This is my first year growing parsnips. Is it true that parsnip canker shows itself on the leaves first as a redish brown colour then a yellowing of its leaves as this is whats happening to mine.
But it is early for canker and the symptoms might point to carrot fly.
Check the roots for small maggots tunnelling.
It is also possible that drought might be involved if the soil is light or they are in a raised bed, which can dry out severely.