Passion Flower has yellow leaves

I have several passion flower vines and all of their leaves are yellowing. They range from light green to yellow! Some of the leaves are also stippled. I have checked for mites, etc., but don't see any. They are all outside in bright sun and get plenty of water and some fertilizer. I don't over fertilize because I have read that the fertilizer will go to the leaves and inhibit flower and fruit production. They are actually all blooming nicely. Does anyone have any ideas as to why the leaves are yellowing? We have had a very hot summer here in Pittsburgh; could that be contributing? I have thought of trying epsom salts. Does anyone have any ideas and suggestions. I would really like to see dark, healthy leaves. Thanks!

Nutrient deficiencies can cause yellowing of leaves, also thrips and virus.
Have you checked for thrips? Hold a white sheet of paper under the tip of a vine and lightly flick the vine with your fingers and the thrips will fall onto the paper where you can see them crawling.
Thrips spread virus.
P. Incense all have a virus. Which variety do you have?

Thanks, I was thrips! Actually my P. Incense is the only one I have that is nice and green!

Mine are doing the same. I contribute it to the end of the season as there are no bugs on mine either. We are getting much cooler nights & I think they like warm warm warm. I noticed this happening a couple of weeks ago, when the nights were getting cooler.
It was about 34 here this AM, guess I'll be moving plants to the GH today. Phew! Glad it didn't freeze, I have about 50 plants out that need to get in the GH.

I have about 50 to bring in, too. Only, I don't have a greenhouse! Oh, well, another winter of tripping over plants, I guess.