Phlox eelworm

Symtpoms
Stems appear stunted and swollen, with leaves at the shoot tips much reduced in width. Such shoots dieback early, having failed to flower. Please note that cold weather snaps in the spring can also cause distorted foliage, but this is usually less severe and there is no abnormal thickening of the stems or early dieback.

Spot them

The eelworm is invisible to the naked eye, buts lives within stems and foliage.