By the late 17th century, potatoes had become common as a supplementary rather than a major food. The diet was mainly around butter, milk, and grain products. Potato became a base food of the poor, especially in winter. The lack of genetic variability among the potato plants in Ireland caused the emergence of "Phytophthora infestans" which had devastating effects in Ireland. Another factor is that holdings were so small that no crop other than potatoes would be able to feed a family.