Mycorrhizal Fungi

When using mycorrhizal fungi inoculants, it is important to consider what effect your fungicide treatments will have. It would make no sense to inoculate with mycorrhizal fungi, only to kill them with fungicide application. The obvious question is, “Can fungicides be applied at all when mycorrhizal fungi inoculants are being used?”

Exhaustive tests have been performed to determine if  the VAM (endo) fungal spores in our products are viable not only throughout the production stage, but all the way to the distributor shelf.  This involves testing viability after production, packaging, shipment and storage conditions.  This bulletin describes some of those trials.

Mycorrhizal fungi all reproduce by means of spores. However, these spores differ dramatically in their size, shape, color, longevity, dormancy, and manner of dissemination. This technical bulletin will consider the major commercially significant differences between spores of ectomycorrhizal fungi and VA (endo) Mycorrhizal fungi.

Plant roots are intimately associated with soil, both physically and chemically. Soil salinity (saltiness) is an important parameter that affects plant growth and nutrition. Plant roots must draw water and dissolved nutrients from soil. If the soil is salty, the roots must work harder, since salt tends to draw water from the root. The saltier the soil, the stronger is the challenge to the root’s draw. Soil can contain various salts, such as sodium chloride (NaCl, or table salt), or calcium chloride (CaCl2), among others.

Five lots of aged VAM (endo) fungi spore cocktails were tested for viability. The tests were performed on representative samples removed from VAM cocktail batches being shipped from the production facility in Texas to our lab in Pittsburgh.  The samples of each lot have been stored in an inoculant storage room since they were prepared. The inoculant storage room had a wall-mounted A/C in addition to the building’s central A/C.  The temperature fluctuates between 18 and 22 C in the room.* The cocktails are stored inside zip-loc bags on the shelves which are covered with black plastic sheeting.  The spore collection includes representative samples of all lots of VAM fungi spore cocktail shipped to Pittsburgh since 1998.

Mycorrhizal fungi are well known for the beneficial services they perform for plants, specifically regarding increased absorption of water and mineral nutrients. A recently discovered organic substance made by VA (endo) mycorrhizal (VAM) fungi has been shown to have significant positive effects on soil structure. This material is called “glomalin”, named after the Glomales, the taxonomic order of fungi to which VAM fungi belong. So far, VAM fungi appear to be the only producers of glomalin, which was discovered in 1996 by Sara F. Wright, a scientist with the USDA Agricultural Research Service.

Since commercial mycorrhizal fungi inoculants for the landscape industry were first introduced in the 1990’s, a lot of competing companies have entered the market with knock-off versions of PHC products.  With several choices available, it is important to recognize quality. The best test of quality is objective testing and experience. However, there are several reliable clues that can be gleaned from the label. A few of the most common labeling tricks are discussed here.

Quality control of biological products is a tricky matter. After all, such products involve live ingredients. So not only is there concern about proper formulation, one must also be concerned about viability, that is, the survival of the living organisms in the mixture during production, packaging, shipment and prolonged storage. You can mix the product faithfully according to a prescribed recipe. But if the living components are dead by the time the customer opens the package, all that careful attention to formulation goes right down the drain.

A drought is a particular stress that can have short and long-term effects on tree health. These effects should be taken into consideration when planning a fall fertilization treatment subsequent to a severe drought.

In natural settings, the vast majority of plants associate with mycorrhizal fungi. Mycorrhizal plants occur in all terrestrial environments where their host plants grow. This includes soils with varying pH from as low as 3 to as high as 9.5. However, at the extremes of this range, the occurrence of plants is much more limited. Most plants grow in soils of pH 4.0 to 8.0.

One of the most well-documented benefits from mycorrhizal fungi is the increase in the uptake of phosphates by the host plant. Mycorrhizal fungi increase the amounts of phosphate appearing in host plant tissue, and radio tracer studies have confirmed that this phosphate is being provided via the mycorrhizal fungi.

VAM fungi colonize the roots of host plants and perform absorption services for the plant. Various studies have demonstrated that plants associated with VAM fungi show increased uptake of various materials from the soil, including water, and macro and micronutrients compared to non-VAM plants. As a result, VAM fungi improve their host plants’ ability to grow under conditions of drought stress or in mineral deficient soils.

There are several reports circulating the industry regarding mycorrhizal products that are not viable. This is due, in part, to the use of short-lived root fragment inoculum for commercial purposes, and to the unfamiliarity with the extended time requirement for assaying spore-based inoculum.

Commercial products that supply mycorrhizal fungi for landscape or agriculture must be treated differently compared to traditional fertilizers. Since you are attempting to introduce living organisms into the root zone, you need to be careful to keep the fungi alive not only during storage, but also during application. This technical bulletin discusses the temperatures that will promote or inhibit mycorrhizal development. Since mycorrhizas form in soil, the temperatures we are dealing with are soil temperatures, not air temperatures. In summer, forest soil tends to be much cooler than the air, whereas in winter, the soil is usually much warmer than the air. Throughout this bulletin, author citations are provided (Authors, year) wherever facts are quoted from published literature. Full citations are listed at the end of this bulletin. Temperatures are given in both Fahrenheit (°F) and Celsius (°C).