Looks like a nitrogen deficiency. How often are you watering? You should wait until the top two inches of soil is dry before watering. Then drench the soil until you get some good run off from the drain holes. Stick your finger to you second knuckle. If it comes out dry, then water. Next time they are due for a watering, give them a feeding with a fret that has a good amount of soluble nitrogen. Too high temps can cause blossom drop, but I think overwatering/underfeeding is one of your issues. Do you have bees visiting the flowers often? Pollination could also be a problem. If there are no bees, you will need to hand pollinate with an artists paintbrush.

Squash borers will come. I always take part of the vine further down the plant and root it. Giving it more rooting points along the vine seems to keep production away from die back. I sometimes just cut the infected part away once I know the plant has additional roots. Literally, just mound up a little dirt over any part of the vine and it will take care of itself. It wants to live, too. I've got borers--but more squash than we can eat, so I just get over the appearance of the vines. (We also perpetually have white mildew from our humidity). Last year I had considerable cucumber die back, but not until I canned three batches of cucumbers. For me---the earlier you get the squash in the ground, the better off you are to beat the bug attack. Then, when it comes, it is too hot, I've gotten all the harvest I can handle and my friends are sick of my food, too, and I'm ready for the break the die back gives me.

It is normal for old zucchini leaves to yellow up and die, but it doesn't seem like this is the problem your having. It's hard to say what's going on here, for sure you are having some kind of bug problem. What I'm not sure is if the bugs are causing the leaves to die or if it is a combination of bug and some kind of disease. Also how's the drainage in those tubs? If your zucchini are sitting in water constantly they will die. If it were me I would remove all the dead foilage, and throw it in a garbage bag. If there's any dead leaves on the ground pick them up also and throw them away. Then go to the store and buy some habernero peppers or any hot chiles. Pick up some garlic too. Cut up or mash up in a processor and throw the contents into an empty milk jug. Fill the milk jug with 10% skim milk and the rest water. Throw the cap on it and sit it outside and let it sit for a day. Then spray your leaves with this solution. This should help treat any fungal disease if there is any and also keep the pests away. You will have to re-apply when it rains or if you splash it off when watering.

I planted my cucumber plants in a 30 inch pot (there are two plants) in mid May, about a week after I got them. They had perfect, green leaves for a while after I planted them. But, lately, I've noticed the leaves have holes in them:and the edges are drying up:but, it's still producing cucumbers. In fact, it's the second time it's produced. I had know idea cucumbers can reproduce. These are the cucumbers: I water the plant when it needs it, and try not too get the leaves wet. Does anybody know why the leaves are partially drying up?