Yellow and mushy at the base sounds like something is attacking the bulb. If it is an insect, then Provado will probably help. If it is a fungus, you will need to remove the leaves, dig the bulb up, and perform surgery on it, to remove the rotten parts. Dust the cuts generously with wettable sulfur and cinnamon--to stop any bacterial diseases--let the cuts dry for a day or two, and replant, preferably in a different spot.