Lesson learned. Next time I'm harvesting them as soon as the tops are mostly dead. For one thing, it was hot, sweaty work digging them up when it was over 100 degrees. For another thing, they didn't look so good when I did get them out.
Some of the potatoes had already started to sprout, as you see in this picture on the left. On the right is a good example of the many potatoes I dug up that had holes in them from bugs tunneling into them. Yuck! Some of them had also rotted. I threw those away, but kept the sprouted ones, and the ones that has some holes in them, if the rest of the potato seemed OK.
Last time I grew potatoes, I hardly got out more than I put in. Central Texas isn't a very good potato growing area, but I've read that you can expect about a 5-fold yield here. However, that's much less than in good potato growing areas where you can get a 10-fold yield.
This year I planted 3 pounds of two varieties, Purple Viking and Red Pontiac, and got about twice that much back. That's an improvement, so I hope that as I get my soil better and better I might finally get up to that 5-fold yield.
I've decided I'm going to stick all these potatoes in the fridge and use them as seed potatoes to plant this winter for next year's harvest. Potatoes you want to eat shouldn't be kept in the fridge because it makes them convert their starches to sugars and taste bad, but for planting that doesn't matter. The Purple Viking and especially Red Pontiac potatoes look so bad from being left in the ground too long that I really don't feel like eating any of them anyway. Since they're already starting to sprout, I think they'll make fine seed potatoes. Putting them in the fridge should make them go dormant, and then when it's time to plant them this winter, they should sprout this fine. Maybe next time the soil will be improved enough for me to get an even better harvest, and next time I'll dig them much sooner before they start to rot or get eaten by bugs in the ground.