Answer: Morning glories are usually quite trouble free, but one possible cause might be over-watering. These plants are very undemanding in terms of culture and the old rule of thumb of "an inch a week" from either the gardener or the sky should be sufficient. (A weekly deep soaking is also better than a daily light sprinkling.) In my experience they in fact resent any actual sogginess.
Also check that any mulch is pulled back away far enough so that it does not touch the vine, and that there is adequate space around them both for good air circulation and to allow sunlight to reach the base of the vines.
Another possible cause might be some sort of insect attack; you might wish to examine them closely for any sign of sucking insect on the undersides and along the stems.
Good luck with your morning-glories!
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