Causes of internal discoloration and potatoInternal Discoloration . rot, late blight, and suboxygenation (blackheart), also cause brown to black internal discoloration . . . day or two cause a dark discoloration. . when pierced with a fork and the internal . . during the growing season may cause some potatoes to . . Do not use potatoes with severe internal discoloration. What should I do if I find a rock in a bag of potatoes or find the . describes the rust-brown discoloration, in the form of ... more