Looks like bad case of PM to me.
Thanks, Oscar and Guanabanus. In the past few weeks I treated my mango seedlings twice with a general fungicide that seems to have arrested the spread. It's hard to tell, though, because the infected leaves don't recover; it's only the new growth that doesn't appear to be infected.
I am learning that to be a successful fruit farmer I must get my plants out of the shadehouses and into the ground as quickly as possible. The powdery mold, aphids and citrus canker spreads so easily in the shadehouse, but is rarely a major problem in the field. The shadehouse is a disease factory.
The alternative is to manage the shadehouses as a chemistry lab, with treatments available for whatever parasite or disease appears. I'm trying not to do this, but wonder how the commercial nursery operations manage all the pests and problems.
John