Question
Hi,
I have about 100 phalanopsis and on 15% of the crop  the leaf tissue becomes soft inside without any discoloration to the leaf, is this from overwatering and or lack of ventilation? I also just sprayed them with physon 20 am I on the right patyh to ther recovery.
Thanks Jeff

Answer
Jeff, this sounds like a case of mesophyll collapse.  This is usually related to physical stresses on the plant rather than a viral, bacterial, or fungal infection. In such an instance, use of physan or another fungicide is not likely to be effective.  You can obtain a sampling of theories as to the cuase of mesophyll collapse be googling this topic.  One of the first web sites to have an article on this is the Americam Orchid Society web site.

The most predominant explanation for mesophyll collapse is exposure of the plant to cold or heat for a brief period which stresses the tissues.  This can happen through focused sunlight which heats the inside of the leaf before any external damage occurs or is visible. Exposure to droplets of cold water is also thought to be a cause(especially when the leaf is hot).

Regardless of the cause, I recommend removal of the damaged leaf by making a cut with a sterile blade just below the damaged area.  Once a leaf is damaged, its structure is compromised, leaving that leaf open to future infections.  In addition, I recommend the use of dynagrow's "Protek" which can strengthen leaf structure in case of future stressing of the plant.  Be careful to follow the instructions exactly on the Protek because too strong of a dose can lead to brittle leaves.  Finally, be careful to avoid future exposure of the leaves to cold water or sunlight as might be focused briefly from a window on a particular leaf for a brief period.