Question
I received a very large healthy Peace Lilly as a gift.  It seemed to be doing very well untill I noticed black spots on the leaves.  They would start along the edges and then spread till the whole leaf was involved.  This also happened on new leaves.  I dusted it for insects and covered it with a large bag for a day to kill any insects but this did not help.  I recently purchased another small peace lilly and now that one is doing the same thing.  How can I save these plants.  They are in well lit areas and are watered regualrly.  The rooms they are in are very large and airy.  Thank you.  Kathy

Answer
HI Kathy,

First, a photo of the plant would be very, very helpful. If you would like, you can send a photo to send me a photo at : [email protected]  

Some people mistake scale insects for "spots" on the leaves or stems.  Scale insects are usually flat, oval to circular, disks that attach themselves to the leaves to feed.  They produce a sticky excretia that attracts sooty mold, which appears black and can be wiped off the plant with a cloth.  This does not sound like your situation.  But, examine your plant very closely, and try to remove one of the spots by rubbing.  If it comes off, then you know it is a scale insect.

Burning or darkening of the leaf margins can be caused by chemicals such as chlorine in the water, or by accumulated salts caused by fertilization over time, or by overfeeding.  These are usually minor problems, and I don't think that accumulated salts could be the problem if the plant has been recently acquired from a nursery.  To avoid chlorine burn, allow the water to stand for approximately 24 hours before watering.  

These symptoms can also be caused by a problem with the conductive tissue providing water to the various parts of the plant.  Most serious of these are the bacterial and fungal root rots.  Once these pathogens get into the vasculature tissue, they can rise to very large numbers in a very short time, causing collapse of the tissues and cells from which the bacteria obtain nutrition.  Such lesions often appear black and water-soaked or mushy.  

You can examine the vasculature (both stems and roots) for signs of these rot-causing pathogens by cutting a cross section of an affected stem.  In a healthy plant you should see a creamy white to off white color, perhaps even a slightly green "ring" around a core of tissue.  In diseased plants, there may be very dark tan to brown tissue.

You can also examine the root system in the same manner.  If you encounter any mushy, foul-smelling odor when the root system is exposed, then you have a severe problem with root rot.  These are almost always caused by excessive watering.  Remove all tissue that appears to be either burned (chemicals) or mushy (root rot).  Cut back the roots until they appear "healthy", i.e. no discoloration or apparent damage.

I have had limited success in "curing" a plant that has a serious problem with root rot, but there are steps that you must take if there is to be any chance.  ONLY continue to this step if you find evidence of root rot.  Salt accumulations can be removed by several soaks/rinses with distilled water.    

1.)  Stop watering the plant - allow it to dry out a bit.  2.) Remove the plant from its pot. 3.) Remove all the soil from the plant's roots. 4.) Examine for diseased tissue and remove all of the dead/damaged roots. 5.) Dip the root system in a 10% bleach solution for 2 minutes.  6.) Soak roots in distilled water for 5 minutes. 7.) Repeat the sterile water rinse. 8.) Allow to air dry. 9.) Repot with new, bagged potting mix. If the potting mix is not moist to the touch, water just enough to provide a little moisture to the mix.