Oops. Apparently rustic art forms remain unappreciated.
The issue with this query is: What is the issue? Is it causing a draft? Do you feel as though there might be a problem with pests emerging from the holes? Is your concern purely aesthetic, i.e. you just don’t like the pattern? Or does the thought of an indefinable black void in your quarters feel a little creepy?
Also, will your landlord allow you to smear plaster over his property?
Based on the information given, your options are limited. In terms of filling the holes, the options are putty, plaster, and (the extremely tacky, but quick, method of) filling the hole with aerosol packing foam. Just point and spray.
On the other hand, you could cover it with a piece of furniture or a wall-hanging such as a tapestry. unless it’s a “rustic” one, apparently