Cattleya walkeriana with black spots on leaves

[Jasonmc89]

Hi there, I've had a Cattleya walkeriana for a few months now and it seems do be doing okay. It's been putting out nice healthy roots over the last couple of months and most of the pseudobulbs are nice, fat and smooth. The plant is on a west facing window ledge, planted in medium bark, in a 9cm clay pot with 1cm holes all around it. I water the plant when the potting mix becomes just dry by sitting it in room temperature water up to pot level for about 10 minutes. I feed every other watering. I also have a small fan in the area circulating the air, but not blowing the plant directly.

The black spots started to show up a couple of weeks ago on one leaf and grew quite fast. I decided to remove the leaf with a sterilised craft knife and covered the cut area with cinnamon. Since then another couple of leaves seem to be getting the same black spots.

The spots don't seem mushy or watery but are a little sunken and they are a dark blackish purple colour. I'm pretty sure it's not sunburn as it's winter at the moment and it's pretty dull and cloudy. It's not cold though.

Does this sound like classic black spot or could it be something else? I have attached a photo.

Any help would be greatly appreciated!

Cheers.

[wintergirl]

If the leaves and the spots are not mushy I most likely would not cut them. Your plants may need some calcium. Many times catts will get black spots when they are low on calcium.

[Jasonmc89]

Thanks for the reply!

That's interesting, how would you suggest I provide calcium?

[wintergirl]

There are several threads on here about adding calcium. Crushed egg shells work over time but there are quicker methods for right now. Here are some threads:
How do I calcium?

B. cordata... Why is the baby leaf which it has black on it.

Adding calcium to orchids

[silken]

I've seen a lot less black spotting since adding a Cal/Mag supplement. I just bought it at a plant or hydroponics store and add some in with my other fertilizers in the water.

[estación seca]

This also tends to happen when it's a little cooler than the plants like. Where they're from in Brazil winter nights may be in the 5C range, but days can go into the upper 20s C of higher.

There may be solubility issues between calcium/magnesium supplements and other fertilizers, depending to some degree on water pH. It is probably best to use them on different days and not mix them together.

[Jasonmc89]

Do you think a deficiency in ca or mg would cause spots to appear and grow so rapidly?

The black spot in the photo I uploaded grew by about 25% in one day.

[estación seca]

The spot is caused by an infectious organism that may grow rapidly, not by calcium deficiency. Calcium deficiency weakens the plant and predisposes it to getting infected. It takes weeks to months of feeding calcium to replenish the plant and get it back to tip-top shape. In the mean time the plant is susceptible to further infection.

[Jasonmc89]

Right, okay.

Would you agree that the best thing for me to do would be to treat the plant with a systematic fungicide and at the same try to provide some additional minerals?

Thanks a lot for your input.

[Nexogen]

Pretty crappy, fungicides may affect the plant. It may be a sign of toxic chlorine or something ...