Why Do My Watermelon Seedlings Have These Black Spots?

Question

I am a newbie gardener and unfortunately selected watermelon as my first go at growing fruit, without knowing how fragile they can be. I germinated them indoors in the same starter pots you see now, they had a strong grow light and water every day. I have been keeping them outdoors as I prepare for transfer into soil, and I just cut the second, weaker seedling from each pot to give the stronger one room. The seedlings are developing black spots that eat up the leaves and leave them dry and dead, it sort of resembles a burn. This began when they were still indoors. I would love some advice on basic watermelon care as well as treatment of the black spots! These are the crimson sweet variety and I live in North Texas. Much thanks.

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Answer 1

I hope these two articles will help:
https://www.gardeningknowhow.com/edible/fruits/watermelon/watermelon-disease-control.htm
https://www.gardeningknowhow.com/edible/fruits/watermelon/watermelon-myrothecium-leaf-spot.htm