Light adaptation takes several weeks, rather than days.

However, in my experience at least, excess light causes yellow or white bleached patches, not black spots. Nor do these spots appear on the edges of leaves...are you sure this is burn? Any photos?

Hello again,

Thanks for your response. Here is a picture of the black spots, which you can see clearly on the upper-left-most leaf and (though it's a bit harder to make out) along the top edge of the right-most leaf.

{{gwi:144902}}

If the spots are not caused by the light as you suggest then it is somewhat of a mystery, as they have all emerged within the last 2 days and only on cattleyas (again, dends and oncid are fine).

Unfortunately those spots look like some sort of rot :( Sorry to say it - keep monitoring them and see what happens. If they continue to grow in size, then some sort of intervention will be necessary. Were these plants moved to new conditions, or watered differently recently?

I can't say I'm surprised as they have been in very low light conditions (i.e., indirect north window light) for about 6 months. I've watered only once a week since summer, but I'm not sure how great the roots are since they probably don't dry out too quickly without evaporation from brightish light. Since the move haven't gotten a spike from any of the new growths (out of, say, 6 or 7 on 3 different plants) due, I assume, to the low light.

So I guess the question is why, though, the spots -- let's say they're rot spots -- have just appeared under the new light? I can't imagine there has been THAT much additional rot over just the last 3 days?

A 250 watt HPS bulb 18" away from the foliage should not be strong enough to cause a problem. You introduce a plant to increased light intensity by starting them away from the light source and moving them closer, not by the # of hours in the light.

Circle the black spots. If the spots increase beyond the marks you have a bacterial issue. If they spots stay within the lines, it isn't a problem.

Brooke

I have grown orchids under 400W metal halide bulbs for six years. For several of those years, catts were the type of orchid I was primarily growing. Eighteen inches from the top of the leaves is really quite far away for catts.

You are getting good advice from the posters above --

the black spots you have are not burn from the lights, and are most likely rots.

acclimate your catts to the higher light by starting them further away from the lights and gradually moving them closer every three or four days or so. Leaves should feel cool or barely warm to the touch. Any orchid in light that is too strong can easily burn in an hour or even less.

As Calvin pointed out, light that is too strong causes the leaves to turn a harsh yellow. Actual burn might be white or could be mushy brown, but would appear on the flat part of the leaf exposed most directly to the light, not on the edges.

As to why the spots should suddenly appear when you first put them under lights-- probably just a coincidence. Rots always seem suddenly to appear out of nowhere. As you suspected, rots are more likely under lower light conditions. Prevention includes higher light and more circulating air. So you are increasing the light just in time.

You have to watch closely to make sure the rot doesn't spread. It can spread very fast. Spray the plant with a dilute solution of Physan 20, or gently rub cinnamon on both sides of the leaves. You might have to cut off the affected portions if it continues to spread.