If you are using AtoM 2.5 or later, then we have support for Markdown throughout AtoM's records.
First, navigate to Admin > Settings > Markdown, and make sure that "Enable Markdown support" is set to yes.
Note that if you are making this change (i.e. if the setting isn't already on), then you will need to reindex your site. In the command-line, run the following from AtoM's root installation directory (typically /usr/share/nginx/atom if you have followed our recommended instructions):
Now we can use Markdown to style your content in descriptions, including in the "Sources" field of the Control area on a description.
We can add italics by encapsulating our words either in asterisks, or underscores - i.e. either *like this* or _like this_. In the image below, the box in grey shows what you would type in the edit field, while below that you see how it will render in the view page: