Scary looking lesions on Dendrobiums?
 

My Dendrobiums are losing leaves to this crazy looking black spotting. I figured it was some sort of fungus.. Neem isn't helping, systemic isn't helping, just resorted to liquid copper today, hopefully that does something.

Any idea what this is specifically and how best to treat it?

Tell us about your culture and especially about your watering habits. These may be older leaves or in my experience, underwatering. We need more information.

And the temperatures you grow at in your culture.

Hi Pynn,
I thought of temps initially too but the substrate looks suspiciously bad on your picture.
To me most of my problems get fixed by fixing the rootzone first. It really can't hurt at this stage.
There are probably mould spored growing down there, it looks rotted so worth airing and throwing all that old stuff out, cut away any affected leaves with sterilized scissors and give it gentle ventilation to prevent any more growths forming as easily

Ps: It is a den phal so temps should be 20 C minimum, it needs to be kept warm

I can't see the substrate well enough. I asked about temperature because a year or so ago I had a Den phal gifted to me by an orchid buddy at a show in February. Never had one before. Got it home and a couple of days later it had a leaf that looked just like that one leaf... the reverse chickenpox looking one... yellow with green "pox."

It freaked me out, and of course immediately thought VIRUS! I called my friend, and he said It must have gotten cold. I said Not really, other than pretty chilly for part of the car ride home. He said that could be enough to cause it. I'd never seen anything like it. He mentioned something about stomata with that chickenpox look, but can't remember what he said.

Guess we'll have to await OP to chime in with some more details.

I've had them do that from lack of water. Also, old leaves.

Same here over the years. We’re still shooting in the dark until OP chimes in.

Pynn ...... possibly fungal related.

But ---- whatever it is, to cut down on that sort of thing ......... provide the orchid with warm temperature, and gentle air-movement. Gentle air-movement around leaves, stem, and even the roots and through the media.

Provide adequate amount and level and duration of light for the orchid too.

And every once in a while - provide some fertilser and some mag-cal.

Hi everyone! Sorry for slow response, I got caught up in the Thanksgiving weekend kerfuffle.

All of my plants are in an indoor grow room under LED's where I have a box fan running 24/7 to create some gentle wind throughout the room. 68-78 degrees depending on time of day. Humidity lower than ideal, but also not terrible? Usually 60-65%, can bottom out at 50%. I water all the Dens about once per week.

What's weird to me is that the leaves that are turning and falling aren't the lower leaves, they're often top leaves on mature canes, so it makes me want to rule out over/under watering. I've lost plenty of leaves in the past to under watering, and in my experience it's the lower leaves that turn yellow and drop with no real spotting. New canes seem immune to the issue. I took a closer look at the media, there's nothing rotting or molding, but it does look like the bark is past its prime - it's a darker black-brown. Liquid copper seems to have largely cured the leaf spotting, so I was getting more sure that it was fungal.. until I saw this leaf today.

Plants in question are Den. gouldii x Den. tangerinum, and Den. spectabile, btw.

Thanks for all the info everyone!

Today's leaf makes me think of spider mites. They are common in low-humidity times of year and indoors under lights.