I thought the same thing & added more worm castings and seeweed/fish fertilizer with all of the rain we had. My Gold Yukon's that are bigger than my hand
have "hollow heart." I had never heard of this before. The inside was split and brown. They say its from too much fertilizer and too much rain/water. We got over 10 inches in June here. The taters are still yummy, I just had to cut out the middle. Has anyone else ever had this happen?
sounds like you use the same fertilizing methods as I do .... I have not grown potatoes this year, but the rest of my veg are looking superb (and huge) .... I use worm humis, seaweed/algae extract and fish hydrolosate
I also use stuff called rootgrow rootfood (it's not the normal rootgrow that is Mycorrhizal Fungi, it consists of humic and fulvic acids)
I also add weekly feeds of actively aerated compost tea that is made from worm humis, molasses, forest tpsoil, alfalfa etc etc