What are the most common types of mushroom contamination?

The three you will meet most are green mould (Trichoderma), which starts white then turns bright green; bacterial wet spot, which is slimy with a sour smell; and cobweb mould, which is grey and wispy and spreads fast. Others include black or blue-green pin moulds and yeasts. Learning to tell your white mycelium from these early is the key skill.