Symptoms:

bulb discolors and rots, spreading upward through the inner scales, turning them chocolate brown
a pale pink fungal growth may develop on the basal plant and among the affected scales
bulbs in storage turn uniformly brown, dry out, and become mummified
those in the ground usually rot off completely, and by the end of the season there may be no trace of them
leaf symptoms on growing bulbs include yellowing, stunting, and sparse foliage
most damaging during or following hot summers
after about one month in storage, the basal plate of the bulb becomes soft and brown, then rots. The deterioration spreads to the inner scales, which turn dark brown, and a pale pink, fluffy fungal growth may then appear between the scales and on the basal plate. The bulb gradually dries out and becomes mummified. Bulbs that become infected in the ground may, if not lifted, rot away in the soil. In some cases, foliage symptoms of yellowing and wilting occur first.

Causes:

infection occurs at the basal plate (the flattened area at the base of the bulb)
persists in the soil and on infected bulbs in storage
the soilborne fungus infects bulbs through the basal plate;
the fungus is encouraged by high soil temperatures
If the infected bulb is not lifted, it may infect adjacent bulbs
stored bulbs that are infected but do not show symptoms also act as a source of infection when planted

dip bulbs in a fungicide solution, before summer temperatures are at their highest, and within 48 hours of the bulbs being lifted
dust with sulfur
early planting in autumn
check bulbs for signs of disease before storing, and discard any that are damaged.