Zucchini, eggplant, melon and cucumber pest/disease?

Question

I'm trying my hand at growing some vegetables on my balcony. I'm in Firenze, Italy, it's currently 30-35 degrees Celsius (about 95 Fahrenheit?) and I have full (10+ hours) sun on my balcony.
The problem is with my zucchini, melon, eggplant and cucumber plant. All four show the same issues: yellowing leaves with brown edges with holes, and its as if the plants starts to die/yellow/wilt from the bottom upwards. Some pictures that might describe this better below. I have the suspicion all suffer from the same thing as there are large similarities between the start and spread of the disease. I have no idea whether it's because of small insects, a disease, or (by now I just start guessing) something that's in the soil and infects the plants? I do not see many insects around my plants - sometimes some tiny flies (white and some black/white ones) but not many or something that is clearly noticable.
Maybe relevant information: my mint plant shows some similar damage, though much less; my tomatoes are completely unaffected as well as my basilicum, my lemon tree, oleander and lavendel. So far I tried a general insecticide spray as well as a spray based on neem oil. Neither seemed to solve the issue. I also fertilized the soil once every month or so with a general potted plant fertilizer that you add to the water.
I really hope someone can help me with this. Any suggestions, ideas or thoughts are much appreciated!!!
The pictures:
First, my zucchini is definitely in the worst shape of them all. I cut off some heavily infected/damaged leaves, hence the shape. But it just looks terrible at the moment:

Additionally, my young eggplant plants start to show some leaf damage:

Whereas the older one is still somewhat decent:

The same goes for my cucumber plant:

Brōtsyorfuzthrāx · Answer

It looks like it could be Alternaria, by the symptoms (especially the yellowing lower leaves). Alternaria is a genus of fungus that infects plants, and can even cause issues for humans. It's the closest thing in appearance I know about to what I'm seeing on your plants. Anthracnose is another possibility (it's a similar fungus). They're both common fruit rot pathogens, but they can also both affect foliage.
Anyway, Jimmy Fix-it could maybe be right about powdery mildew, though, but normally, it causes a white substance to form on the top of the leaves. I haven't looked into the symptoms much beyond the powder, but I have had it on shaded plants some years ago.
It's probably a good idea to get new plants and soil for next year, as you planned to do. I would also recommend washing/sterilizing the containers if you re-use them.

Jimmy Fix-it · Answer

If you look at the underside of the leaves visible in your second picture and at the stem in the bottom right of the last picture, you have what looks to me like powdery mildew. All four of the plants you mention are susceptible to it, in my experience.
Counterintuitively, hot dry conditions are perfect for it. My zucchini have it bad this year and I am combating it (with some success) by spraying the plants every 3 days with diluted solution: 1 tbsp. baking soda, 1 tsp. liquid hand soap, 1 tsp. vegetable oil, in 1 gallon of water. Mix well (I shake up my sprayer) and spray affected leaves top and bottom and any areas of affected stem.
If you use too much baking soda you will burn the plants, I killed a large healthy spaghetti squash plant by experimenting with a stronger solution.

Zucchini, eggplant, melon and cucumber pest/disease?

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