Cereus has a point; it is natural for older, mature leaves to turn
yellow and then fall off. The problem arises when it is new foliage
doing this.

Spider mite and scale can also be the cause for yellowed foliage and
premature leaf drop.

As can fluctuating temperatures.

Maintain even moisture at the roots, although an occasional drying out
does not hurt them (as long as they don't wilt) and don't
over-fertilize as too much can result in loss of bloom (but lush
foliage).

If insects are the problem, use an insecticidal soap following package
directions. As a preventative as well as control, you can also use
Bayer 3 in 1 for Roses. Follow package directions to the letter on
this one!

Hemma