Without a photo, I'd say the most likely culprit is powdery mildew.
Soap spray is usually effective vs.
soft-bodied insects (aphids, ants, and others).
I personally use "the milk solution" vs.
powdery mildew. Start with 10% milk, 90% water. Make a pint or a quart (in the U.S.). In the mathematically aware world, make a liter. Then add 1 Tb. (15 to 20 mL) plain yogurt with live cultures, stir it in, and wait 30 minutes to an hour for the lactobacillus bacteria to multiply. These are the actual agents of powdery mildew's destruction. Stir the whole thing again, put it into a spray bottle, and apply to the squash plant on the top of and underneath the leaves.
If 10%/90% doesn't do the trick, increase the proportion of milk to water, but don't exceed 50%/50%. It's my experience that you want some fat in the milk (low fat is fine). Skim milk didn't work for me, but some people here say that it does work for them. Maybe it was the addition of the yogurt (a 2010 modification) that made the difference.
There's also a baking-soda preparation that can be used vs.
powdery mildew, but I haven't used it. The "Search the Forum" function, just to the left of the "Log Out" option, can help you find the specifics.
But this is all based on a hypothesis which is, ATM, unsupported by evidence and supported only by the mathematical probabilities....
Sunset Zone 17, USDA Zone 9