Mycorrhizae: Evaluating The Products

Greetings again, plant people. Dr. Bob Linderman here, a retired research plant pathologist with a 50-year career focused on ornamental and nursery crop diseases and emphasizing the epidemiology and control of soil-borne, root-infecting, fungal plant pathogens, and the biology and application of beneficial microorganisms, especially mycorrhizal fungi and antagonistic rhizobacteria.

In a previous article, I described what mycorrhizae is—the symbiotic relationship between specialized soil fungi and the roots of plants—and how this relationship benefits the growth and health of plants. Given that these relationships have been helping plants grow for some 460 million years, they definitely have proven their worth. An important point to remember is that there are three main types of mycorrhizal fungi and each type forms with different plant groups. The largest group, the endomycorrhizal fungi, form an association with many different plants, including most crop plants.

The second group, the ectomycorrhizal fungi, colonize roots of pines, firs, oaks, eucalyptus, hazelnut and birch. The last group, ericoid mycorrhizal fungi, associate only with ericaceous plants like rhododendron, blueberry, azalea, etc. And don't forget that some plants simply don't form mycorrhizae, so don't bother to inoculate cabbage, broccoli, beets, turnips, radishes and carnations.

The most important part of the last article was to describe the benefits that mycorrhizal fungi give to their host plant partner: improved root development, improved transplant success, increased yield and quality, greater tolerance to plant diseases, improved soil structure due to aggregation, improved fertilizer-use efficiency, improved tolerance to soil drought and improved tolerance to soil toxicities, like salinity. So now, let's address how to evaluate mycorrhizal products from the label, how to inoculate your plants and what cultural practices may enhance or harm the formation of mycorrhizae and thus the benefits you seek. These comments are largely for endomycorrhizal fungi.

Evaluating Mycorrhizae Products

There are many mycorrhizae products on the market. How do you decide what to buy? Some of the claims are completely bogus, some are misleading. For example, microbial content and spore numbers of mycorrhizal fungi are often confusing. Some products include both endo and ectomycorrhizal fungi, plus a lot of other bacteria and fungi.

Depending on which plants you plan to inoculate, such as tomato transplants, for example, only the endomycorrhizal fungi will associate with tomato, and the ectomycorrhizal fungi are wasted. On the other hand, if you are planting hazelnut trees, the ectomycorrhiza part is needed and the endomycorrhizal fungi are wasted. And don't be deceived by the spore numbers listed, which are often a combination of all the fungi. Ectomycorrhizal fungal spore numbers are usually much higher than endomycorrhizal fungi numbers. To determine the endomycorrhizal fungi numbers for your tomato plants, consider how many spores per gram of product are listed.

I have seen labels listing several endomycorrhizal fungi but with a total of less than one spore per gram—not enough to do much good! Different endomycorrhizal fungi species have different soil preferences, so providing a mixture of species allows the plant and its soil environment to determine the best of the lot.

Often the label includes a long list of other bacteria and fungi, supposedly ones that could help your plant. The truth is that most of those microbes are thrown in to convince you that the combination would be good. Many on that list may be of little or no benefit to the inoculated plant.

Products that are truly holistic are the ones where the endomycorrhizal fungi are produced in such a way that the end product contains a team of microbes grown up with the fungi from the beginning. I call that the mycorrhizosphere phenomenon and the resulting benefits to plant growth and health are the results of the team effort.

Back to spore numbers, sometimes the spore number listed can be in the hundreds of endomycorrhizal fungal spores per gram. Such high spore numbers are not likely the product of pot culturing, but rather the spores are produced in vitro (on roots grown under sterile conditions); those spores may form mycorrhizae but without any plant benefit, as would result from spores produced on plant roots. This topic is the focus of current research, but initial studies seem to indicate that higher spore count doesn’t always mean better results.

How to Inoculate Plants With Mycorrhizae

I have heard stories of people being told to coat seeds with endomycorrhizal fungal spores. In fact, that is bad advice. First of all, the fungi don't produce millions of spores, so what might get stuck on seeds would be very few spores at best. You don't want the spores stuck on the seed coat, anyway; you want them to be near or in contact with the plant roots. To deliver the spores to the roots, you could use a root dip approach with soluble products. That method gets the inoculum right to the roots.

Or you could dust the roots while transplanting with granular products. Finally, you could place granular products under or around the root ball of transplants or in the furrow under the seed at planting. Those methods seek to place the inoculum where new roots will make contact with spores and initiate the mycorrhizal symbiosis as quickly as possible. In that process, spores near the roots will germinate, and the germ tube will grow toward the root, contact it, and grow into the root. The fungus will spread inside the root, but will also grow out from the root into the surrounding soil to mine the soil for mineral nutrients and water. The microbial associates then begin to assemble the team due to the selective influence of food leaking from both the roots and the endomycorrhizal fungus.

If the product already has those team members included and waiting, the team can form and function right from the get-go. Products that have only endomycorrhizal fungus spores take a long time to assemble a good team of microbial associates. Failure to form an effective team could mean failure to reap the potential benefits of the mycorrhizal association.

Once mycorrhizae have formed, the fungal partner will go where the roots go. So the earlier the association is established, the sooner plant growth will be enhanced, and the sooner the plant will yield its products. In these cases, more inoculum is better than too little, so use enough that will ensure early and thorough root colonization.

Some people want to inoculate plants already in the soil or potting medium. This can be difficult because you cannot easily deliver inoculum to as many roots as with pre-plant inoculation. Drilling holes around the plant and filling them with inoculum mixed into the soil could result in some mycorrhizae establishment, but not as much as any pre-plant treatment. Injecting fungal spore suspensions into the root zone is also possible, but mycorrhizae formation would be limited to the injection sites, and it would take time to spread to other areas of the root system. The fungus will eventually spread, but it takes longer before plant benefits are recognized.

Once you have inoculated your plants with mycorrhizal fungi, there are some things to keep in mind as you manage your plants, specifically fertilization and pesticide applications. For most crop plants grown in soils or potting mix, fertilizers with a high amount of available phosphorous can be inhibitory, so choose fertilizers with low phosphorous in their formulation.

This is truer for potting mixes than for soil. Better yet, choose organic fertilizers instead of inorganic fertilizers. And if you think you might have some root diseases, choose a fungicide that will not inhibit the mycorrhizal fungi; if pythium or phytophthora root rots are involved, most of the fungicides that target those fungi do not harm mycorrhizal fungi. For other root diseases, however, the fungicides of choice are often inhibitory. I can tell you which ones are good and those that are not good, so contact me. And if you want to know if you have formed mycorrhizae, I can help you there, too. We analyze roots sent to us to determine the level of mycorrhizae formation.

If you suspect a root disease, I can analyze roots for pathogens and recommend treatment (if possible) that will not interfere with the mycorrhizae