black rot, aka gummy stem blight...fungal disease

Some kind of fungus growth.
It happens to bottle gourds while curing but not with severely pronounced deep pattern like that. I will wash it with vinegar (use a brush) to kill the fungus, possibly.

It's just brown etch. An overgrowth of lignin on the skin. Not rot or fungus. Totally harmless. The flesh is still fine. Answering (yes I know it's 10 years later) because this still comes up in a Google search of butternut etch and the false answers can cause people to throw out perfectly good crops!

The squash in the OP's picture from 10 years ago is for sure showing signs of black rot/gummy stem blight. nc-crn gave the correct diagnosis.

Search results for "brown etch on squash" brings up mostly Australian websites and articles, a couple of which mentioned black rot as a cause.

Rodney

https://extension.usu.edu/vegetableguide/cucumber-melon-pumpkin-squash/gummy-stem-blight

ha ha .... old age ... like when i look in the mirror . [ten years ago post]