The RHS (Royal Horticultural Society) sells little packets of mycorrhizal powder that you can put into your garden if you feel it is lacking beneficial fungi. Another more natural route is to bury a kilo of cooked white rice near a very healthy tree, where the soil is soft and 'healthy' then retrieve it after a week. It will be mouldy, but with the right type of mould. Mix that into compost, grow tomatoes in that compost, then when they are finished, chop up their roots, mix it into the compost again, add fresh compost from your compost bin to make seed compost. Mix that seed compost into whatever you want to 'infect'. Some people grow just the fungus using sprouted barley and add the mouldy sprouted barley to their compost.