Not too much is really known about what causes crown rot. I have
heard that it is caused by bacteria, fungus, and nematodes. All of
these reported causes of crown rot are found naturally in the soil
in many southern regions but only occasionally seem to affect a plant.
Attempts have been made to produce crown rot in healthy daylilies by
innoculating them with infected tissue from sick plants but rarely
has this been successful. This leads me to believe that daylilies
have a natural resistance to crown rot and only when they become
stressed do they succumb to the disease. This is similar to the way
some people tend to get sick when they are under stress. Stress in
daylilies can be caused by such things as prolonged high temperatures,
lack of sufficient moisture, and serious insect problems. Even blooming
and seed production takes a lot out of a plant and can stress it.
The first symptom is a yellowing of the leaves. If not treated immediately, the entire fan will soon rot off at ground level. It may affect a single fan or the entire clump all at the same time. You can be fairly sure however, that if only a single fan is infected, the others in the clump won't be far behind.
There are a few agents that are reported to be successful in treating this disease. Subdue and Banrot are multi-purpose fungicides reported to be effective. Agristrep is a bacterial agent and regular household bleach is supposed to be effective. In any case, they only work if you catch the infection before it reaches the crown of the plant. Once the crown is infected, there is very little chance of saving the plant.
I dig up the infected plant, wash off the soil, and remove any infected roots (they will be soft or slimy). Then I soak the plant's roots in water with subdue (12 drops/gal) or bleach (20:1 ratio) for about an hour. Then I pot them up, placing the pot in a shady location. If the plant survives, I water it with subdue mixture until I replant it (see culture page for planting information).
Because I have a 'day job', I don't always have time to do this procedure. However, because crown rot kills so quickly, something must be done as soon as symptoms appear. My alternative method of treatment is to completely soak the ground around any infected plant with either a Subdue or Banrot mixture. I have had fairly good success with this method also if I catch the problem early enough. See my hidden pot trick to see how I make sure I get the roots completely soaked in a short amount of time.