The symptoms of nonpathogenic distress such as fertilizer or insecticide burn, water-logged soils, dry air, etc. One helpful way to distinguish between disease and cultural or chemical injury is to look at the line between healthy and diseased tissue. Usually, if this line is sharply delineated, the cause is of a nonpathogenic nature. Houseplant diseases caused by bacteria usually produce angular lesions frequently surrounded by yellow halos or water-soaked areas. Fungi produce a wide variety of symptoms including root rots, stem lesions, leaf spots, mildews and rusts.
Diseases caused by viruses often appear as abnormal or distorted growth.
Nematodes are microscopic worms that mostly feed on roots, causing the plant to become stunted, yellowed, and generally unthrifty. Avoid over-misting to prevent powdery mildew and leaf spots. Purchase healthy, vigorous appearing plants. Beware of bargain plants. Kenneth R. Bolen, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.