How do I stop my knock out roses from the leaves turning yellow?

Here's an article that tells you the different things it could be: overwatering, under-fertilizing or a fungus: http://homeguides.sfgate.com/troubleshoot-yellow-leaves-knockout-roses-49451.html

Usually never. Its a sticky thing to deal with.

Roses love potassium, I throw our banana peels under the backs of them, also you can get food sticks for rose bushes, water them generously

You can mix a squirt of Dawn dishwashing liquid with water ( 1Tlbs Dawn to 32 oz. water bottle) and spray your Roses. Leave on at least 2 hours then rinse with water hose. Re-apply and rinse again in 2 days. Make sure you don't spray in direct sunlight. That's what my husband does to his Knock Out Roses where he works. They are lovely.

They may just need air. I have noticed with any roses, I have grown, if they have plenty of space all around them for the air to circulate, the leaves don't turn yellow. Roses don't like to be crowded and they don't like wet feet.