Yes, that does look like carrot rust fly damage, which in turn opens the door for carrot canker which is what turns it the rust color. Sometimes insects will attack one plant but not the other in the same family. Not sure why that happens. Avoid planting in early spring and fall as this is when the fly lays its eggs. Do not leave the celeriac in the ground once it is ready to harvest. Harvest it and move it to storage. There are no approved chemicals that homeowners can purchase to deal with carrot rust fly. So the only solutions are organic. You can cover the crops with a fine mesh netting, remove damaged plants immediately as they give a stronger odor which could attract the flies and you could companion plant with a strong smelling plant like onions which will mask the celeriac smell. For prevention grow less susceptible varieties, such as 'Fly Away' and 'Resistafly', which are low in phenolic acid. During the winter, thoroughly fork through the soil where susceptible plants will be grown. This should expose overwintering carrot flies to frosts and predators.

Regards