| ||Common diseases
Harmful micro-organisms are always present to some extent in the soil, the air, the water and on plants themselves, but a healthy plant has natural defenses against them.
Foliar diseases caused by parasitic micro-organisms are seldom a problem within the home because of the relative dryness of the air. Fungi and bacteria need prolonged periods of moist air conditions, and moist or wet foliage, to spread from plant to plant. The most common diseases of house plants are those involving the roots. Because the soil is almost constantly moist, the roots are most susceptible to plant parasitic bacteria, fungi and sometimes nematodes. However, it is usually only when the plant has been weakened through improper cultural practices or insect damage that it becomes susceptible to disease. For example, a cactus thriving in a hot sunny window may suddenly wilt or develop a blight if moved to a humid shaded location. The same thing could easily happen to a shade-loving fern if moved to a dry sunny location.
Plant diseases are generally more prevalent in warm, humid conditions, particularly when the ventilation is poor, or the plants are overcrowded. Diseases can usually be prevented if plants are treated with care:
Disease symptoms are easily confused with the effects of simple cultural problems, such as edema, underfeeding, overfeeding, low humidity or drought. Correct identification of a disease problem can usually only be made by a plant pathologist because of the microscopic size of bacteria and fungi and their amazing diversity of form. Once disease has set in, it can spread rapidly through the plant. Under certain conditions it may move easily to other plants, although most diseases are specific; they affect only one species or variety of plant. The micro-organisms are carried on air currents, by insects, on tools or through handling. They can even be splashed from leaf to leaf if the plant is watered carelessly.
- Always use sterile potting soil or rooting medium.
- Clean and disinfect old pots with bleach before reusing.
- Keep any gardening tools clean.
- Water plants carefully. Water standing in the crown of a plant encourages fungal or bacterial growth, which may cause stem and crown rot. Water left standing on leaves, flowers and buds may encourage the development of botyritis blight or other diseases.
- Do not overwater plants. Roots which cannot get enough air will die, weakening the plant and making it susceptible to disease.
- Provide adequate ventilation and avoid overcrowding so that each plant receives fresh circulating air.
- Protect plants from cold drafts or temperature fluctuations.
- Remove faded leaves and flowers because they can provide sites for disease development.
Isolate plants you suspect are diseased to reduce the risk of spreading infection, and always wash and tools after touching diseased plants.
Treatment of disease is a different problem. Evidence of plant disease usually indicates a susceptibility of the individual plant, and removal of damaged parts or chemical treatment may not lead to recovery. If damage is severe, it is best to discard the plant and start over with a new plant and fresh, sterile soil.
This can be a problem with house plants, particularly in humid, poorly-ventilated conditions. It appears on any above-ground part of the plant as a white, powdery growth. In the early stages it spreads slowly and will not cause much harm to the plant. Move the plant to a drier location with proper ventilation, and remove damaged parts. A suitable recommended fungicide can be used to control this disease. Powdery mildew spores are spread easily by air currents; they are one of the few fungal spores not requiring free water to germinate. Begonias, African violets and roses are often affected by powdery mildew diseases.
Some types of fungi and bacteria enter the plant roots through the soil and interfere with the water-conducting ability of the plant. Leaves wilt and become pale, eventually turning brown; growth is reduced and the plant may soon die. In this instance the plant may have been infected prior to purchase, the change of conditions having made them more susceptible to disease. Healthy cuttings can sometimes be taken from the upper parts of the affected plant, and rooted in a sterile medium. The pot should be disinfected before reuse.
Crown, stem or root rot
The micro-organisms that cause these problems thrive in moist conditions, so plants which have become unthrifty as a result of overwatering are highly susceptible to infection. At first, the leaves droop and the plant appears to need water; stems and roots usually turn dark, mushy and rotten; lower leaves become dark and waterlogged. The entire plant may collapse. If detected in the early stages, further damage may be prevented. Place the plant in a dry, ventilated room and withhold water until progress of the disease is halted. You may need to remove the damaged portions, rerooting the top part if necessary. Cacti and succulents are particularly susceptible to stem and root rot. They should always be kept fairly dry, particularly during winter months.
Botyritis blight (grey mould)
This fungal disease can develop on any above-ground part of the plant as a result of high humidity or overcrowding, or if dead leaves or blossoms have been left on the plant. Symptoms vary from plant to plant but the disease generally appears as a fuzzy grey growth. Remove old leaves and flowers, increase ventilation and keep in drier conditions. Treatment with a recommended fungicide may be necessary.
Leaf-spots, flower spots and blight
The spores of these fungi are spread by air currents or water. Infection only occurs under moist air conditions or if standing water is left on the foliage for prolonged periods.
This covers a number of soil-located fungi which attack seedlings and cuttings, causing them to rot at the base. Affected plants collapse and die. Damping off fungi also cause seed decay. This should not occur in proper conditions. Use a sterile porous rooting medium; avoid overcrowding of seedlings; do not overwater and choose a well ventilated area.
Viruses multiply only in living plant cells. These parasitic micro-organisms are spread by certain sucking insects, on tools or through handling. They cause a variety of symptoms; stunting, ring spots, mosaic vein banding and abnormal leaf growth. They seldom cause serious problems with house plants.
These are difficult to control, and can only be properly identified through laboratory diagnosis. They cause root and stem rots, wilts, leaf spots and galls on the different plant parts. To reduce the chances of bacterial infection, soil sterilization and good cultural practices are essential.
Nematodes are microscopic pests that burrow into root tissue or foliage. Root nematodes may cause lesions, swellings or galls to appear on the roots. Infected plants become stunted and unthrifty, and are prone to wilting. Leaves become discolored, as if suffering from mineral deficiency; flowering is reduced, and the whole plant collapses as more roots are damaged. When repotting plants, always check for signs of root nematode disease.
Foliage nematodes move invisibility in moisture on leaves and stems; they feed from inside the leaf. Some species cause dark brown markings on the upperside of leaves, and small brown spots and water-soaked margins on the undersides of leaves.
Nematode diseases are extremely rare in house plants, and again can be identified only through laboratory diagnosis.
Adapted from Agdex 285/636-1.