Gosh, I was just reading about this last night in "Orchid Pests and
Diseases"--a small book published by AOS. Here's what it says:

"Calcium deficiency is most evident on cattleyas, paphiopdilums and
other orchids during the spring and summer months, when the plants are
in active growth. Young, expanding leaves start turning black at the
tips, and as the blackening spreads downward onto the leaf blade, it is
preceded by a yellow "halo." In severe cases, in may encompass the
entire leaf blade. In Cattleya, it also can cause a complete blackening
and death of the developing leaf...several fertilizers used by orchid
growers do not have any calcium -based units, thus calcium must be
added to the medium to alleviate the problem. A light dusting of lime
over the surface of the medium usually will suffice.....calcium
deficiency is sometimes ovelooked because it has been called sunburn
or it is confused with the damage caused by excessive use of dry
fertilizer. An overfertilized cattleya will first have blackening of
the younger foliage (not just the new leaf, as in calium deficiency)
and possibly some of the odlder leaves as well. In severe cases, root
injury and the death of the plant may ensue.

Good luck!



In article , Geir Harris Hedemark
wrote:

My Blc has just contracted something. One of the leaves has a
1/4in/6mm round black spot near the tip. The tip itself has not been
affected, there is some yellowing around the area, the area itself is
thin and filled with necrotic tissue which looks "wrinkled".

The plant is sitting in about 20-25kLux of light (where it has been
for most of the summer) in a steady 22 degrees and 45-55%
humidity. The humidity has started rising as winter approaches. There
is less air exchange - we are regularily hitting 0C during the nights
now. The only thing that has changed is that I have been redecorating
the hall and there is dust more or less everywhere. Could the dust be
a carrier of something? There is no visible molds where I have been
working.

Any idea what I should do about this? I can't get hold of any brand
chemicals, but I can mix my own from whatever is used in chemistry
labs if someone has a recipe. There are no labs around I can send a
sample to for analysis. How do I stop this from happening again?

It is on a new leaf as well.

Geir - unhappy