Infected catt
 

My C. Portia var. coerulea 'Mrs. Carl Holmes' HCC/AOS shows signs of an infection (possibly black rot). The plant initially appeared healthy until I set it outside in the afternoon sun. Before I bring it inside for the night, I notice black splotches on the leaves. Fearing black rot, I cut off the infected parts using a sterilized knife and spray the entire plant with Physan 20. The day after, I set the plant outside. I check on it later and more of the plant has turned black (it wasn't there when I set it outside)! I'm beginning to fear that the entire plant is infected and I have emailed the vendor (Hausermann's orchids) as this was a recent acquisition. I have been treating my orchids with calcium/magnesium supplements (thanks Silken!) so I know it's not an infection due to nutrient deficiencies. So is this infection black rot?

What comes to mind is sunburn on the leaves since you just started taking it outside. I am not any type of expert but I sun burned one of my cats also this year. I even had a curtain up and had it in the shade in a small greenhouse. But it had gotten to much sun and marked my Catt.

I thought it was sunburn at first, but none of my other catts were sunburned and the late afternoon sun shouldn't be strong enough to burn the leaves.

I hope you figure out what is going on with it. Someone will jump in and let you know I am sure. We have some very good people here that come on the board and can give you some information.

Direct afternoon sun is still pretty hot. The easiest thing is to NOT put it outside and see if it continues to get black! There could be just one ray of sun hitting the leaf for a long enough time to toast it.

It is sunburn.
With Cattleyas, it is important to remember that the different species come from quite a variety of growing conditions. There is the aclandiae that grows where it is hot and sunny and then some of the laelias that grow in high elevations where it is cooler. Different hybrids have the influence of these different ancestors and, thus, can react differently to light and heat (and need different conditions). That is why your others might suffer no ill effect and this one does.

I'd watch, keep it separated, and set in a dry and aerated place.

It doesn't look like black rot or sunburn.

Are the patches watery? (don't touch)

Do you give your catts any calcium? I know your black marks are pretty extreme but I had black marks come up over night from low calcium. I had been watering with distilled water and not really fertilizing. Once I started giving calcium I never got another. Some catts seem more sensitive to this condition. Just a thought. It may be sun burn, that's my first thought but I thought I'd mention this too. Here are a couple of pictures of the black that suddenly appeared on my plant.

http://i187.photobucket.com/albums/x...o-0117%201.jpg

http://i187.photobucket.com/albums/x...o-0116%201.jpg

It is great to see all the responses we are receiving about these black spots on your Catts. Some thinking it maybe also sunburn like I did at first. Others not sure what it could be but advise to remove it from the other orchids and watch it carefully.

With mine I have it in a plastic greenhouse and the sun was hot coming through the plastic. I had a curtain up and it still burned so you never know just how much sun they can handle specially when you just begin sticking them out side. Now I have curtain and 60% shade cloth over the roof of the greenhouse. Its done so much better. No more burning on my leaves.

I agree with Wintergirl. It may be sunburn but in case it's blackrot it's very important to give orchids enough calcium.