I know the mycologists think that gardeners are stupid for using mycorrhizal spores but there are some circumstances where it might be appropriate to use them. I am no particular fan of RHS but they still promote the use of mycorrhizal spores when planting trees and shrubs.
Most plants seem to be naturally infected by some kind of fungi and these seem to give plants advantages. The amount of research into the mutualistic symbiosis of plants and fungi is remarkable and ongoing. It may well lead us to understand the world of plants as that of vast organisms encompassing whole forests and acres of soil. Infection happens naturally. The biochemistry is fascinating and involves exudates from both roots and fungi. Both fungi and plants are fundamentally changed; unique genes are switched on and new structures are formed both by roots and fungi.
While this description seems to paint a very cosy relationship just between the fungi and the plant, this hides the interaction of the rest of the soil organisms. Bacteria, both pathogenic and benign, are trying to gain entry to the root. Fungal spores are being transported by nematodes and earthworms; hyphae and spores are being eaten by herbivorous nematodes, arthropods and fungivourous collembola and the whole lot is reliant on the constant addition of dead organic matter.
What is more there seems to be an incredible chemical communication between plants, fungi and animals living in the soil. I don't know why I am amazed at this because it has long been known that plants produce exudates in their aerial structures. Flowers produce scents and nectar to communicate their presence to pollenating organisms. Resins and exudates are often produced by stems and trunks.
So if this all happens naturally why bother with adding mycorrhizal fungi spores to planting holes or growing medium?
I would suggest that if you left wood chippings in a plastic bag they would eventually become infected with fungi. The likelihood of them being oyster mushrooms is not that great, so using spores to impregnate the wood seems to be appropriate. There are many species of fungi all inhabiting different niches. Some are saprophytic heterotrophs, others are parasitic and many are mycorrhizal. There are slimy ones, microscopic ones and enormous ones. Most reproduce using tiny spores that are ubiquitous. I would conjecture that there are oyster mushroom spores floating around my allotment in both air and the soil solution just as there are mycorrhizal fungi. Their fitness will depend upon finding an appropriate habitat. If I, as a gardener, alter the environment to favour these fungi then I am likely to accelerate their success.
So, when we add mycorrhizal fungal spores to planting holes, we are just speeding up the process and making sure the appropriate fungi are in the growing medium. Lots of commercial growing mediums are relatively sterile. Organic gardening uses the understanding we have of the natural world and turns it to our advantage.
In cultivated land hyphae may be broken up and killed so adding spores to planting holes seems to be a reasonable thing to do. Also if you are trying to reclaim degraded soil where there may be a lack of diversity, adding spores and improving the fungal environment by adding compost and woody shreddings as mulch seems to be a valuable thing to do. Gardening is an intervention and to some extent gardeners degrade the soil. We must attempt at every opportunity to increase diversity and improve the habitat of soil living organisms by adding lots of organic matter - something that Robert Pavlis questions too. http://www.gardenmyths.com/tag/compost-too-much/