What can I do to get rid of nematodes?
If it is nematodes, there is nothing that can be done to help the affected plant officially, but I would give neem oil a try. It affects many types of pests and might work on these. Also, good soil can help reduce the rate of the nematode spread and can slow them down in terms of the damage they can cause.
I would have an extension service verify that it is indeed nematodes before doing anything beyond what I just mentioned above.
Here is additional information on nematodes: https://www.gardeningknowhow.com/plant-problems/pests/root-knot-nematode-disease-a-stunted-plant-growth-cause.htm