How to recognise it
Narcissi (daffodil) bulbs in storage suffer a softening and browning of the basal plate (where the roots come out), followed by a brown to red/purple rot spreading through the inner bulb. Pink/white mould is often found between the bulb scales and on the basal plate.
With crocuses the signs of infection start while the plant is growing, with the foliage becoming yellow and dying back. The corms, when lifted, have a rot spreading up from the base of the corm which appears at the top as rings of dark, sunken spots with pale centres. The scales of the corm may be olive green in colour. As with narcissi, a pink/white mould may be associated with the rotting and may in fact envelop the entire corm.
Why it’s a problem
The fungus transfers from the soil to the bulb or corm, via the roots or adhering to the outside of the bulb/corm when it’s lifted. The fungus can rot completely through the bulb/corm, thereby killing it. Any infected bulbs/corms cannot be ‘cured’ and should be destroyed.
Where you are likely to find it
Affects narcissi, crocuses and, occasionally, lachenalias. Symptoms usually appear on narcissi 4 to 5 weeks after the bulbs have been lifted and occur most often after a hot summer.
How to prevent it
Ensure that bulbs/corms come from reliable sources so you don’t introduce the infection to your garden.
Handle bulbs and corms carefully to prevent injury, which can provide an entry point for this fungus, and don’t allow them to be damaged by exposure to direct sunlight. Any damaged bulbs/corms should be destroyed and not stored. Keep stored bulbs/corms in a cool, dry place with good air flow. Dusting with sulphur powder may help protect stored bulbs/corms.
Hot water treatment can control the fungus in narcissus bulbs. Immerse them in water heated to exactly 44.5°C for 3 hours.
Narcissi from the triandrus, jonquilla and tazetta divisions are highly resistant to basal rot.
If an area has been contaminated by the fungus then you should keep it free of bulb/corm-forming plants for three years.
How to get rid of it
If any growing plants show the symptoms, remove the bulb/corm and a spadeful of the surrounding soil. Destroy the bulb/corm and discard the soil.
Is it good for anything?!
Other useful information
Similar symptoms can be caused on cyclamen corms by a closely related fungal infection.