Eryngium 'Jos Eijking' flower stems grown up with buds, then some , but not all develop a brownish-black mark about half-way up the stem which affects the structure of the stem and it collapses. Whatever the pest is, it also seems to infect and affect some of the flowers. A similar dark mark appears in the centre of the flower and prevents it properly developing and colouring up.
Any ideas ? Plant is a favourite but I acknowledge that it appears to be congested where it is. Somewhat anxious to split it in case I kill or weaken it. I have cleared the area around it to provide more air and light as most, [not all], eryngiums prefer dryish sunny conditions.