Agree thin it down to three plants max and 2 would be better IMO. You can tell the difference between the 2 varieties by the leaves and assuming you aren't saving seeds you can cross pollinate them by hand with each other if needed. Dave

Link below is to several discussions of the same question that the search pulls up and will answer your question. Looks like you have both powdery and downey mildew problems and both are discussed in the link. Dave Here is a link that might be useful: Squash leaves turn yellow discussions

I didn't think zucchini or summer squash plants split off but I think they are capable of putting down more roots. Maybe that's what happened. Blaketaylor, I'm sorry about your plants. Do you know what happened? My first 3 plants were started indoors but got touched by that late frost in May then stressed by the long cold spell. I always use fish emulsion when I transplant and use it if a plant looks like it's ailing. After mixing it up I pour the mix all over the foliage, not just around the base and roots. The plants looked so bad I decided to plant 6 more hills, 3 each of two different varieties. Eventually, the sick plants got healthy and all plants started producing at the same time. Last weekend I shredded a large zucchini in my food processor then made a recipe. I put the rest of the shredded zucchini in the frig. It has kept nicely and doesn't take long to mix a batch of cookies or muffins in the morning while it's still cool. Borrowed my DIL's mandolin so I can try a refrigerator pickle recipe and a summer squash and potato gratin. BTW my vegetable beds are a combination of lasagna gardenings and square foot. I am very happy with the lasagna method since it made it much easier to start a veggie bed on old sod. You don't have to remove any grass. I think the loose texture of the soil and the higher level of dirt compared to grassy paths has helped make my garden healthier than some neighbors during the awful cold, rainy spell we had. I know people who just gave up. We had areas of the yard with long standing puddles (also have a high water table). I thought I would have trouble this year because last fall we topped the two new beds with a thick layer of fresh from the barn steere manure. In spring it was hard and hadn't composted well. Since I planned winter squash in one and zucchini and tomato plants in the other, I decide to just dig a shallow hole, add some better compost so the seeds could get a start. Things have done well. I did not use a lot of compost or lasagna method in the beds by our kitchen porch. I don't like the soil texture at all. It seems packed down. I plan to add grass clippings, etc this fall to improve the texture.

I find the holes in the stem interesting. Were the stems intact when you planted? I cannot imagine a pest finding your plants so quickly and doing such damage. It almost makes me wonder if the plants were infested when you bought them. Normally, (and I'm in southcentral OK, so I might see pests earlier than y'all do further north) I don't see squash bugs until May or June, and it can vary from year to year. Squash vine borers are more likely to make their first appearance in June. Having either of them active this early in April just seems unlikely unless they traveled with the plants from wherever they were grown to the store and then home with you after you purchased the plants. That doesn't seem real likely either because most plant growers use pesticides in order to deliver healthy plants to the stores/consumers. Normally the SVBs don't even show up until vining cucurbits have been in the ground a while and the vines are beginning to run. There's always a possibility that striped or spotted cucumber beetles might have attacked the plants. They are notorious for showing up and attacking young plants. Either of them would have been about the size of a lady bug and either spotted or striped---we have both here. I did find a striped cucumber beetle on a plant in my garden (a weed) a couple of days ago. One thing about the cucumber beetles is that they will feed on a wide variety of plants while they wait for anything in the cucurbit family to arrive, so they could have been lurking in your area just waiting for some member of the cucumber/squash/pumpkin/melon family to pop up. I often see cucumber beetles even a month before the normal planting time for cucumbers and squash in my area. They often feed on wildflowers, cool-season ornamentals and even corn plants very early in the season. Here's some info from Clemson on cucurbit pests, and it includes a listing of pesticides. If you still have the plant remains, you might want to slit into the stems near the holes and see if anything pest-like is inside the stem. You might find a beetle or a grub inside that would provide a clue. A small whitish grub might be an indicator that you have the earliest squash vine borer infestation I've ever heard of. I'm not saying SVBs cannot appear this early, but just that it would be shocking if they have. Of course, we have had some very hot weather very early this year, so never say never. Cucurbit Pests Hazel, Wrapping them with foil works about the same as wrapping them with other recommended wrappings (nylon stockings, row cover fabric, ace bandages, gauze, etc.), which is to say it will work for a while, but as the plant grows and more and more stems and leaves become available, the SVBs will find their way in. The only way I have had any real success controlling them is by excluding the pests by growing them underneath row cover or netting firmly attached to the ground so the pests cannot fly, creep, crawl or tunnel underneath the covers. If I plant summer squash and zucchini very early (in late March or early April), I usually can get a harvest before the SVBs show up, but once they show up, the plants are on borrowed time. Instead of fighting the SVBs so hard, I just enjoy the regular summer squash early in the year, and grow Korean summer squash later in the year. The Korean squash are C. moschata and have solid stems that thwart the efforts of the borers to bore into them and through them. I love squash, but it certainly is a pest magnet in our climate, and for me, the ultimate solution is to grow the C. moschata types of summer and winter squash. I don't know why the C. moschata summer squashes have not caught on widely in the southern states where SVBs can have multiple generations and destroy the plants for months. The Korean summer squash is about the best kept secret in southern gardening that I've ever seen.