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: