seysonn-do you think the yellowing is normal? It just seemed like it started when the powdery mildew came on. I'm relieved to hear that you think it looks normal! I've been pretty worried about it but like I said, all the new growth looks good.

Looks like powdery mildew. I get it most years, but oddly, almost always after the plants are already weakened by SVBs. My healthy plants never seem to get mildew.

Plants can be amazingly resilient; do give these a chance. If leaves are badly damaged, as in your fifth photo, go ahead and remove and destroy (don't compost) them, as they probably don't have enough chlorophyll-containing tissue to contribute to the upkeep of the plant. However, there is no need to remove the leaves if they still have a lot of undamaged tissue - just squish the borers inside the tunnels in the leaves; you might even be able to see them if you look closely. I know, yuk, but it's them or your zukes! Besides, that's what garden gloves are for, and they're inside the leaves anyway. I trust you made drainage holes in the bottom of that bucket; if not, you need to do that now. Be careful with the Epsom salt; don't over-do. The only yellowing I see in your photos is on heavily infested leaves, and may be a by-product of that infestation. On another social media site, someone shared a site with multiple charts illustrating how plants exhibit nutrient deficiencies. I'm posting it below in case Prof. Google hasn't already shared this one with you. (By the way, I've not had a chance tor review this yet myself.) http://bigpictureagriculture.blogspot.com/2015/12/plant-nutrient-deficiency-leaf.html Looking forward to more photos and wishing you the best of luck ... I've not even planted my zuke seeds yet! Heeled in some tomato plants in that spot and they appear to have taken over.

Another lesson from this is not to plant seedlings right on top of each other. If you put six seeds in one hole, as it appears you've done, you're going to have six plants coming out of that hole, each of which would fill that whole pot and a lot more. That will make separation of the plants without damaging the roots very hard to do. The roots of your plants are all tangled up. If you plan to put a bunch of plants in one pot, with the idea that you're eventually going to separate them, at least spread them out. I suspect that they're so tangled up that separation won't be smart. Snip away.