In a little black bowl.

Hello,

This year's harvest of yellow Agria potatoes has developed some small black spots that become visible after peeling. Each spot is hard and extends only a few millimeters into the flesh.

There are no significant visible problems on the outer skin. The inside of the potatoes is completely healthy, but the flesh just beneath the skin is covered with small dark spots.

What disease or condition could this be, and what causes it?