I had the same problem with a few of my lilies. Like you, I've grown numerous varieties for many years flawlessly, so I was quite surprised, and saddened to see this. At first I thought the top soil we recently added to one of our beds was the culprit, but then a few lilies in another bed showed the same signs.

So, I asked around and was told that it was due to Botrytis, which is a fungus. I did some reading on it and found that Botrytis appears where fogs or heavy dews are accompanied by warm temperatures. This sounds about right as I live in a valley between two mountains. Late evening watering could also play a factor, which my husband is very fond of doing. Something else I learned is that I may need to separate them as I've really just let them grow and grow and they could desperately using some space around them for air to circulate.

Handling of the existing problem includes stripping and properly removing all the foliage from the infected plants. Also, I was advised to treat for insects, as they will carry the fungus from plant to plant. (My petunias were also affected - all bunched up and unable to flower.) I've pruned my petunias back and they seem to be coming out of it. As for the lilies, I cut the infected ones back to just below the wannabe buds, but I can't bare to cut the ones around it that are blooming so beautifully right now, especially since they don't appear to be infected. It is my understanding that the bulbs are not affected, only what is above the surface. I did read that you could use Benlate, also known as Benomyl, to stop an infection after it has started. Or, if you prefer something more natural, here is a recipe for organic fungicide taken from the plantlilies.com website (where you can also learn more about this disease):

FUNGICIDE RECIPE
1 ts baking soda
1 litre water
1 ts dish soap or safer's soap

Mix together, spray on. Be sure to cover the complete plant, including stems, leaves and top of soil. Also be sure to spray both sides of leaves. Spray every 3-5 days to control the problem, especially while weather conditions favor fungal diseases. Use preventatively every 2 weeks.

Hope this helps!
Sunny