I have similar but less pronounced appearance on the leaves of the Oriental liles mix grown in full-full sun with couple of weeks of hot weather (90s degrees F /32oC in the shade), dayly watering. Stargazer is in the part shade, and perfectly OK.

My guess is:
1. Too much sun.
2. Frequent watering removes fertilizer from the soil and adds salts.

What can be tried:
1. Add some shade (shade cloth, rose collar (cheap!)
2. See link below about yellowing
3. Gypsum for irrigational salinity.

Just a thought...

Here is a link that might be useful: See at the bottom of the page - yellowing

Thanks for the advice! I'll give it a try!!
Suzanne

It looks like it could be either iron defiency or chlorosis. I assume it's the latter as I have the exact same symptoms with my own lilies because the pH of my soil is too alkaline (7.8). A soil test confirmed this; I also had chlorotic problems with my roses that are grafted on acid loving rootstocks. Lilies prefer slightly acidic soil to take up nutrients. A soil test would benefit you greatly. You may acicify your soil with pine needles or shredded leaves (mulch) during the fall gathered from your own natural environment. Adding agricultural or elemental sulfur to the mix would achieve long term benefits, though; I highly recommend a soil test first to be sure :)