Root knot nematodes are tiny 'eelworms' that live in soil and become plant parasites when they use celery roots as their nurseries. Celery requires more water than other crops, so when nematodes interfere with moisture uptake, plants begin to wilt, new growth comes to a standstill, and leaves may take on a yellowish cast because they are not getting enough nutrients from the compromised roots.
Good crop rotations prevent nematode buildup in many gardens, but root knot nematodes may be unavoidable in sandy soils in warm climates. Nematodes are not very active in cool weather, so sometimes winter celery will be successful even in soils where some nematodes are present.
Pull up affected plants and dispose of them in the bin. Mark the area where the troubled celery grew, and do not grow carrots, celery or okra there again.