The potato is a starchy, tuberous crop from the perennial nightshade Solanum tuberosum. The word "potato" may refer either to the plant itself or to the edible tuber. In the Andes, where the species is indigenous, some other closely related species are cultivated. Potatoes were introduced to Europe in the second half of the 16th century by the Spanish, and have since become an integral part of much of the world's food supply. It is the world's fourth-largest food crop, following maize, wheat, and rice.The green leaves and green skins of tubers exposed to the light are toxic.Wild potato species can be found throughout the Americas from the United States to southern Chile.
I would say it’s the seed. Seed is the most important key to growing any organic crop, and that’s especially true for potatoes. There’s a wide variation in seed potato quality, and certified seed potatoes will always do best in terms of yield, storage and overall quality.