What would cause my nectarines to go mouldy before they are ripe? The dwarf tree is growing in a pot.

This is Brown rot caused by the wet weather conditions. Wet weather at the time of flowering helps spread the fungus. It is too late now to do anything, other than picking off infected fruit and collecting up fallen fruit, dispose of in the rubbish, rake up leaves and burn or throw in the rubbish to stop the disease spreading and overwintering in the soil. Spray with a suitable fungicide in late winter early spring before and during bud burst. Make sure the tree is pruned with an open canopy to improve air circulation and reduce conditions fungi love to multiply in.