I also have the same problem. It may be due to lack of air circulation.
I am planning to do like the following things :

1. I did a custom made order for special tray. The tray has bottom lot
of opening. The tray will have 4 legs of one foot high, so that
there will be circulation from bottom. I am going to pick the
special trays next week.

2. I have good back yard. I will keep the tray in the back yard so that
it will have air breeze as circulation.

3. I am planning to post the same question @ wheatgrass.com.

4. I am open for any new suggestions.

--- In wheatgrass@yahoogroups.com, "wacklinda" <wack01@...> wrote:
>
> Hi, I have been growing my own wheatgrass in trays but often have
white
> mold growing at the root base. Can someone tell me why that happens?
> Thank you.
>