Possible bacteria infection on Cattleya dowiana leaf.
 

I recently noticed this large spot on a Cattleya dowiana ('Maku' x self) that I acquired 3 months ago. The plant tests negative for CymMV and ORSV by an Agdia Immunostrip test. I am guessing that it is a bacterial (Pseudomonas) infection.
Orchid Diseases

I will treat with Bonide Liquid Copper Fungicide (copper octanoate 10%), 8 ml per liter.

I see that the Bonide Copper Fungicide label says that it is "For Organic Gardening". I wonder if I could get copper acetoarsenate approved for "Organic Gardening".;)

Scale in photo is in centimeters.

Could you just cut off the leaf? Or just cut down until you have good leaf? Then treat with cinnamon? I have used this method for years with good effect. I have brought back quite a few orchids with severe fungus and bacterial infections using this method. Give it a try. By the way I work on CAMPENdleton. Any connection? Just curious.

I came within seconds of lopping off the infected portion, I had the shears at the leaf and was about to squeeze the handles when I reconsidered.

If this was an old leaf on a well established plant I would not have hesitated to remove it but this is a young plant with only 5 pbulbs/leaves and the leaf in question is the most recent and largest. I feel that it would be quite a setback to the plant to loose that much leaf. If I can stop the infection from spreading the leaf can still be of use to the plant. I took the photos of the leaf with the scale to document the current extent of the infection.

The leaf is already compromising the pbulb. The stress is already present. If you can, cut out the portion of the leaf that has been compromised. then seal it. If allowed to spread, it will affect any possible flowering. But if it was going to flower it would already have produced either buds or a sheath. The least stress on the plant would be to cut off the affected portion. I do it all the time and the pbulb, if it is going to send up flowers or sheath does so anyway. I'll see if I can find some pics.

I had similar-looking circular spots on my cattleya.

http://farm9.staticflickr.com/8295/7...e62b21721c.jpg

I debated whether to cut off the entire leaf because it was the biggest leaf on the plant, but I did so anyways. I noticed no more black markings since, and the plant has put out a new growth.

I just don't feel comfortable leaving questionable markings on the leaves.

Hmm...My advice would be immediate isolation and a virus test. That looks like Oncidium Ringspot to me.

---------- Post added at 12:43 AM ---------- Previous post was at 12:42 AM ----------

Oh, I noticed it tested negative. I would get a lab opinion on that. the home test kits I have used have never been that accurate with around a 65% accuracy

www.icogo.org/.../Dr._Ching-An_Chang ...

Page 44 has a picture that looks similar

Maybe this link will work:
https://docs.google.com/viewer?a=v&q...ytSMVoTJPeHy2A

I would doubt that that is correct for the Agdia Immunostrips. What brand were you using that only had 65% accuracy and how was the level of inaccuracy determined?

With the C. dowiana pictured above, I did cut off the disfigured leaf 6 months ago. None of the leaves now show any disfiguration so I would say that the problem was not ORSV.