It's literally a mildew that, when the leaves dry (and the humidity lowers) turns into powder (hence…the white color). The bugs walk across it on their way to their garden party (sorry…couldn't resist)…and track it to the next plant. Most 'maters and peppers are resistant, but your herbs: especially the sage, might be the next on the "hit" list.
How to get rid of it??
Lots of different ways really, it just depends on how organic, concerned about your ground water and bees, you are…..versus….the ole attach the noxious poison in a bottle to your hose end, add water, and just hope it works…..that I hope (since you have added organic sail)…you aren't.
There are lots of remedies:
1: only water in the morning. This allows the plant leave to dry throughout the day, limiting the fungal production
2: cut and remove any heavily affected leaves of the plant. i know they are pretty when they are full, but a squash plant with 4-5 healthy leaves will produce far more product than one that is dead from the fungus.
3: dispose of those leaves as far away from your garden as possible. DO NOT COMPOST!
4: Dry rub the remaining, healthy(ish) leaves with paper towls to remove the loose spores that can be caught in the wind or tracked to other plants by vagabond critters.
5: Keep an eye on everything else too….and especially (although I didn't mention it before): get rid of the Japanese Beetles!! They are the absolute WORST for spreading this….
Now…..There are some fungal control agents that are organic, (and the ones that aren't….but I don't use them so….)and of course both are available at the usual outposts: Lowes/Home Depot. Some people do this funky milk wash down thing, but…. Personally, I've always had the best luck when I subscribe to the clip it back, wipe it off, water at night, thin out your space to increase breeze methods….and I've even tried wiping the leaves down with oil if they weren't too far gone….I can't swear that the oil thing helped…but is sure didn't hurt.