Most spring-flowering bulbs are sold in autumn. This is a time when mould can thrive, so check them carefully when you buy to make sure there are no signs of it – an infected bulb can easily spread the fungus to other bulbs. Lily bulbs in particular don't have a natural protective layer and are bruised easily, which can lead to rot.
Patches of mould start to appear on the bulb. Sometimes it's tucked in between the scales (in the case of lilies), or it can be found under the papery tissue around the neck.
In general, the bigger and plumper the bulb, the better. It needs to be firm and undamaged. If there's brown rot at the base of the bulb, don't buy it.
Blue mould may appear on tissue that has been slightly damaged, but you can cut it away with a clean, sharp knife and dust the wound with fungicide.