Jackmanii were bred to be as tough as nails. its probably not a disease or a soil issue. is it in full sun? are the leaves that are turning white shaded by the upper leaves or by some other type of larger plant off to the side? the lack of direct sunlight will start to have an effect on them. we have a very large Jackmanii where the lower half is shaded by the top half during the height of summer when the sun is directly overhead. they turn yellow, the white. eventually dropping off. the upper leaves are fine however. this may be a factor for you...?
