I`m as sure as one can be from a photo of that size that you`ve got blackleg there. Dig it out and burn it along with any others similarly affected.
Unless you`ve had blackleg in your patch before, it is almost certainly the result of infected seed. I lost 20% of my crop to it for several years until I changed my seed supplier, since when I`ve not seen it again.
With normal crop rotation it`s unlikely to cause problems in the future.