How to Grow Mycorrhizal Fungi for a Healthier Garden

For eight years, The Rodale Institute and the USDA-ARS have worked together to create an On-Farm AM Fungus Inoculum Production System. This system would allow more farmers to benefit from the environmental and economic benefits of mycorrhizae. Over the years, our tests have mostly been about finding a cheap way to do things and finding the best ways to make propagules. We’ll tell you everything you need to know to start your own on-farm system based on what we’ve learned over the years.

When it comes to agricultural ecosystems, arbuscular mycorrhizal (AM) fungi are the most important because they live on most crop plants. AM fungi are “obligate symbionts,” which means they have to attach themselves to plant roots in order to live. This connection starts a relationship that is good for both the fungi and the plant. In exchange for sugars from a plant, fungi’s long, thread-like structures, called hyphae, act as an extension of the plant’s root system and make it easier for the plant to get nutrients that don’t move, like phosphorus (P), zinc, and copper. The plant’s root hairs go about 1-2 mm into the ground, but the mycorrhiza’s hyphae go deeper and can reach up to 15 cm from the roots. The relationship between mycorrhizae and crop plants often makes the plants grow faster and produce more. But even when the plants don’t grow faster, mycorrhizae are still responsible for most of the plant’s P uptake. Some people say that mycorrhizae can also make a plant more resistant to disease, help it grow in dry conditions, and improve the structure of the soil.

Standard farming methods like heavy phosphorus fertilization and frequent tilling hurt mycorrhizae, but many environmentally friendly farming methods can help native mycorrhizal fungus populations. Mycorrhizae can be found in soils that have been intensively managed for a long time. Cover crops, a diverse crop rotation, and growing plants that work together with AM fungi can all help these populations grow.

Inoculation with mycorrhizal fungi offers another opportunity to take advantage of the benefits of mycorrhizae. Mycorrhizal spores, pieces of crop roots that have been colonized, and live mycorrhizal hyphae are all active propagules of AM fungi that can be used to “infect” other plants.

Under certain circumstances, using inoculum may be preferable to using management practices to bolster AM fungi populations. In extreme cases, like when fungicides were used in the past, it may be necessary to inoculate the soil to bring back AM fungi that have been severely damaged. In less severe cases, inoculum can be used to make seedlings that are already colonized with mycorrhizae. This way, they can start benefiting from mycorrhizae right away when they get to the field. Past research shows that this competitive advantage can have a positive impact on yields. In a 2008 study, strawberry plants that were inoculated with AM fungi before they were outplanted produced 2017% more fruit than controls that were not inoculated (Douds et al. 2008). One study with peppers saw a yield increase from 2014 to 2023 in plants that were inoculated and grown in compost-amended soil and a yield increase of 2034 percent one year after inoculation in plants that were treated with chemical fertilizers (Douds and Reider 2003). Several other crops, such as tomatoes, potatoes, onions, peanuts, watermelon, garlic, and celery, have also seen their yields rise.

While commercially produced inoculum is available, it comes at a cost to farmers. The cost of commercial inoculum includes the greenhouse or lab space needed for production, as well as the labor and time needed to separate AM fungi from the original medium and/or mix the spores with a carrier substrate. These costs, as well as shipping and handling, are all passed on to the farmer (Douds 2010).

Choosing a host plant

The most important factor in choosing a host plant is selecting a plant that supports mycorrhiza growth. Other plants that don’t get along with AM fungi are spinach, sugar beet, lupine, and mustard family plants. A dependable host for the majority of AM fungi species, bahiagrass (Paspalum notatum Flugge) has been used extensively. Pathogens should not be spread, so the host plant should be from a different family than the infected crop. Since the inoculum system is aimed at vegetable growers, bahiagrass, which is a grass, is a perfect general host. Lastly, bahiagrass won’t become a weed in the field because it is a tropical plant that dies when it freezes.

As of yet, bahiagrass seedlings are not commercially available. In order to use bahiagrass as a host plant, farmers must establish their own seedlings. We just plant bahiagrass seeds in vermiculite or seed starter and let them sprout. Then we move the seedlings to cone-shaped plastic pots that are filled with a mix of sand and soil. Taller than the typical greenhouse flat, these conical pots produce seedlings with a long root system. When planted into the bags of diluted compost, the long root ball will touch the propagules deeper in the bag more quickly than seedlings grown in pots that aren’t as deep. In the conical pots, we use a 1:3 soil:sand (volume basis) mix of sterilized field soil and coarse swimming pool sand mixed together. We have found that the bahiagrass seedlings lose iron when they are grown in regular greenhouse potting soil.

Usually, the process of germinating bahiagrass and growing seedlings begins in the greenhouse four months before the last frost date. This way, the plants can be moved as soon as possible after the last frost. (See our Quick and Easy Guide to On-Farm Mycorrhizae Inoculum Production for a printable timeline and ideas on where to find these things.) ).

Finding the right mix of media

If the host seedlings are going to be planted outside, they need to be moved to the right medium first. The nutrient availability of the medium has a significant impact on the number of mycorrhizal fungus propagules produced. Plants that are grown in places with lots of nutrients, especially phosphorus, can stop the fungus from colonizing their roots and spreading. The idea of using pure soil, pure compost, or soil that has been mixed with vermiculite has been thrown out because the AM fungus spore production was low on these media.

Select dilutions of compost, however, have been successful in producing mycorrhiza inoculum. Compost is a very rich in nutrients and has many microbes that help keep the soil healthy and keep plants from getting diseases. It gives bahiagrass all the nutrients it needs to grow. But because compost has a lot of P, it needs to be mixed with a substrate that is low in nutrients, like vermiculite, perlite, or peat. Another benefit of this dilution is the resulting light weight medium that can be easily recovered and utilized.

A study we did shows that the best rate for diluting compost depends on the type of AM fungus used and the biomass. In 2003 and 2004, experiments showed that some levels of dilution helped overall mycorrhiza growth more than others (Douds et al. , 2008). In this study, a yard clippings compost and a dairy manure and leaf compost that were high in N, low in P, and moderate in K had a high chance of working at dilutions of 1:2 to 1:4 [compost:diluent] based on volume. One type of controlled microbial compost that was high in P, low in N, and moderately high in K worked best when diluted 1:19 or 1:49. Most of the time, dilution ratios between 1:3 and 1:9 work well for us, and we’ll usually use a 1:4 volume-based dilution with yard waste compost from city composting facilities.

We also tried different types of diluent and measured how inoculum production changed with perlite, vermiculite, and peat-based potting media (Douds et al. 2019). , 2010). All of the media amendments can be used to successfully make inoculum. Spore production by all the AM fungi that were studied was not significantly different among mixes that used different diluents. However, a most probable number bioassay, which is a way to find out how many infectious propagules there are, showed that vermiculite did tend to make more overall propagules than peat-based potting media. Our theory is that the laminar sheets of vermiculite might be a great place for mycorrhizal hyphae to grow and stay put. The similar spore populations and root colonization between the three diluents support this theory.

This dilute compost mix is used to fill seven gallon plastic bags three-quarters full. As we will talk about below, field soil will be mixed into each bag to act as an inoculum starter. The bahiagrass seedlings will then be planted in this mix. The total amount of media needed is based on how many plants will be grown from seeds next spring. You can print out our Quick and Easy Guide to On-Farm Mycorrhizae Inoculum Production, which includes steps for making 200 or 400 ft3 of inoculated greenhouse potting medium. This can be used as a starting point to figure out how much raw material you need for all sizes of inoculum production.

We usually grow isolates of a certain species of AM fungus for research purposes, but one of our first tests showed that the on-farm method could also be used to grow native mycorrhizal fungi. In this early trial, contaminant fungi were found in what should have been a single species inoculum. Most likely, these harmful fungi came from soil that was mixed into the compost when it was turned over the previous year. This unintentional spread of native AM fungi showed that the on-farm method could be used to make an inoculum that was not only cheaper than commercial inoculums of a single species, but also had a wide range of locally-adapted mycorrhizal fungi that could be used to increase the native populations of a farm (Douds et al. 2005).

Research has shown that using a multi-species inoculum with local isolates is important for a number of reasons. There are many types of AM fungi that are different from each other in ways like how they colonize, how they look for P in the soil, how they can cause plant species to grow differently, and how they make glomalin. Typically absent from commercial inoculants, but present in healthy soil, Gigaspora spp. are significant producers of glomalin, which aids in soil aggregation. Also, some research shows that the native AM fungi are better at helping plants grow in their native soil than the introduced species.

Field soil can be mixed into the weak compost mix to provide native AM fungi that are well-adapted to the area and diverse in terms of taxonomy (Douds et al. 2010). Soil should be taken from a natural part of the farm, like a wood lot or fence row, to get a wide range of mycorrhizae. The soil from these places is better because it should have a lot of different kinds of healthy mycorrhizal fungi that haven’t been changed by farming. You could also get soil from a production field, but the field shouldn’t have been used to grow the crop that will be inoculated in the last two years. This precaution is to avoid introducing pathogens to the inoculum. Also, because mycorrhizal fungi can be spread out in different ways, it is best to combine four or five samples. Since most mycorrhizae are in the top 10 cm of soil, samples don’t need to include soil below this level.

Once a pooled sample has been collected, sieve out any rocks or roots. Put 100 cm3 of soil from the combined sample into each seven-gallon bag that is full of medium. Make sure the soil is well mixed, and then put four or five bahiagrass seedlings into each bag. To give the host plant the most time to grow, this step should be done as soon as possible after the last frost.

During the growing season, the bags only need 5 to 10 minutes of work per week. They should be watered as needed and weeded so that weed seed doesn’t get into the inoculum. As the host plant grows, the mycorrhizae will proliferate. At the end of the season, senescence of the host will prompt the mycorrhizae to sporulate. The spores will spend the winter in the compost and vermiculite mix, just like they do in the field. In the spring, the inoculum will be ready to use.

Infectious propagules can be spores, pieces of colonized roots, or viable mycorrhizal hyphae. All three are made using the on-farm system. There will be a lot of hyphae and colonized roots from the previous season’s growth. The number of spores can vary depending on the type of AM fungi and how close the medium was to being fully diluted. Studies in the past found that a typical 1:4 mixture of yard clippings compost and vermiculite produced 30 spores cm-3 (Douds et al. 2006). Based on the number of colonized root pieces and hyphae, the farm system creates hundreds of propagules cm-3 using the 1:4 dilution. This number is higher than the goal “mass production” inoculum density of 80–100 propagules cm-3 (Douds et al. 2005).

The spores and hyphae are mixed with the compost and vermiculite medium. The roots of the host plant must be cut up so that the mycorrhizal vesicles inside can be used. Produced by most AM fungi, vesicles are spore-like, globular organs that contain energy reserves. In the spring, the mycorrhiza can regrow from these vesicles. Using the on-farm system, bahiagrass typically have 70-80% of their root length colonized by mycorrhizal fungi. In this case, even small pieces of root contain AM fungi and can be added to the medium to increase the number of infectious propagules.

Cut off any dead leaves, take the root ball out of the bags, and shake off the medium into a large bin. This is the best way for us to get the spores and live hyphae from the inoculum. The root system can then be cut into pieces with scissors and mixed into the inoculum. The inoculum is now ready to be mixed with potting media in the greenhouse.

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