Huh. I can see why you thought that might be a watermelon seed. It sure looks like one.
It doesn’t look like a fig-leaf gourd seed, or at least not like the ones I have. So, I’m guessing you’re right, and it’s a luffa.
What did Fukuoka say about pruning fruit trees? That seems like an odd thing to say, so I’m curious about what he meant.
As far as I know, apple trees are perfectly happy to grow 40 feet tall and have all the fruits waaaaaaaaaaaaaay out of reach, so all they’re good for is hitting the ground in a spatter of gunk that could possibly injure somebody. We have one like that in a local park. Instead of being a delight, it’s a nuisance.
Same goes for the 30-foot-tall mulberry tree in one of my neighbors’ yards.
And the 40-foot-tall mulberry tree in another neighbor’s yard. And the 40-foot-tall pear tree in that same neighbor’s yard.
And a whole host of others I’ve seen.
As far as I know, unpruned fruit trees eventually become unharvestable and do nothing but create a huge mess that attracts rodents and bugs.