Plant roots (yellow) colonized by mycorrhizal fungi (white). Photo: Premier Tech

The importance of beneficial mycorrhizal fungi, or mychorrhizae, to plant growth is fairly well known. (If this is a new subject for you, I suggest you first read Beneficial Fungi for Plants). We know that the vast majority of plants live in symbiosis with microscopic fungi, that they attach themselves to the plant’s roots and, acting a bit like root extensions, help the plant absorb the water and minerals it needs for its growth, promote rapid root development and allow the plant to better tolerate drought. In return, (after all, this is a symbiotic – that is, mutually beneficial – relationship), the fungi receive small quantities of carbohydrates, such as sugar, from their plant host.

Most gardeners also know of or actually purchase commercial mycorrhizal fungi (my container is labelled “mycorrhizal growth enhancer”): they’ve have been marketed for over a decade now and are widely available. They generally contain Rhizoglomus irregulare, a very common mycorrhizae that most herbacaceous plants will link to, plus sometimes other species.

Newer News: Disease Protection

What is less known about mycorrhizae is that they can also help prevent plant diseases. More and more studies are finding that the presence of mycorrhizae in the soil has an inhibitory effect on the development of pathogenic fungi such as fusarium and sclerotinia, Through a process intriguingly known as “suicidal germination”, fungal disease spores in contact with beneficial fungi are forced to germinate, then die before they can make contact with a host plant. It sounds more to me like fungal murder to me!

The tomato plant to the right suffers from fusiarum wilt.

I’ve been using mycorrhizae for years to help protect my tomato seedlings (Solanum lycopersicum) from soil diseases. Why tomatoes? Because they have a whole host of diseases (fusarium, verticillium, alternaria, early and late blight, sclerotinia and others) that survive from year to year as spores in the ground. Plus, pretty obviously from the dismal results I was getting, one if not several had set up shop permanently in my veggie garden and my crop rotation was no longer working. I attribute the healthy foliage and impeccable fruits of my tomatoes over the last 7 or  8 years to the fungi I apply combined with growing in pots in fresh soil each year and with a conscious choice to grow only naturally disease-resistant varieties. Even late blight (Phytophthora infestans), which wreaks havoc every year on my neighbor’s tomatoes just the other side of the fence, has not, so far, harmed my plants. (If I was the slightest bit superstitious, which I’m not, I would add here “touch wood”!)

Obviously, a personal experience is a single backyard proves nothing, doubly so in that I‘ve been taking other precautions, but I’ve been reading the book Myccorhizas: The New Green Revolution by scientists J. André Fortin, Christian Planchette and Yves Piché, and they have put together an impressive amount of information on the subject of how mycorrhizal fungi help prevent plant diseases.

Applying Mycorrhizae

It takes only a pinch of mycorrhizal inoculant to treat seeds.
The potting soil I use already contains mycorrhizae.

To apply mycorrhizal fungi, simply apply a pinch to the furrow or sowing hole just before you cover the seeds with soil… or start seeds indoors using a potting soil already containing mycorrhizae (my preferred method). You can also apply them by sprinkling commercial inoculant on roots of your plants as you transplant them.

Already Present?

It’s very possible that the soil in your garden already contains natural mycorrhizae, but activities as simple as as working the soil, applying pesticides or using concentrated fertilizers can reduce their population. They certainly won’t be present in potting soils (unless you buy one with added mycorrhizae).

Mycorrhizae: they may be useful in helping you prevent diseases, or maybe not. It’s up to you to decide whether you want to try them it for that purpose!20170204a

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