Unless the culprit is identified, and appropriate preventive steps taken, you're likely to run into the same problems when new plants are added to your collection. The following information and problem identification tips just might help solve some of your houseplant headaches. Although some contagious diseases can be found on houseplants, if plants are purchased disease-free, fungi and bacteria generally cause little trouble indoors. Most homes and offices lack the high humidity necessary for the development of these diseases. Some diseases such as powdery mildew, can develop on plants growing in a bathroom or kitchen window since these rooms may have periods of high humidity.
Cultural problems arise when houseplant are subjected to adverse growing conditions.
Light, fertility, water and temperature imbalances are most often involved. Yellow, brown, or black spots can indicate a fungal or bacterial leaf spot disease. Leaf spots vary in shape and size depending on the organism and type of plant involved. A grayish-white powdery growth on stems and leaves is typical of powdery mildew. Controls for foliar diseases include removal and destruction of affected leaves and maintaining adequate spacing between plants to promote air circulation.