I regularly use neem oil in my garden to help suppress the presence of white flies, aphids and certain soft-body caterpillars. I'm curious if neem oil can be effective in addressing powdery mildew on cucumbers or pumpkins. I'm in NY zone 7a and grow in raised beds using organic methods (best I can).
It can be used for treatment of powdery mildew, but there are other, useful non chemical treatments too, such as milk. 1 or 2 parts dairy milk to 9 or 8 parts water, mixed in a sprayer, or alternatively, baking soda and oil mix. Further information on these types of control here https://www.growingformarket.com/articles/powdery-mildew-solutions#:~:text=Neem%20oil%20is%20labeled%20for,to%2014%20days%20is%20recommended., but note these are treatments for powdery mildew, not downy mildew.
The best way to get rid of powdery mildew is to spray the leaves with Lysol. I have used Lysol for fungal problems on many plants plus I have used it to get rid of the botrytis mold that can often grow on the surface of the soil of houseplants. Lysol is generally safe on most plants.
Another way to suppress populations of insects, like aphids, is to mix a few drops of dish washing liquid with water (I use distilled water) in a spray bottle and spray the plants thoroughly. I usually spray infected or susceptible plants about once a week.
Neem oil is excellent for powdery mildew. It's indicated for that usage on the bottle and is mentioned in articles on powdery mildew prevention. I used it for the first time this year and for the first time, my squash and cucumber foliage was powdery mildew-free. It's also excellent for organic gardening, unlike some of the other suggestions here. (Please don't spray your vegetable foliage with Lysol. Please.) And it's affordable. It was a few sprays per leaf with a 24oz bottle that retails for $10USD on Amazon.