I don’t think it is ever a case that someone can patent the only crop you can grow. Parents are not issued for such broad categories. You cannot patent corn or potatoes as a whole. What is patentable is a specific variety of corn that you develop anew. This means that you can always grow the traditional varieties that are not (and cannot be) patented. If a seller of patented variety quadruples seed price, you can just go back to these if patented variety is not worth the extra cost.