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Mycorrhizal fungi

Also known as Mycorrhizae, Arbuscular mycorrhizal fungi (AMF), Ectomycorrhizae, Orchid mycorrhiza

Beneficial or harmlessBeneficial
Arbuscules of arbuscular mycorrhizal fungi inside flax root cells Photo: Msturmel · Public domain

In short

Mycorrhizal fungi live in partnership with the roots of most plants, trading water and nutrients, especially phosphorus, for sugars. They are naturally present in most garden soils; inoculant products are most useful in sterile or soilless container mixes and heavily disturbed soils.

How it looks

Mostly invisible without a microscope. Arbuscular fungi grow inside root cells, forming branched arbuscules and swollen vesicles; ectomycorrhizal fungi form a sheath around root tips and may produce mushrooms near trees.

Signs

No damage. Plants with a working partnership may take up water and nutrients more efficiently and tolerate drought, salinity and some diseases better.

What favours it

Living roots of a compatible host. Colonisation falls sharply when phosphorus is high, after years of heavy fertiliser, manure or compost.

Prevention

  • Avoid heavy phosphorus fertilising, which reduces colonisation.
  • Disturb garden soil as little as practical.

Treatment

Start with the first step and move down only if the problem persists.

  1. In established garden soil, inoculants are usually unnecessary.
  2. For soilless or pasteurised container mixes and transplants, an inoculant placed in contact with the roots may help; choose a product with the fungus type your plant uses.
  3. Store inoculants as directed; viability declines in storage.

Pros and cons

Pros

  • Extend the effective root area, improving uptake of water and phosphorus.
  • Associated with better tolerance of drought, salinity and some root diseases.
  • All orchids need a fungal partner for their seeds to germinate in nature.

Cons

  • Many commercial inoculants fail to colonise roots (one review found about 80% of treatments gave little or none).
  • Useless for plants that do not form the partnership, such as cabbages, kale, radish and other brassicas, and also beet, chard and spinach.
  • Products are often unnecessary where native fungi are already present.

Look-alikes

Root rot
Rotting roots are brown or black, soft and smell bad; mycorrhizal roots look healthy and firm.
Trichoderma
A free-living soil fungus that suppresses root pathogens rather than forming a nutrient-trading partnership inside roots.

Where you find it

Most vegetables, ornamentals and houseplants (arbuscular type); oaks, birches and conifers (ectomycorrhizae); blueberries and rhododendrons (ericoid); orchids (orchid mycorrhiza).

Sources

  1. Do Home Gardens Need Mycorrhizal Inoculants? — UConn Home & Garden Education Center
  2. Mycorrhiza — Wikipedia

Written in our own words from these sources. Check a product label before using any treatment.