Question
Some of my Orchids are getting a black patch or streak which eventually kills the shoot or whole orchid. I have been spraying with white oil and on occasions an insecticide with some results but the shoot still dies or is that damaged no flower shoots appear. The black patch ends up as a hole and in the worst cases the shoot lose rigidity and falls over. In some cases I have cut off the shoot but the new shoot eventually developed the some problem, Any ideas?

Answer
Hello Ar,

Without actually seeing the problem on your orchids, it is a little difficult to determine what it is although I have some thoughts about what it might be.  Also, you don't mention what kind of orchids this has been happening to, some orchids are more susceptable to certain diseases.  

My first guess is that the black patch you are seeing might be a fungus.  Fungus will have a black sooty appearance and usually can be seen at the base of leaves where it attaches to the main stem or bulb.  Fungus can seemingly appear overnight.  What causes fungus on orchids?  The usual cause is water getting on leaves when you water the plants in late afternoon or evening and the water remains overnight.  Fungus spores in the air love dark cool and moist conditions and once they find water droplets on a leaf can rapidly grow.  If the fungus is removed within a day or two of forming on a leaf, little damage occurs but if left lone for days, it will slowly infect the leaf and slowly kill the leaf.  

Using white oil does little to prevent this.  To eliminate of minimize fungus, water your orchids in the morning to allow enough time for water droplets to evaporate before night fall.  Also, having some light air movement around them will speed up the evaporation process.  

Black patches may also be due to a bacterial or fungal infection that has penetrated into the orchid itself rather than only on the surface.  To prevent possible infection from spreading or infecting other plants near by, remove the infected area by cutting it off with a sterile cutter for each cut.  Once you cut a leaf off, do not use it again for another cut as it is possible to spread the infection via the sap on the cutter blade.  I use industrial grade single edge razor blades for this purpose, make one cut and throw away.  Most paint and hardware stores carry these blades for use in paint scrapers.  Lastly, when cutting out infected plant parts, cut beyond the infected area and look at the cut edge for any signs of black spots or streaks in the cut.  If there is even the tinyest black present, cut more off until no black is visible.  

Hope this has helped.  Good luck.

Jim Kawasaki
San Jose, Ca.