Several candidates, but more info is needed. Is that leaf from the older parts of the stems, or from the new growth? Has fertilizer, or manure been applied recently? Is it in the ground, or in a container?

It's in a pot old growth

It looks horribly like fusarium wilt disease, then. It is less common in pots than in the open ground, but it can happen if the potting compost or the pot has been contaminated with natural soil, or if the pot has been sitting on natural soil. The fungus is currently without a cure, unless you can buy something containing Streptomyces lydicus in the UK.