ARS Feature Article:
Mycorrhizae: What Is It?

Inside Roses

a personal investigation into the anatomy and physiology of the rose

by Dr. Gary A. Ritchie • 8026 61st Ave. NE • Olympia, WA 98516

Or, more correctly, what are they? Mycorrhizae is plural; mycorrhiza is singular. You can determine the meaning of the word mycorrhiza by breaking it down into two parts: "myco" means "fungus" (mycology, mycologist); "rhizo" means "root" (rhizome, rhizosphere). Put the two together, and you have "root-fungus." That's right, mycorrhizae are fungi that are specifically adapted to live on or within the roots of plants.

While many fungi that colonize plant roots (e.g. fusarium, pythium, phytopthora and others) cause disease, mycorrhizae do not. In fact, mycorrhizae and plants form important symbiotic relationships. The plant provides carbon (food) to the fungus, which cannot manufacture its own food. In turn, the fungus provides certain critical services for the plant. It is not completely clear what these services are, however. Some evidence indicates that mycorrhizae assist in the uptake of mineral nutrients, phosphorus in particular, from the soil. Other evidence suggests that they aid in water uptake, while still further research suggests a role in disease resistance.

Whatever they do, mycorrhizae are very important. A large number of plant species support mycorrhizal associations. Being fungi, many mycorrhizae produce fruiting bodies in the form of mushrooms. If you are a mushroom hunter, you'll recognize the names of some of these: laccaria, hebeloma, rhizopogon and telephora are all mycorrhizal fungi. Mushrooms of mycorrhizal fungi abound on the forest floor during spring and fall.

Because of the importance of mycorrhiza, plant scientists have studied their potential use in agriculture, horticulture and forestry for many decades. Unfortunately, results of much of this research have been inconclusive and/or difficult to repeat. For example, I once read a report of a study done by scientists at the British Columbia Ministry of Forests. They inoculated several species of greenhouse-grown conifer seedlings with several types of commercially available mycorrhizal fungi to see if they would colonize the root systems. The study was repeated across three consecutive years.

They obtained different results with different conifer species and different results for each of the three years. Sometimes inoculation was successful, sometimes it was not. In one year, the non-inoculated controls had higher infection rates than the inoculated seedlings. In one case, inoculation reduced the amount of mycorrhizae present on seedlings. This study underscores the difficulties researchers have experienced working with mycorrhizae in horticultural systems.

In other studies, workers have found that non-mycorrhizal seedlings, when planted in nurseries, immediately become mycorrhizal because of the abundance of mycorrhizal spores in the nursery soil and in the air. Mycorrhizae that are present on plant crops in the nursery often disappear rapidly after the plants are moved out into a field environment. This is because the native mycorrhiza quickly overwhelm the mycorrhizae carried from the nursery.

So, what does all this mean to us rose growers? Based on work with other plants, my guess (unsupported by data) is that the roses we buy at the nursery are already mycorrhizal, having picked up the fungi naturally in the fields in which they were grown. Further, I suspect that soon after we plant them into our gardens, they become colonized by the native mycorrhiza in our garden soil. So I would be reluctant to plunk down much of my hard earned cash on rose mycorrhizae without first seeing good, hard scientific evidence of a benefit. I have not yet seen such evidence.

This article was provided to the TVRS as a courtesy by the American Rose Society.