Allium problem

JulieH6

Hi I planed a number of allium bulbs last year.  They started to look promising, ranging from having grown about 1ft to about 3 ft in height, some with the flowers just starting to open and fairly suddenly an area of their stem (usually about a couple of inch, about half way up) has gone black and then withered.  The soil they are planed it is quiet heavy clay.  Any help with this problem would be welcome.