(The Communicator, April 1997)
Houseplant diseases are caused by the plant cells being infected by micro-organisms such as bacteria and fungi. Besides keeping your plants in good health with proper care (watering, feeding, proper light and temperature) the best way to avoid these diseases is to prevent water from sitting on the leaves and flowers and to space plants apart so there is good ventilation around them. Good spacing will also make it harder for any problems to spread to other plants. Overwatering is a major factor in the development of these diseases, especially during the cooler months.
Botrytis blight (grey mould)
This fungal disease can develop on any part of a plant that is above the soil level. It can be caused by dead leaves and spent flowers being left on the plant, too much humidity or overcrowding of the plants leading to poor ventilation. The number one cause is likely too much misting of the foliage and allowing the moisture to stay on the leaves. Symptoms will be different between plants, but this
disease generally appears as a fuzzy, grey mould that covers affected flowers, leaves
and stems.
Crown, stem or root rot
The micro-organisms that cause these problems love moist conditions, so plants that have been overwatered are prime candidates for these diseases. On first observation the leaves droop and you think the plant needs water. But if you water it the stems and roots may turn mushy and rotten. On the other hand, if you notice it early enough, further damage can be prevented by placing the plant in a well ventilated, dry room and holding back on watering until development of the disease is stopped. Cacti and succulents are very susceptible to root and stem rot. Keep them fairly dry, especially during the winter months when they are dormant and do not need much moisture.
Leaf spots
Leaf spots on houseplants can be caused
by bacteria, fungi or poor growing conditions. In the case of fungi, the spores are spread in the air and water. Brown
or yellow spots, sometimes with a wet looking centre, appear on the foliage. At the onset, the spots will be small (about a quarter-inch across) but can grow and join together until the affected leaves die. This infection occurs when water is allowed to remain on the leaves. Cold water can also be a cause of spotting. Use warm (not hot) water and apply to the soil, not the foliage.
Powdery mildew
Plants infected with this fungus disease become coated with a powdery-white mould. It differs from grey mould in color and lacks the fuzzy growths associated with grey mould. The spores are spread
by air currents and do not require water to germinate. In the early stages, powdery mildew spreads very slowly and, if noticed, can be controlled fairly easily. African violets and begonias are often affected by powdery mildew.
Prevention
Most of the disease problems touched on in this article can be prevented by good cultural practice, cutting off the damaged parts, being careful when watering, and by moving the plants to a drier, well ventilated area. If the problem persists you may need to treat the plant with a recommended fungicide. Fungicides are chemicals that are able to prevent or cure diseases caused by bacteria or fungi. They are classified into two types — protectant sprays prevent fungal spores from spreading to the plant and eradicant sprays prevent further development of diseases that have already started. Benlate (benomyl) works well as both a protectant and an eradicant. Fungicide use in the home should be
a last resort. Read the label and follow general pesticide safety rules.