Spores or infective propagules: how to judge a VAM product
A mycorrhiza (VAM) pack can carry thousands of spores and still do little for your crop. What spores and infective propagules are, how each is counted, what India's standard asks for, and what to check before you buy.
In brief
- A spore is the fungus's survival and spreading unit. An infective propagule is anything in the pack that can actually start a partnership with a root: a live spore, a piece of fungal thread or a piece of colonised root.
- Not every spore counts: dead and dormant spores look much the same under a microscope as live ones.
- Spores are counted under a microscope; infective propagules are measured by growing a test plant in a dilution series, the MPN test.
- India's standard for mycorrhizal biofertilizer asks for both: at least 10 viable spores and 1,200 infective propagules per gram.
- Check the label for infective propagules per gram (MPN), buy fresh stock, keep it cool and place it near the roots. A good count is necessary, but it does not guarantee a field result.
Why one number is not enough
Mycorrhizal fungi (AM fungi, sold in India as VAM) cannot be grown on their own: they multiply only on living roots.2 So every VAM product is made on host plants and carries the fungus in some form, usually spores mixed with pieces of colonised root and fungal thread in a powder, granule or liquid carrier. The pack helps your crop only if enough of that material is alive and able to reach a root and start the partnership.
That is why the words on the label matter. A pack can show a high spore count and still do little, because some spores are dead and others are not ready to grow. Researchers who tested 28 commercial mycorrhizal products on three continents, in greenhouses and in fields,3 concluded that the use of these products is held back by inconsistent efficacy, and proposed clear quality criteria and a standard test of viability.4
What is a spore?
A spore is a unit built to survive hard times and to spread: drying, heat or a season without a host. Many fungi make large numbers of small spores, and some bacteria, such as Bacillus, form tough resting spores inside their cells.
Mycorrhizal fungi make unusually large spores, several kinds of them depending on the fungus, formed singly or in clusters in the soil and on roots. A spore can be in one of three states: alive and ready to germinate, alive but dormant, waiting for the right moisture and temperature, or dead. Under a microscope all three can look much the same.
What is an infective propagule?
An infective propagule is any piece of the fungus that can, on its own, start a partnership with a root. For mycorrhizal fungi there are three kinds:5
- Live spores that germinate and grow a thread towards the root.
- Pieces of fungal thread (hyphae) in the soil or carrier.
- Pieces of colonised root that still hold living fungus, often with its oil-filled storage bodies (vesicles).
So every infective propagule is alive, but not every one is a spore, and not every spore is an infective propagule.
Which kind matters most depends on the fungus. In one study, Glomus and Acaulospora species colonised roots from spores, from threads and from root pieces. Gigaspora and Scutellospora species started mainly from spores, a little from root pieces and not from threads.5 This is one reason a spore count can say too little about one product and too much about another.
| Spore | Infective propagule | |
|---|---|---|
| What it is | A survival and spreading unit | Any unit that can start a partnership with a root |
| Alive? | Alive and ready, alive but dormant, or dead | Always alive and able to grow |
| Includes (VAM) | Spores only | Live spores, fungal threads, colonised root pieces |
| How it is counted (VAM) | Washed out on fine sieves and counted under a microscope | Grown with a test plant in a dilution series (MPN test) |
| What the number tells you | How many spores are in the pack | How many units can actually start colonisation in a pot test |
How each is counted
Spore count. A weighed sample is stirred in water and poured through fine sieves; the spores caught on the sieves are counted under a microscope.6 This counts spores, alive or not, and misses threads and root pieces.
Infective propagules: the MPN test. The product is diluted step by step, tenfold each time, and each dilution is mixed into sterile soil in which a test plant is grown. After some weeks the roots are checked for colonisation, and the pattern of colonised and uncolonised pots gives a statistical estimate, the "most probable number" of infective propagules.7 Because it uses a living root, it counts every kind of propagule and only the ones that work. In the soils where the method was developed, it found propagules of some fungi that a spore count missed completely.7
Bacterial biofertilizers and fungi grown in the lab. Products such as Rhizobium, Azotobacter, PSB or Trichoderma are counted as colony-forming units (CFU) on lab plates. Mycorrhizal fungi cannot be grown on plates, so a CFU count does not apply to them, and the numbers on a bacterial pack and a VAM pack cannot be compared.
What India's standard asks for
Mycorrhizal biofertilizer is covered by India's Fertiliser (Control) Order. Since the 2021 amendment, the standard asks for at least 10 viable spores per gram and at least 1,200 infective propagules per gram, measured by the MPN test with tenfold dilutions, at a pH of 5.0 to 7.0.8 In other words, the rule itself treats spores and infective propagules as two different things and asks for both.
On the pack, look for both figures: spores per gram, and "IP/g" or "infective propagules per gram (MPN)". A pack that gives only a spore count tells you less.
What a lab count cannot tell you
An MPN count is the best lab measure of how much working fungus a product holds, but it is measured in pots with a test plant. In a field, the result also depends on the soil's phosphorus, the fungi already living there, the crop and how it is grown. Across 54 maize fields inoculated with the same fungus, the growth effect ranged from 12% less to 40% more.9 A good count is the minimum, not a promise.
How to choose and use a VAM product
- Read the label. Look for the FCO registration, spores per gram and infective propagules per gram (MPN), the name of the fungus, the batch number and the manufacturing and expiry dates.8
- Buy fresh and keep it cool. The fungus is alive; heat and sun shorten its life. Store the pack in a cool, dry, shaded place and use it soon after opening.
- Put it where roots will find it: on the seed, in the nursery, as a root dip or in the furrow at sowing. A propagule helps only if a root grows past it. For how roots signal to the fungus, see how mycorrhiza finds the root.
- Go easy on soluble phosphorus. High doses weaken the partnership; across field studies, phosphorus fertiliser reduced mycorrhiza by 32% on average.10
- Skip crops that do not form mycorrhiza, such as mustard, cabbage and beet (see types of mycorrhiza), and do not mix the product with fungicides unless the label allows it.
Protective coatings, such as the hydrogel beads described in our VAM field note, aim to keep more propagules alive between the factory and the field. Whatever the coating, the number that counts is how many infective propagules are still alive when the pack is used.
Editor's note: this article is based on a technical note by Dr. Abhishek Mathur. NanoKheti's editors rewrote it in plain language, checked each statement against the published research listed under Sources, and added the summary, India's standard, the buying checklist, questions and links.
