Question
Wayne,

thanks for offering this service. have been growing phal for 30 yrs. new problem last 2 years

1. surface of lower leaves show light and dark green areas
2. under side of leaf shows dark, blackish, non-wet areas sunken in. not larger than a dime. irregular shape
3. almost looks like something is chewing off the cambium layer. problem never breaks thru to surface of leaf

have been treating with malathon and D-2780 but little success. Collection of 100 plants

Thanks for any help.

Answer
Bill, thanks for your detailed description.  My initial feeling is that your plant(s) that show these symptoms suffer from mesophyll collapse. This is most commonly attributed to temperatures at or below 50 degrees F. and includes watering with tap water or other cool water that comes into contact with a leaf. Damage my appear weeks after exposure. This mesophyll collapse, however can be caused by other agents such as thrips or viruses. Any plants exibiting these symptoms should be separated from the unaffected plants.  Parts of leaves showing the symptoms should have these affected parts cut away with a sterile cutting tool. Newly emerging leaves should not be affected.