I bought a lovely gardenia tree. Not only were there many yellow leaves, but some of the leaves had a dot or two that look like something had bored into the leaf. Thank you for your help with this.btw all the gardenia trees had the same problems.
I can't make an identification from your description, but can recommend treating your Gardenia with Neem Oil. Neem Oil will treat most sucking insects and will also treat most fungus issues.
Neem Oil is organic and safe for people, pets and bees.
Here are some links with more information.