Don't know if you have noticed but the deer like daylilies. If you look in the garden catalogs that flood our mailboxes throughout the year, the growers seem to think that daylilies are deer proof. Real life says something different!
I have loved this particular perennial for many years. It will grow in just about any soil or under any light conditions. When I was a senior at Virginia Tech, I took a class that went on a field trip to Andre Viette’s Nursery near Charlottesville. I was in heaven because I knew my mom would love them. Took the plunge and bought $72 worth of daylilies. (I only remember this amount because it was the first thing I EVER charged on a Visa!)
Nearly 30 years later, those plants are still in my mom's yard as well as my yard, my sister's in Baltimore, my sister's in Takoma Park and my brother's in Alexandria, as well as many neighbors' yards. They are nothing if not prolific.
For the longest time I thought that this plant was disease-free. Turns out, one of my neighbors had a patch which always comes up so nice and light green in the
spring but has streaked leaves every year. (see picture) This patch has less
and less blooms over time. This year, there was a plethora of buds, which is a great thing, until the deer chomped on it the past week.
Looking up the diseases of the daylily, there really is very few, which is probably why you see so many plantings along highways and byways in Virginia. The big two are Leaf Streak and Daylily Rust.
See this link to read more about these diseases. http://www.daylilies.org/ahs_dictionary/diseases.html
The best way to control these fungal diseases is to remove the dead material to prevent overwintering on the soil. You can also look up the appropriate fungicide to treat the soil and the leaves to kill off the spores before they spread. You should also isolate any new daylilies if you already have the rust or leaf streak and/or if you are not sure your new plants have the disease as well.
My mother's daylilies have the Daylily Rust which has made for a not so good season this year for her. We will split up the plants and treat the rhizomes this fall. It is just so sad to see diseased plants in your garden, isn't it?
Keep the garden clean and this will control a lot of your problems.