That's a pretty bad case of powdery mildew. I'm kind of amazed one plant has it so bad and the other is untouched... what are the varieties? I doubt you can do much for that plant... It may still produce a little while but it is in its decline. If you have the season for starting a fresh plant I say go for it.

It is completely safe to eat the fruit.

You'll be fine eating the squash. That plant on the right is a goner. Maybe get some Daconil or Serenade and start treating the plant on the left (Zucchini) before its infected.

Mine got it last year at the end of the summer and killed it, but it was already October so it was no big deal. Fruit is safe to eat, but the plant will die slowly. new leaves will grow, get covered in the white stuff, then die. the cycle will repeat until the plant is killed. :(

Plant a new one now!

Thank you everyone, I live in Southern Calif. Our weather has been very odd this year, this white stuff has spread very quickly. It is producing great Squash, but I was VERY concerned about how safe it was to eat as I am constantly making bread and didn't want to make anyone sick.

Whoa! I've never seen such a bad case of powdery Mildew! I was literally in shock to realize it was actually a squash plant and not some velvety kind of holiday decoration or something lol...
Powdery Mildew is just a fungus and it's not toxic at all. I would suggest rinsing it all off the fruit before eating it though... the plant is pushing out fruit because it's dying and trying to create seeds (inside the fruit) to survive. The mildew will creep down the stems of the leaves and kill the plant, and being so early in the year still, the death of the plant will happen before the frost comes around. Not sure where you are, it would be a good idea to edit your name and toss in your zone so advice can be more specific, especially for the spring and fall when the weather changes.
The plant on the left is about to be over taken by the mildew, my advice would be cut out the plant on the right immediately and toss it in the garbage (NOT IN THE COMPOST) because fungus just creeps along and spraying the plant on the left while leaving contact with the one on the right is really counter productive.
The plant on the left is still salvageable. Go to the garden and get a "natural" spray... I used one with sulfur as the active ingredient and it worked well. It doesn't so much kill the existing mildew, but prevents it from spreading, so you spray the healthy leaves. Cut off the infected leaves at the base of the plant, be careful not to hurt the main core stem of the plant. Harvest all the fruit from the plant you can to give the plant the energy to push out more healthy leaves, faster and with less stress.
Your plants are in the shade and against a wall, and very thick and healthy. That's wonderful until moisture gets trapped in the soil and creates humidity around/under the plant and doesn't get a chance to evaporate so the plant can remain dry, in which case you get powdery mildew. You need to thin out the leaves of the plants... this will not hurt the plant, just thin it out with the largest leaves, leave as many adolescent, healthy leaves as possible. The spaces between the leaves will encourage airflow and help keep away powdery mildew. Also be mindful not to over water on cooler days. You may also want to spray the wall with your mildew treatment, as it may be sitting happily on the moist beautiful wall after you water the plant, encouraging spreading from plant to plant.
Take care!

Miss_Moose...............thank you so much, we are not professional growers, just a older married couple that like Zucchini & Squash bread and baking for our church. Your post was very informative, we will do as you have suggested...thank you again!!

You bet, Nickie, good luck!

Nickie, it's just been a bad spring for powdery mildew here in SoCal I think. I've already pulled and replanted all my squash due to rampant PM, which usually isn't a problem until fall for me. Even my kale is showing signs of PM, never had that happen before. I think it's the weird weather we've been having. Anyway, you should have plenty of time to replant some more if you wanted to.

Yes Humsi, we have decided to do that come Sunday, it has also spread over onto my climbing rose bush, so I will be pulling that also, which just happens to be near all my tomato plants, will be keeping an eye on them. Hopeful it wont get that far. I will edit my location, I just signed up for this site today. Thank you for responding.

I didn't think PM of squashes and PM of roses (or tomatoes for that matter) are the same fungus... but I could be wrong. They might be unrelated.

"Whoa! I've never seen such a bad case of powdery Mildew! I was literally
in shock to realize it was actually a squash plant and not some velvety
kind of holiday decoration or something lol..."

That was similar to my reaction too. I only took a quick glance at the photo as I was more interested in reading the text first and thought it was a non-edible plant in a flower garden.

That's a horrible case of PM. As others have said, pull the squash plant that's covered and spray to prevent it from taking over the zucchini.

Rodney

Thank you, we have pulled all plants, tilled up the soil and replanted today....it spread fast let me tell you that much!!

If you don't want to saturate your plants with chemicals, you can make a powdery mildew spray from milk and water, or from baking soda and water. There are many versions of this on the internet so it's easy to google and get the proportions. You have to make sure you also spray the undersides of the leaves, and do it fairly often until the problem is under control.