Hi Asclepius,
I occasionally have small problems with nematodes too. And they only bother my tomatoes and not any of my other crops. Interesting. So I have learned to always pull up the roots on all of my tomatoes at the end of the season so I can discover the problem before it gets out of hand. If it only infected one or two plants in a bed then I have just poured a big bucket of boiling water in the spot. That works. If it has effected a small group then I turned the dirt over every day for a week to dry them out and kill most of them, Then I solarized the whole area and left the plastic on it all winter to keep the area super dry. I only had them come back in the same spot the following year once and so I left the area fallow for the following summer. That worked too. And I always rotate my crops so I won't plant tomatoes there again for at least 3 years.
Anyway, I think these will give you the info you are looking for about marigolds. Happy gardening.
https://ipm.ucanr.edu/PMG/PEST...
https://www.patwelsh.com/pests...
https://www.lsuagcenter.com/pr...
