Propagules include spores (center) and root fragments that contain vesicles.
MYCORRHIZAL FUNGI ARE clearly important and valuable tools for every grower: farmer, horticulturist, landscaper, turf specialist, silviculturist, or home gardener. Every growing environment is different. Mycorrhizae can be temperamental, and a mycorrhizal pairing may work in the greenhouse but not in the field. Different conditions within an environment can produce different results. You will need to experiment to learn what works best. There are, however, some general rules you can follow for the best results.
Mycorrhizal associations are host-specific. Although most trees associate with ectomycorrhizal fungi and flowers and shrubs associate with arbuscular endomycorrhizal fungi, some variability occurs. Using the right fungus or fungi can be important to obtain the best results. Benefits of mycorrhizae depend on conditions but can also vary according to the particular species of fungus and host plant.
Using a mixture of various species of mycorrhizal propagules often results in the greatest benefits and usually increases the probability of your getting a good match. Each fungal species may impart its own unique abilities to offer the most benefits in total.
Not only is the proper mix important, but the delivery system makes a difference. Liquid formulations may be the best application for lawns, but they may not be appropriate for tree seedlings or some farm crops. Read labels carefully for advice regarding the best plant associations and to determine which particular species of propagules are included in the mix.
Make sure you are using viable propagules. Propagules include spores, fungal hyphae fragments, and root fragments with vesicles, but some commercial mix labels list only spore counts. Commercially produced spores last for a couple of years, and root fragments with vesicles are viable for about a year. Although they won’t all die at once, the number of viable propagules decreases with time. You can inoculate and then test the soil for their presence, or you can send propagules to a lab to test them for viability. The manufacturers of these products may also provide batch test viability results.
If the soil contains too much phosphorus, a potential host plant will not create, partner with, and form mycorrhizae. To ensure continuous colonization, avoid the use of fertilizers that contain phosphorus. If you send soil to a lab for analysis, look for phosphorus levels of less than 70 parts per million. Some suggest that phosphorus levels should be lower, in the range of 30 parts per million.
Add mycorrhizal propagules to soil mixes, including compost mixes, and use these when you’re transplanting, even if plants have already been colonized. Tests show that early colonization maximizes benefits, but colonies do not necessarily transfer to new roots in a new container if they are not replenished. The more propagules there are, the greater the chances that fungal hyphae will encounter roots.
Give the plant a head start so that the benefits of mycorrhizae are available immediately and continuously from germination. To start the formation of mycorrhizae as early as possible, roll seed in propagules. When you purchase a plant, add propagules to the soil and/or roots before you transplant.
Pesticides, herbicides, and fungicides impact both the mycorrhizal fungi and the soil organisms that interact with them. Adding the wrong–icide can kill mycorrhizal fungi. Consult the product label or online information to determine how a substance may impact precious fungi. On the other hand, you may be surprised to see which chemicals aid mycorrhizal colonization.
Because mycorrhizal fungi and networks are fragile, you must treat them with care. Disturb soils as little as possible, and avoid tilling, deep plowing, leveling, or other mycelium-destroying activities. Do not burn fields to remove stubble; high temperatures can kill the arbuscular mycorrhizal fungi and spores. Because fungi need oxygen to survive, avoid practices that result in soil compaction and poor drainage. If you add propagules to compost teas, add them immediately before application.
Store propagules within a range of temperatures that will keep them alive: 65–75°F (24–30°C) is best. Temperatures above 120°F (49°C) will damage or destroy propagules. Keep them cool, dry, and contained to avoid contamination. If you purchase and use a propagule mix, consider transferring the package to a clean, sealable glass jar; if the package is too large to fit inside, sterilize the jar and pour the mix inside. Then store the tightly sealed container in a dark, cool location, and keep the label on hand so you won’t forget what’s inside and so you can periodically check the viability.
A view of arbuscular spores and hyphae through an electron microscope shows the fragility of these organisms.
Although mycorrhizae were discovered more than a century ago, their full potential is still not known. A tremendous amount of research continues, as scientists look for ways to use mycorrhizae to increase production of food and other crops. Many universities conduct mycorrhizal research, and new studies are published regularly and available via the Internet. Several sites offer abstracts of scientific papers related to mycorrhizal fungi.
Using propagules to create mycorrhizae is a relatively new process for growers and begs for citizen science. Experiment to find ways to optimize existing processes and improve your results. Remember that results vary, even when scientists study the fungi. For example, a USDA study reviewed 568 combinations using 66 host plants and 74 fungal species. Of these combinations, 267 showed positive results and 253 showed no impact. Some studies showed that inoculated plants developed more mycorrhizal associations when no fertilizer was used, but in others, associations were best with the addition of fertilizer. Some host plants establish more mycorrhizal associations when bare roots are planted, but others do better with inoculation of the soil in which they are grown.
We still have a lot to learn. Experiment to find the best fungal combinations for the plants in your soil. And then share your results.
