garlic rotting off

Whay have some of my garlic bulbs rotted away?  They were Autumn sown and some just have a mouldy base and the roots have died!!

Does it spread easily?  How did my garlic get it?

If it is whiterot Slow worm then it's in your soil already ang could take 10 years or more to get rid of it (by not planting anything from the alium family anywhere near the garlic patch).
   Have you grown onions before and if so were they anywhere near where your garlic is planted because if so then you should have seen evidence of white rot on them aswell.

I have very bad whiterot in one bed. There seems to be more and more of this around, probably due to to 'knowledgeable' gardeners advising folks not to move their onion bed (seriously, I'm still hearing this)

 I'm using the method described under 'management decisions 'on this site to hopefully reduce the non-allium period to 3-4 years:

http://www.ipm.ucdavis.edu/PMG/r584100511.html

I've found a cheapish source of garlic powder from an equestrian supplier. I've not applied it yet but will in June.