On both arugula and romain in garden.
That looks a lot like leafhopper damage. They come in many shapes, colors, and sizes. I would use an insecticide like azadirachtin. This is an extract from neem oil, and is my go-to.
https://www.gardeningknowhow.com/plant-problems/pests/insects/leafhopper-control.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/azadirachtin-vs-neem-oil.htm
Half my salad leave plants died in spite of being regularly watered. Obliging them up I found whitestuffround the roots. Is it a blight of some kind? If so, how can prevent it?
It could be white mold. Was it also at the crown of the plant? These articles tell more:
https://blogs.cornell.edu/livegpath/gallery/lettuce/white-mold-aka-drop-on-lettuce/
https://www.gardeningknowhow.com/edible/vegetables/lettuce/diseases-affecting-lettuce.htm
https://www.gardeningknowhow.com/edible/vegetables/lettuce/sclerotinia-symptoms-in-lettuce.htm