some type of mechanical leaf injury ... opening up the skin surface .... which allowed a problem to develop ... viral or ??? .. just blanked out ... fungal

see this a lot in my hosta.. after slug or hail attacks ... the original injury to the leaf.. followed by the secondary problem later ...

snap off the leaf.. and dont worry about it ...

sun scorch would not be limited to one leaf on one plant ...

but i am no expert on greenhouse management .. nor pathogen ID ...

where is rhizo when you need her

ken

Thanks Ken, I bet you're right and I appreciate the reply!
Donnie

It looks like it got too much water that didn't drain off well to me.
Sherry

Hi Sherry, do you mean on the foliage itself or the soil in the pot? I think it has something to do with the municipal water, however I try to avoid it on the plants themselves, but it has happened. Maybe a little damage of some kind then the water doing the secondary damage?? Hope to have a reverse osmosis system pretty soon. Thank you!

M. grandiflora isn't really a swamp tree, Donray. You don't see them growing naturally on permanently soggy soil, although they can grow close to creek banks in soil that gets real soggy for short periods of time. The type magnolias that grow on permanently soggy soil down here are m. virginiana/sweetbay - they grow right in the bottom of the hollow here, and I see them growing in bogs on the coast.
I'm also surprised at how thin and sandy the soil can be and/or how dry loamy soil can get and mag grands still don't show any signs of stress.
The bad place on your mag grand looks like the spots I get on my cactus when I overwater them. I don't know if they're caused by overhead watering, but I doubt it - I imagine they're caused by long standing 'wet feet' without any period of (relative) drying out, which could be caused by your overwatering them or your pots not giving them proper drainage. I'm always surprised at how wet plants can still be at the roots even when they seem fairly dry near the top. You can get a more accurate reading of the pot's wetness by sticking your finger in the drainage holes in the bottom to several inches.
Of course, it's possible that your mag grand's bad spot was caused by an external injury, but if I were you, I'd be careful with the watering.
Sherry

Thanks Sherry! I have noticed the greenhouse has been staying pretty damp, so the pots have been holding more moisture than they normally would. I need to back off the water on everything a little I think. Haven't had a lot of sunshine and the temps have been cooler. That I can fix pretty easily. Thanks again!
Donnie