Diseases are most often a result of overwatering, poor soil conditions, too much or too little humidity and lack of air movement. General actions to take include isolating or quarantining diseased plants, removal of diseased tissues and possibly chemical treatment. Root, crown and stem rots occur when the soil is kept too wet or the plant is damaged by cold temperature. In these cases, remove diseased tissue or take a cutting of healthy tissue to start a new plant. Leaf spots can be caused by fungi, bacteria or growing conditions.
Water drops remaining on leaves or high humidity are necessary for infection.
Whatever the cause, the affected leaves should be removed and the probable cause corrected. They come into the home on new plants or with plants which are put outside for the summer. Insects may come in on clothing or when the door is open. Scale insects are among the most difficult to control. These insects are mobile when young, but then settle and secrete a protective covering that can't be penetrated by insecticides.