Mycorrhizae: The Good Guys in the Ground (Part 2)

Even More Fun Facts about Mycorrhizae + How to Encourage it in your Soil

You may remember in my last post that I talked about several benefits of mycorrhizal fungus and Rhizobial bacteria.

Today, we’ll discuss some more cool facts about mycorrhizae, and then I’ll give you a few tips to help you encourage mycorrhizal proliferation in your garden.

Let’s get started…..

Plants Get to Call the Shots in the Mycorrhizal Relationship

Just as with any relationship, things can occasionally go wrong. When the other half of the relationship is a fungus or a bacteria, the likelihood of this is pretty high (Kinda like if your daughter picks up her new boyfriend in a sleazy biker bar and his name is Snake. Just sayin’) However, unlike your daughter, plants have a built-in safety mechanism to keep things from getting out of control.

Plants contain jasmonic acid, which plays a major role in limiting the relationship between host and fungus, keeping it beneficial and not allowing the fungus to grow out of control. To that end, it’s been shown that jasmonic acid levels increase in the roots and shoots of colonized plants, which in turn protect the plants from an out-of-control fungus or bacteria.

It’s also been shown that mutant tomato plants insensitive to jasmonic acid are not able to regulate the extent of mycorrhizal colonization in their roots, which could lead to the mycorrhizae becoming a parasite of the host.

This is particularly dangerous in soils with high phosphorous levels, as we talked about in the last post (again, this is quite rare in “wild” conditions, so most research in this area has been done in a lab).

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Mycorrhizae are Olympic Competitors

In the initial stages of colonization, it’s been proven that certain beneficial fungi can compete with other fungi in the soil. 

Some beneficial bacterial species such as Pseudomonas can also grow so vigorously that they take up all available space on the host plant’s roots, not giving the pathogenic organisms anywhere to live. Sorry dude, no vacancy!

Another thing Pseudomonas can do is alter the iron in the soil in such a way that the pathogen can’t use it anymore.

Multiple species of Trichoderma are also currently being used or being tested for use as biological control agents because they can outcompete or kill pathogenic fungi in the soil. 

One unintended and unwanted effect of application of Trichoderma in certain situations, though, is that it doesn’t allow other mycorrhizal fungi or Rhizobial bacteria to colonize host plants where it’s been applied. 

However, this negative effect only occurs if the Trichoderma gets there first.  If the mycorrhizal fungi have already been established, the Trichoderma has no negative effect. 

As with mycorrhizal associations, Rhizobial associations can also be adversely affected by Trichoderma applications. 

When Trichoderma and Rhizobium were introduced in peanuts at the same time, root nodules on the peanuts were significantly smaller and fewer than normal.  However, this particular strain of Trichoderma is known to grow very quickly and may prevent other species from colonizing simply because it uses up all available space on the roots. 

Host Plants are Really Good Recruiters

In response to an attack by a disease or insect, many host plants can actively recruit beneficial bacteria or fungi to come defend them.

What they do is change the amount or chemical makeup of the carbon they secrete from their roots, attracting the good guys to them (remember that the beneficial fungi and bacteria associate with their host plants because they receive carbon from the plants as a benefit).

When attacked by a leaf disease, Arabidopsis (a plant species often used in laboratory testing) has been found to produce malic acid in its roots, which makes it more attractive to B. subtilis, a beneficial bacteria. 

Similarly, when sweet pepper plants were attacked by aphids, increases in a specific strain of B. subtilis were detected in the roots, although scientists have not yet been able to figure out how the peppers recruited the bacteria to come to their aid.

Mycorrhizae and Rhizobia are Bad News for Pathogen Health

Interestingly, mycorrhizae and Rhizobia can have direct effects on the health of different pathogens, whether these are diseases, nematodes or insects.

Tomatoes showed a benefit from mycorrhizae against nematodes, in that substances secreted by the colonized tomato roots paralyzed the pests and decreased their penetration into roots. 

In general, mycorrhizal protection of roots has been demonstrated against certain fungi and bacteria in tomatoes, against fungi in barley and against nematodes in bananas. 

Even parasitic plants are sometimes foiled by the action of beneficial fungi and bacteria. When searching for a plant to latch on to, parasitic plants hone in on a particular chemical exuded by the host plant.

However, when mycorrhizae have colonized a plant, that plant often reduces its production of this chemical, making it less likely to end up parasitized.

Bad bugs are also affected if they feed on colonized plants. Beet armyworms that ate the leaves of tomato plants colonized by a beneficial bacteria weighed significantly less than those that fed on noncolonized plants. 

The larvae of noctuid moths were 13% less likely to survive when they fed on colonized grass, versus feeding on grass that hadn’t been colonized.

The black vine weevil had only 50% survival rates when they fed on strawberries colonized by a beneficial fungus (Glomus spp).

It has also been found that colonized tobacco plants will transmit more nicotine (which is harmful to the bad bugs) to their leaves and roots than noncolonized plants.

BUT, Mycorrhizae and Rhizobia are Great News for Beneficial Insects

When colonized by mycorrhizae, tomato plants attract many more parasitic wasps than uncolonized plants. 

In bean plants, predatory mites were more attracted to colonized plants and helped control harmful mites. 

It has also been shown that mycorrhizae increase the number of pollinating insects that visit their hosts, possibly due to increased flower size or changes in pollen. 

Although not directly related to plant defense, improved plant health may result from mycorrhizal associations, which can indirectly allow a plant to better defend against pests and diseases, and attract beneficials as well. 

Plants Connected by Mycorrhizal Networks can Lend a Helping Hand (or branch?)

As you now know, mycorrhizae produce hyphae, which are filaments that run through the soil. These hyphae also connect plants to each other underground, often at greater distances than you might imagine. These are called common mycorrhizal networks (or CMN’s).

If you don’t believe that a fungus can possibly have much impact via its connection between plants or trees, check out this article on the Humongous Fungus (yes, really!) in Oregon. It weighs more than 200 gray whales and covers over 3 square miles and is believed to be the largest living organism on earth! This particular fungus (Armillaria) is parasitic and is killing the trees to which it’s connected, but is an amazing example of what a fungus can do, given the opportunity.

It’s already known that nitrogen fixed by legumes can be shuttled to other plants that can’t fix their own nitrogen, and water can also be transferred plant-to-plant, particularly in times of drought. 

When tomato plants colonized by a beneficial fungus were infected by a pathogen in the lab, healthy plants connected via mycorrhizae to the infected plants were found to have an average of 81%, 74.1% and 122.6% higher levels of peroxidase in their leaves.  Peroxidase is an enzyme related to plant defense. 

No changes were seen in plants grown directly next to each other but with no mycorrhizal connection.  Thus, the mycorrhizal connection somehow transmitted the danger signal to the connected tomato plants and allowed them to start mobilizing their defenses.

The healthy plants connected to the infected plants were then infected with the pathogen and disease severity was shown to be 50.2% and 63.8% reduced in these plants versus noncolonized plants that were also infected, showing that the network had stimulated an immune response even before infection began. 

How To Encourage Mycorrhizae in your Own Garden

Encouraging abundant mycorrhizae in your own garden is simpler than you might think.

Adopt a No-Till Mindset

First and most importantly, stop tilling! As I discussed in my No-Till Gardening post, tilling breaks up and destroys the hyphae produced by mycorrhizae. Hand digging on occasion will also do some damage, but not nearly as much as mechanized tilling.

If you’ve applied a nice deep layer of mulch and you’re diligent with hand weeding (and by diligent I mean going through the gardens once a week and pulling up whatever has appeared, maybe an hour a week of work), mechanical tilling really isn’t necessary. I haven’t tilled any of my gardens in over 10 years and don’t have extensive weed problems anywhere (and I have a lot of gardens!).

Keep that Soil Covered

You may remember this picture in my previous post of the hyphae I uncovered when I disturbed some decomposing straw to mound up around my potato plants.

This wasn’t an isolated incident, I’ve noticed this repeatedly over the years. Covering your soil with organic mulches, whether that be straw, wood chips, or grass clippings, will encourage the development and growth of hyphae.

You’ll also notice an increase in beneficial insects within the soil (such as earthworms and spiders). I have to admit I really should have had gloves on when moving the straw just because of the number of spiders I unearthed **shudder** (Spiders aren’t my favorite critters, even though I know they’re really beneficial in the garden).

Growing a cover crop on any unused garden areas is also a great way to encourage beneficial fungi and bacteria. Growing a legume like clover or vetch is particularly good because it will associate with Rhizobia (just like peas and beans) and fix nitrogen in your soil. Plus, the bees love the flowers!

Diversity is the Name of the Game

Because different fungi and bacteria are particularly associated with specific plants, growing a variety of flowers and vegetables will encourage the best diversity of organisms in your soil.

Practicing good crop rotation will also help DIScourage harmful pathogens, and rotating heavy feeders like tomatoes into areas previously planted in legumes will benefit your crops as well.

Organic is the Way to Go

If you’re here reading, you already know this, but specifically, I’m talking about organic fertilizers. When soil nutrient levels get very high (particularly phosphorous, nitrogen and sulfur levels), mycorrhizal fungi and beneficial bacteria can be inhibited or killed off.

Using compost tea or a good organic fertilizer (if you need a recommendation, see my Fertilizer post here or you can find it for sale here) is the best option. Because organic fertilizers are lower in nutrients than their nonorganic counterparts, you’re less likely to overfertilize your soil this way.

It’s still possible to overdo the fertilizer, but you probably won’t, especially if you’ve had your soil tested and you’re following those recommendations. If you have received recommendations from a soil testing lab, why not split your fertilizer applications over the course of the season, doing 2-4 applications instead of just one? This is less likely to be harmful to the life in your soil.

Pro Tip: Because sulfur is antibacterial, you want to be very cautious about adding it when trying to encourage beneficial fungi and bacteria. If you live in the Northeast, the chances that your soils are deficient in sulfur are very low (due to years of factory emissions that spewed it into the air, where rain washed it into the soil). In other areas of the country, get a soil test before applying sulfur, and if you find that you must, use gypsum or potassium sulfate instead of elemental sulfur.

You can purchase mycorrhizal fungi to add to your garden. There are some here, although I haven’t purchased any so can’t give you product recommendations. I can tell you that, as with probiotics for human use, the higher the number of bacteria or fungi, the better.

I hope today’s post has been interesting and helpful for you. Again, if you haven’t read Part 1, you can find it here. Also, if you’re really a glutton for punishment and want to read the full paper I excerpted these two posts from, you can find the link for that here.

As always, there are some pinnable images below, and if this post has been helpful, please pin it to your Gardening or Soil Health boards so you can find it later. Otherwise, as always, smile and have a crazy organic day!

Posts Related to Mycorrhizae: Good Guys in the Garden (Part 2)

• Mycorrhizae: Good Guys in the Garden (Part 1)
• Growing Organic Tomatoes (there are 10 other veggies under “Crop of the Week” in the “Outdoor Plants” heading in the menu)
• No-till Gardening
• The Best Organic Fertilizer
• Grow a More Sustainable Garden
• Subscribers Resource Library
• Ultimate Garden Journal