If the foliage is yellow, the leaves are no longer feeding the bulb. You should be able to twist the stem out of the bulb. This task is easier if the foliage is completely dead but I very much doubt if the professional growers wait that long!

thank you, pardalinum. they are still green, very green, and one of my vines appears to have been touched by frost. it should be soon, i would think. there isn't even a touch of yellow in the asiatics.