I don't have an issue with fungus in my small fruit pots, such as blueberries, currant, gooseberries, cherries, etc. I am very pleased with the results I get from using pine bark as a mulch and will continue with it. It suppresses weeds, helps maintain a slight soil acidity and looks nice.
However, I do not want it growing in semp and jovi pots. So I use fired Fuller's Earth, chicken grit or decorative gravel in semp and jovi pots as a top coat and try to keep a fast draining mix. However, it has been so unusually wet/humid this summer that I have had various types of mushrooms pot up in some of the semp/jovi pots. I take that as a sign to move the pot to an area where it can thoroughly dry out. Unfortunately, the winters here can be very drying. Lately, if you ignore last winter, we've been having winters with little or no snow cover. That makes the semps and jovi's dry out too much, and some winters I've had losses due to drying. As such, I'm a little reluctant to make the soil mixes be any faster draining than they are. This winter, I'm going to move some of my plants into one of the hoop house rows, and cover with plastic. Since the plants will be shielded from the wind as well as the snow, and will be in contact with the ground, I hoping that the drying out issue will be resolved.
I had not thought of using lava rock. Around here, it is available as pumice, which would be too fine or chunks about 1-2", which would be too large. However, I'll check around to see if there is a size in between. I may also try crushing some and screening.
Thanks for the suggestion. I'm always willing to give new ideas a try.