Black spots on a Dendrobium and a Vanda hybrid
 

Back in October I noticed the first black spots on both these orchids. They didn't seem to spread so I didn't think much about it. Fast forward to today, and there are now far more, and spreading to other leaves.

The Den is a Den Hibiki, and after killing several I finally have one that is doing really well for me and doesn't mind the heat (I ordered it from Italy this past summer, thinking that if they can grow it in their climate, it should do fine with me). I would really hate to lose my Hibiki....

https://shutterbug.ponzio.net/_data/...aae4467-me.jpg


The Vanda type is Vanda cristata x Ascocentrum curvifolium. I got it from the same vendor in the same order, but I don't think it's a vendor issue since both were super healthy on arrival.

https://shutterbug.ponzio.net/_data/...2d3c17e-me.jpg


I'm not sure why they got diseased, my humidity is generally pretty low (no more than 50-60%, usually around 30% this time of year).

What can I treat the plants with?

I think it's Gugnardia / Phyllosticta.

Phyllosticta Leaf Spot

They may have been infected when you bought them.

Very bad news if that's the case... I'm going to move both plants off the shelves and away from the rest of my collection. If it's not too late...

If so the lesions represent fruiting bodies. I have read of people covering these with clear nail polish to prevent spores being released.

I don't think the spores are very active at lower humidities and temperatures. I would consider cleaning the leaves and stems of the other plants with soap and water to remove any spores.

Funny you mention cleaning the leaves of the other plants, yesterday I spent 4 hours doing a thorough cleaning (both sides of the leaves, stems, and generally all the nooks and crannies) with rubbing alcohol to get rid of mealies before spraying with insecticidal soap! I guess the rubbing alcohol may have taken care of any spores as well.