Most of the problems that occur on indoor plants are not caused by infectious plant diseases, but are related to improper cultural practices or unfavorable environmental conditions. Cultural and environmental factors relate to moisture, light intensity, growing medium, pot size, temperature, humidity, fertilization, air circulation, and other factors.
An infectious disease problem that can occur when cultural and environmental factors are unfavorable is root rot. Several root rotting organisms of concern include species of the fungi Pythium, Phytophthora, Rhizoctonia, and Fusarium. These fungi can attack seeds, seedlings, cuttings, and the root systems of established plants. Most often they attack after a stress has weakened the plant's defense system. One of the most common stresses that predisposes plants to root rots is an excessively wet growing medium.
Plants infected with a root rot organism usually wilt, often from the bottom leaves upward. Leaves may yellow and drop off. When the roots are examined, they appear darkened and collapsed. The outermost root tissue may slough off, leaving a thread-like appearance to the root.
An examination of the foliage symptoms and the root system will give a good assessment of the presence of root rot organisms. In order to determine which fungus is present, however, microscopic examination of the root system or laboratory isolation is required.
The best method of combating root problems is to prevent them. Avoid over-watering. The growing medium should be pasteurized if possible, well aerated, and easily drained. (Water should drain out the bottom of the pot after watering.) Know the needs of each of your houseplants. The amount of moisture required is dependent upon the type plant, the size of the pot, the type of growing medium, and other environmental factors. Also, avoid over-fertilization. If a root rot problem develops, try reducing the frequency of watering. Repotting may help.
This article originally appeared in the December 9, 1994 issue, p. 160.