Sweet Potato Pie with Pecan Streusel

Sweet potatoes are the edible roots of a plant in the morning glory family. Some have tan skin and pale yellow flesh and a dry, fluffy texture when cooked, similar to that of a regular potato. Others have dark reddish or purplish skin and deep orange flesh and a soft, moist texture when cooked. The latter are sweeter and are known in the United States as yams, although they are not true yams, which belong to a different species and are rarely available in the United States. Both types of sweet potatoes can be used for this recipe, although the yam, particularly the Garnet yam, is preferred.