My Favorite Recipe for Making Agar Plates Full Tutorial. Agar plates are beneficial in mushroom cultivation because you can test cultures for contamination by inoculating them with one drop of a spore syringe/LC syringe before attempting to use the syringe on a grow bag. If the agar plate shows clean mycelium after incubating you are good to go but if it shows other contamination growth it is best not to use the syringe. You can use certain techniques to clean up contaminated mushroom cultures w