respect. Most plants of economic importance to man, including other tree nut crops, forest trees, agronomic and ornamental plants, and even marsh grass in an estuary, normally have mycorrhizal roots. Over 95% of all plants have mycorrhizae. There are two major classes of mycorrhizae. The class that occurs on pecan, as well as pine, spruce, fir, beech, hickory, and many other tree species is called ectomycorrhizae. The other class, endomycorrhizae, occurs on 85% of green plants but won't be discussed here.

The fungi that form ectomycorrhizae with pecan and other trees belong to a group of forest fungi that produce mushrooms and puffballs as their reproductive structures. The spores produced in these fruit bodies are carried by wind and water and are the chief means of their geographic spread.

Ectomycorrhizae were first reported on pecan in 1933 by Woodroof in GA and FL. In 1968, my colleagues and I (Powell et al., 1968) reported that a puffball fungus, called Scleroderma bovista, formed abundant ectomycorrhizae on young pecan trees in an orchard in Waycross, GA following application of various fungicides and a nematocide to control root disease. Within a few months the foliar symptoms of the root disease disappeared on most treatment trees in spite of the fact that high populations of the root pathogens (Pythium spp) and nematodes were still present in the soil. Associated with the disappearance of foliar symptoms was the presence of abundant ectomycorrhizae formed by this puffball fungus suggesting a biocontrol effect. Later, we confirmed (Marx and Bryan,1969) that S. bovista formed ectomycorrhizae with pecan after controlled seedling inoculations. We also found the fungus to produce strong antibiotics that inhibited growth of these root pathogens in lab tests. Following these reports, we examined various pecan groves and nurseries in the Southeastern US and in TX, AZ and NM (Marx, 1970; Marx, 1971a and b, Crews et al., 1978). Without exception, all trees and seedlings had some degree of naturally occurring ectomycorrhizae on their roots formed by various mushroom fungi; S. bovista was fairly common on pecans in the South but not in the West.

In the mid 1970's, we developed techniques to tailor pine and oak seedlings in nurseries with specific ectomycorrhizae formed by another puffball fungus, called Pisolithus tinctorius (Pt). We found that pine and oak seedlings with Pt ectomycorrhizae formed in the nursery survived and grew faster after outplanting on reforestation and adverse mined land sites in the Eastern US than did standard, nursery-run seedlings having only naturally occurring ectomycorrhizae (see Marx et al., 1992).

In 1979, we found that we could successfully inoculate pecan seedlings with Pt and improve their growth in the nursery (Marx, 1979). This was followed (Sharpe and Marx,1986) by another report in which pecan seedlings were found to form more Pt ectomycorrhizae and have more growth at soil pH 5.5 than at pH 6.0 or 6.5. Pt ectomycorrhizae also increased N, P, K, Ca, Mg, Cu, Mn and Zn content in the seedlings. Shortly after this was published we began receiving reports from other mycorrhizal researchers on the natural occurrence of various mushroom fungi and Pt puffballs in pecan groves in GA, SC, AL, AZ, TX and CA. This confirmed that Pt was an ectomycorrhizal fungus on mature pecan and not just seedlings.

All of the above information was only of academic interest at that time since there was no practical way of managing ectomycorrhizal fungi on pecan or any other plant. This changed in 1995, when commercial inoculants of not only Pt, but also other mycorrhizal fungi and beneficial root bacteria, became available. In 1995, Plant Health Care, Inc. pioneered the commercial production and development of these microbial technologies. We concentrated our initial marketing on forestry, mined land reclamation, arboriculture, woody landscape plants, flowers, turf and some Ag crops. Because of this, we have only a few reports on pecan using our commercial products. One report is from a pecan grower in Indiana who used our Root Dip (hydrogel with Pt spores) to reduce his transplant loss from 30 to 2%. Another report (Smiley et al., 1997) deals with mature pecan trees in an urban landscape in NC. These stressed trees were inoculated by soil injection with Pt spores and a fertilizer in water. After 7 months this inoculant plus fertilizer increased growth of fine roots and ectomycorrhizae by 4 times over nontreated controls and more than twice that of the inoculant or fertilizer applied separately.

Mycorrhizal fungal and beneficial bacterial products are available from PHC, Inc. to inoculate pecan seedlings in nurseries, root dip seedlings at transplanting, inject the root zone of mature trees or apply through drip irrigation. Unfortunately, we do not have mycorrhizal fungal products that contain spores of the puffball fungus, Scleroderma bovista. However, we have inoculants containing spores of Scleroderma citrinum, a close relative to S. bovista and, of course, various spore inoculants of Pt.

References:

1. Crews, C.S., T E. Crocker, and D.H. Marx. 1978. The potential role of ectomycorrhizal fungi to increase production of pecans. In Proc. Southeastern Growers Assoc. p 101-104.

2. Marx, D.H, and W. C. Bryan. 1969. Scleroderma bovista, an ectotrophic mycorrhizal fungus of pecan. Phytopathology 591128-1132.

3. Marx, D.H. 1970. Mycorrhizae of pecan. Proc 5th Annual Conf., GA. Pecan Growers Assoc. 1:12. May 7-8, 1970, Albany GA.

4. Marx, D.H. 1971a. Root inhabiting mycorrhizal fungi benefit growth of trees. 5th Annual Western Irrigated Pecan Growers Assoc. Conf., Jan 1971. Las Cruces, NM, p 14-18.

5. Marx, D.H. 1971b. Pecan mycorrhizae--a partnership between fungi and pecan roots. The Pecan Quarterly 5:4-7.

6. Marx, D. 1979. Synthesis of Pisolithus ectomycorrhizae on pecan seedlings in fumigated soil. USDA, Forest Service Research Note SE-283, 4 pg.

7. Marx, D.H., S.B. Maul and C.E. Cordell 1992. Application of specific ectomycorrhizal fungi in world forestry. Pg. 78-98. In. Frontiers in Industrial Mycology. Ed. by Gary F. Leatham, Chapman, and Hall, NY.

8. Powell, W.M, F.F. Hendrix, and D.H. Marx. 1968. Chemical control of feeder root necrosis of pecans caused by Pythium species and nematodes. Plant Dis. Reporter 52(7):577-578.

9. Sharpe, H.H. and D.H. Marx. 1986. Influence of soil pH and Pisolithus tinctorius ectomycorrhizae on growth and nutrient uptake of pecan seedlings. HortScience 21:1388-1390.

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

11. Woodroof, Naomi. 1933. Pecan mycorrhizae. Ga. Exp. Sta. Bull. 178:26 pg.