I am growing some alliums for the first time.

A couple of them (I do not know what their name is) are about 20inches tall
with green leaves which are about one and a half inches at their widest.

The tops of some of the leaves are turning yellowy/brown.

Are they short of some nutrient - or is it likely a disease?

http://www.uploadphotograph.com/?v=photoqcq.jpg