This is generally a leaf rot caused by types of fungi that are commonly
classified as Cercospora or Colletotrichum. Many times this rot will start
out as yellow spots, gradually turning to a brown or black color. Note that
certain plants such as most Oncidium hybrids often get several small black
spots on the leaves due to the sun. In this case it is natural spotting and
will not harm the plant.
The best kind of treatment for this problem is to use a
bactericide/fungicide spray such as Phyton 27. After treatment, examine to
see if the spots are increasing in size or number within a week to 10 days.
If you have successfully rid of the problem, the spots should dry up and
turn brown. If there are still signs of new rot, repeat treatment. Leaves
that are heavily covered in rot should be completely removed. Make sure you
sterilize whatever tool you may use to remove the infected leaves with as it
can spread the disease to other plants.
http://www.orchidweb.com/faqs.htm
If, however, black areas appear and grow larger, a bacterial or fungal
disease could be the culprit. Remove any diseased areas, dry the plants and
treat with a fungicide. As you likely know, good air circulation is
important in orchid culture. When you do water, try to avoid splashing water
from one plant to another.
Bacterial rot causes a blister to form on the leaf, followed by the rotting
of the entire leaf. In addition to removing affected parts and treating with
a fungicide, try to lower the temperature somewhat and decrease the
humidity. Given our hot, humid weather, this might be the real culprit.
Black rot is a fungal disease encouraged by poor aeration, too low a
temperature and excessive humidity. This causes dark, then black spots to
appear at the base of the foliage.
Other rots are prompted by a medium that's too wet.
Should you see small spots on the foliage that turn black, a fungus
encouraged by a combination of heat, high humidity and low light could be to
blame. To discourage and/or treat, try to reduce the humidity or at least
dry off plants and apply a fungicide. There's a little more at:
http://www.chron.com/cgi-bin/auth/story.mpl/content/chronicle/features/huber
/huber97/huber0913.html
|
|
| Now that you bring it up....
| Just before my Phalaenopsis began growing the stem (after almost a two
year
| pause), some areas on the leaves started developing 'those black' spots
that
| you speak of. They started small, at the edge of the leaf in a semi
circle
| and proceeded to spread almost towards the middle of the leaf, and than
they
| stopped on their own. I was concerned that I am loosing the orchid, and
| then the stem began shooting up.
|
| At the moment, the leaves are in a sorry state, but the stem is looking
| great and seven flower buds are forming nicely.
|
| Do any of you know what these spots are where they come from and are they
| any reason for concern?
|
| Raven
