Our friend the spud gets a double dose of poison when grown under nonorganic conditions. Growers spray pesticides on the vines above ground, while the soil gets a dose of fungicide to prevent disease where the tubers are growing. The fungicides prevent potato blight, which was the villain in the potato famine of the mid-1800s in Ireland, resulting in the deaths of about one million people. There's no good organic means to combat it, and that keeps the price of organic potatoes to more than twice that of their nonorganic fellows.
The pesticide level for sweet potatoes is a bit lower than for the Irish variety, according to the Environmental Working Group, but it, too, is still high enough to warrant springing for the organically grown ones.