In a bowl, whisk together the sour cream, mayonnaise, cumin, coriander, paprika, garlic powder and lemon juice. Taste and correct seasonings, including salt if needed. Set aside.
Place the husked corn directly onto grill or oven grates. Grill the corn for about 5 minutes, undisturbed, or until kernels begin to brown. Turn over and repeat. When all sides are browned, remove corn cobs.