"I can't really tell if the problem is powdery mildew or downy mildew. Both are, yes, a sort of mould. Apple Powdery mildew is quite a serious disease that starts on the stems and leaves shortly after bud burst and takes the form of a powdery white coating. You can treat it with a simple fungicide. Mildew problems are common in many seed varieties that have not been treated with an anti-mildew fungicide, and are exacerbated by very damp conditions.The very next morning I was fungicide shopping and by noon I'd sprayed every last leaf, both top and bottom. Within hours the plants had lost their dusty pallor. Had I not forgotten them out in the pouring rain--not once, but twice--they might have been cured. Unfortunately, in a clear case of horticultural neglect, two seedlings re-entered the circle of life. There is new growth on the remaining twenty, so hope prevails.
"If it's downy mildew, you will see yellowish spots on the upper leaves (it's probably not this) and can be treated with a copper-based product."
At one point last week I warned the girls that the seedlings might not make it. Their response: "That's okay, Mommy. We can grow tomatoes instead."