Strange black marks on Cattleya leaves

[bogdan]

Good day all, mine has not been a very good one.

I grow my orchids under LED lights and until now I only had this problem with a cattleya lueddemanniana, about which I wrote here a few months ago. It's OK now, the new pseudobulb grew without any marks.
I am building a new shelf and so I had to rearrange the other shelves in the growing room. This means that the plants had no source of light since yesterday afternoon as I had to turn the other lights off. After 6 hours of sudden lack of light, this is what happpened to some 8 juveniles. Same black marks on the newest growths and the ones before them. I do have to mention that 4 days ago I watered them all with a 1:250 solution of Kelpmax.
The temperature these days was quite high, 30-31 degrees C (86-88 F) during daytime constantly and 75 F at night.

What do you think it causing this, lack of light combined with accelerated growth, but could 6 hours mean that much?

[bogdan]

There are a lot of affected plants, each with the last 2 pseudobulbs' leaves afected and the lessions appear by the hour, the more I keep them in darkness. These have grown under LED lights. Somebody should conduct a study regarding dependency on these lights, if it hasn't been done yet. I didn't have any of these problems under T5 lights, I could turn them off for a week, go on holiday and return to anxious for light plants, but they were still green and not covered in purple blotches.
Sorry for my rambling, this is mostly a lot of my time investment and I am quite mad about it.


Do you have any ideas what it might be? I might as well just throw the poor things. Or the LEDs, which do grow well some other "important" crops but then you don't need to keep that plant alive for years and years.

[estación seca]

The only time I've seen something like that, it was sunburn. The leaves became slightly softer than normal. Most fell off quickly. The plants recovered. Are the affected leaves still firm?

Even a few days in darkness is OK; that happens during shipping. I don't know how to explain these black marks on your plants.

[bogdan]

Yes, the leaves are normal, firm, but have these extended black marks. And it happened only to those parts grown under LEDs. It's like they are dependent on the light and if they are not getting it at the established time, they succumb.

The paphiopedilums have no marks whatsoever, they weren't affected, but each and every Cattleya has this. Unbelievable.

I don't think it's black rot either, it couldn't happen to every plant in the same day, actually in the same time frame. Very strange.

[Leafmite]

I have never seen this. I would just watch and if it spreads in the leaf, remove it and see if you get new growth. I can very well understand your frustration. Good luck!

[Nexogen]

If it happened because of the lights it is possible to be a change in the accumulation of anthocyanins.

[mobelle]

Cattleyas can get black spots if there's a calcium deficiency. This is one factor you can consider.

[bogdan]

Thank you for the suggestion. The strange thing is that all of the plants got the same marks at the same time, even though they were in complete darkness at room temperature. They receive plenty of calcium in fertilizer form, as well as the occasional calcareous tap water.
The only thing they had in common was a dose of Kelpak (1:250) - same thing as Kelpmax - administered a few days earlier.

The majority developed dry scars where the marks were, but a few of the leaves got entirely brown and had to be removed.

[Orchid Whisperer]

The lights, Kelpmax, temperature. none of that should cause brown marks like this.

My first floor guess is something bacterial. Watch the leaves carefully under normal care, if the brown gets larger, cut any affected portion of leaf off, sharp flame-sterilized blade.

[Nexogen]

Looking for something about nematodes, I found something that would make you interested. If you look at "Figure 2: Patch-like leaf spots caused by foliar nematodes on Japanese Anemone" you will find something similar to what you presented.

http://ipm.uconn.edu/documents/raw2/...ntalplants.pdf