[Ashurstman]

Our continuing attempts to grow all our own herbs on the patio keeps running into problems with sage!
This years attempt has an "affliction" of greyish powdery substance on both sides of the leaves  but what is it and most important can we do anything to it - and are the leaves safe to use?

[Trillium]

Sounds like you've got powdery mildew on your sage, and yes, sage can get it. I personally would not eat the affected leaves as it's not pleasant.

THIS is what the RHS says about it.

If you're growing sage with other herbs, it's possible your sage is getting too much water and not enough air. Sage is decidedly a drier type plant and even gentle overwatering can be the kiss of death. If possible, pull out the plants, pot them separately for a while so they get good airflow over them and can dry out a bit. They don't want to be totally dried out, just less moist than other plants. And you'll likely need to treat them. When they recover, plant out again but in a drier area and spaced for air flow.

Here in Canada, I can use a spray of 1 cup water to 1/2 tsp baking soda, mix well and lightly spray over the leaves. Worse affected leaves should be pulled and binned (not composted).

[simonwatson]

I had this problem with mine. I pulled it out of the ground and potted it up. Removed all the afflicted leaves. It's just coming back now and starting to look good.

[Ashurstman]

Thanks for that! Got the solution I hope! It was in a separate pot but maybe watered too much or our wonderful British weather!

[Annen]

Mine died last year in all that rain.  They definitely don't like too much water.