Dark specks on the leaves of marigold seedlings – Should I be worried?

Marigolds are not a vegetable, but they are such a frequent “companion plant” that I’m hoping some of you fine vegetable gardeners might be able to help. I have queried the flowers sub and the plant clinic sub and the general gardening sub without much (any) joy.

The last 2 or 3 days I have noticed small dark specks on the leaves of most of the marigold seedlings in my starter tray. These specks don’t have yellow margins and are flat as far as I can tell after examining them with a hand lens. They are visible on the bottom surface of the leaves as well as the top. They appear mainly on the true leaves, not on the cotyledons. They don’t move when touched.

The young plants look healthy from a distance, strong stems, deep green foliage and are not stunted for their age. They are Durango French Marigolds, from Park Seeds, freshly obtained this year. They were started on 10 February (14 days ago) in Jiffy seed starting mix with fine vermiculite sprinkled on top. Used a heat mat and humidity dome initially but removed them after germination/emergence. They germinated in about 7 days. 85 or 90% of the seeds sowed actually sprouted, which I consider good.

Currently they are under grow lights, oscillating fan for air circulation, on a seed starting rack with daytime temps in the 70’s F and 60’s F at night. About 16 hours of light per 24-hour day. I bottom water when the tray gets light and the medium looks tan instead of deep brown. I’m careful not to overwater; am aware of the risks.   

I’ve grown other varieties of marigolds, but not Durango. Do you think these leaf changes are a sign of an early bacterial or fungal infection? I have quarantined them as a precaution.

Have your marigold leaves ever looked like this? Any thoughts or suggestions? Thanks! Location NE Texas.