Mychorrhizal fungi

It probably depends on the product. I have some ericaceous feed that includes MF, and the instructions say to apply it once or twice a year as a top dressing. However AFAIK "straight" MF is generally only used once, when planting.

I think that is due to regular MF does not work with ericaceous plants? So it's not used at planting time.

Rootgrow state that one application at planting time with support the plant for its lifetime.

I also bought some ericaceous-specific MF for potting up my second blueberry bush - maybe regular MF doesn't work for them? Could well be different species, and maybe they aren't as long-lived as the alkaline-loving variety.

With my scientist hat on again, I can attest that there are a crazy number of fungal species (many of them hard to classify), so sticking with the manufacturer's instructions is probably your safest bet.