Just offhand, most likely squash vine borers, which are the absolute bane of squash/cucurbit growers in the south. Just google them, and you'll learn about them. The key would be looking carefully at the vines, to see if they have penetrations surrounded by frass where the borer went in.

But that's what happens. The bug is a moth, that lays tiny eggs on the vine. The just-hatched borer perforates the (previously beautiful) vine near the root, starts eating the vine from the inside out, and the whole thing dies. There are many, many strategies to try to deal with them, none of which are all that good.

As a once-Northern gardener, who used to be buried in summer squash and cukes, squash vine borers are a horrible welcome to the south.

Yeah, that's sure what it looks like. You can do a little surgery on your plants to find out for sure. At the base near the ground, cut open the vine and see if there are white worms inside. You may not even have to cut them open. You may be able to see the crusty looking holes.

Dang, and theres nothing I can do about it?! For future reference, anyway...
The bed next to it (not pictured) has 3 kabocha squash plants as well that looks almost as bad now. However, I had about 10 squashes growing and came back to 5 which all had green caterpillars in them. Is that the borers? ugh!

Next time use something called BT worm killer. Its bacillus thuringensis that once the worms eat a bit they die. Also comes as a powder called Dipel Dust.

is bacillus thuringensis a natural organic product? i am keeping my garden chemical free.

Yes. BT is a bacteria.

As Sienna says BT is a bacteria that affects only caterpillars and is considered safe to use in an organic garden. Some people feel it must be injected into the stem to treat for squash vine borer because as soon as the eggs hatch they bore into the stem. Beneficial nemadoes are also used in this manner. Here's a post about how to do it:

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Posted by bella_trix z6b SE PA (My Page) on Wed, Apr 7, 10 at 15:19

Glib -
This is something I wrote up explaining how I injected BT and Nematodes for SVB control:

For injecting BT into SVB infected squash stems, I make up the BT at 1 teaspoon (liquid BT) to 1 cup of water (more concentrated). I've done this with Thuricide and one other type (can't remember the name). I use a 3ml syringe and a 16 to 18g needle. You can probably use a 20g and be OK. Any thing smaller (meaning larger number) might be too small. Human medical needles are usually insulin syringe needles of about 27g to 30g and are much too small. You can find larger gauge needles in Livestock supply stores (look for Farm and Ranch stores, Tractor Supply Company, Feed stores - Agway, etc or horse supply stores). They should have the larger syringes, too. I inject about 1 ml in the area of the SVB damage, sometimes more. I've also injected BT into the base of healthy squash as a preventative.

WEAR SAFETY GLASSES OR GOGGLES WHEN INJECTING THE BT!!! This is very important! The squash vine can block the needle. When this happens, the needle separates from the syringe and the BT will shoot back with great pressure directly into your eyes. Not a good idea. Also, please buy a sharps container at the drugstore for disposal of your needles. Trash collectors do not like to get stabbed with un-known needles. You can also keep and re-use the needles.

Prior to infection (or if the SVB are still laying) you can spray the outside of the squash stems with BT at 1 tablespoon to a gallon. Again, wear goggles. The BT really irritates the eyes.

I have had injecting BT work very well for the last three years. Two years ago, I injected both BT and beneficial nematodes and saved many of my already wilting squash. I never knew whether it was the BT or the nematodes that worked. Two years ago, I used BT alone on a few sacrificial squash and worked well. I have saved several that were infected and recovered.

Good luck,
Bellatrix
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It seems like there is always something wanting to eat our veggies before we do, but where there's a will there's a way to grow them. We just have to keep learning and find out what we can grow and what is too much trouble. Wishing you all the best in your endeavors.

Here is a link that might be useful: GW veg. growing post about SVB ...

Oh wow, that's quite an operation! Well I have completely given up on the current plants. It has been a real learning season. I think it is time to strip them all up and lay down a foundation of mulch and get a new watering system going. I hate the darn soaker hoses though. But, this will be nice toknow for future reference because i desperately want to grow kabocha squash. I spend about $15 on them every week so it would save me money!

thanks for the help!

My thoughts turned to your comment "I sprayed black jack organic vinegar solution for ants around august 10th" How long after that spraying did your plants look like this? Vinegar is a great organic weed killer.

Well I left town so I can't say exactly how long it was. But, within a week or two. But I did not spray it directly on the plants only on the soil around it (avoiding plant as much as I could). And I sprayed it more on my summer squash plant which is not pictured, and it certainly didn't die AS MUCH as the kabocha plants.

FWIW, the squash vine borers have preferences. They go after vines that are more easily penetrable. I don't know where Kabocha rates on the penetrability scale, but summer squash vines (zukes in particular) are pretty soft and easy to get into, certain winter squash and cuke vines, as well as melon vines are somewhat harder. I did butternut this season, and was pretty successful wrt to borers. Butternut is understood to have a pretty stout stem and is one of the more borer-resitant. I understand that borers like Hubbards so much that people often plant them as trap crops.

Well if the vines have anything to do with the hardness of the vegetable, they should be very durable. Kabocha skin is definitely as tough as acorn or butternut when it comes to the skin and structure of the squash.

Well, if I recall correctly, Hubbards are built like tanks, and they seem to be the ones that squash vine borers greatly prefer. Could be there is some other attractive thing about them to borers.

But as I said, there are many strategies to deal with borers, and all of them are, at best, if not a lot of chemicals, then a lot of work.

Well I may just forget trying the winter squash for now. But im still trying to figure out what exactly a "borer" is. Was that all of the little green caterpillars i saw inside the squash?

"All those little green caterpillars"?

Oh yeah, that's them. That's what the little hatchlings that penetrate the vine turn into. In fact, when they're suitably fattened up, the worms occupy the entire open space in the vines, which are really built like tubes.

One mitigation strategy, in fact, is to delicately slice into a vine where a worm seems to be living, grab it with a little hook, and pull it out. Then bury the wound and hope for the best. This is HIGHLY labor intensive, if you've got any large number of vines. Another strategy is to go and pick off the very tiny eggs before they hatch. That's a tweezer job, and again, HIGHLY labor intensive. Another strategy is floating row covers, which I've not yet tried, but that means you pollinate by hand, and unless you have a drip system underneath, it makes watering really inconvenient.