You need to look at the specific requirements for the major, and check prerequisites for the graduate level courses. You might find that some of the graduate courses are open to junior and senior students who have all of the pre-reqs. Just because there’s no undergrad plant genetics major, doesn’t automatically mean that you can’t take any coursework in that topic as an undergrad. Places with big grad programs also have space for undergrads in their labs, you just have to ask around. Also, pay attention to the USDA labs on or near each campus. What projects are ongoing there? They also have positions for undergrads and recent college grads as trainees.