Potato fruit
These are genuine fruit, but not that common. Usually, potato flowers just drop off. When fruit do form, they’re more likely found on certain varieties, like Yukon Gold. This year, there were fruit on just about every Chieftain plant, here and there on the Kennebec, and none that I noticed on the Yukon Gold…
Each fruit contains 300-500 seeds that don’t come true: planting them doesn’t result in the same potatoes as the parent plant, there’s lots of genetic variation. Potato breeders plant out thousands of seeds, check out the results, then keep replanting the most desirable potatoes for 10 years or so to get stable new varieties—apparently, this is the way new potatoes are bred.
And, the fruit are poisonous, rich in solanine, not for eating (potatoes, tomatoes, peppers, eggplant and tobacco are all members of the “deadly nightshade” family, all prone to having toxic parts). Interesting! Since they suddenly appeared this year on two varieties, I’d guess it was about the weather!