Houseplant Pests and Diseases
When a plant that has been a source of pleasure for months suddenly begins to ail, it is not always an easy task for the layman to determine what kind of disorder it is suffering from and what should be done to save it. Plant ailments can be divided into three groups: diseases caused by viruses, bacteria or fungi; insect pests and physiological disorders.
In the case of diseases it is frequently necessary to spray the plants, often several times, with a specific chemical to overcome the problems. Insect pests are generally easily spotted and readily identified and the plants can be treated accordingly. Always remember that a healthy plant is less prone to attack from pests and diseases than a neglected one. As for physiological disorders, these are generally caused by a mistake in the growing conditions and all that needs to be done is to correct it.
The most widespread diseases are those caused by fungi.
Pests can generally be discovered before they cause any visible harm. It is therefore recommended to examine plants thoroughly at least once a week. Only nematodes will escape attention. We usually take notice of them only when they occur in large numbers causing visible signs of damage such as watery spots which later turn black, or thickening and twisting of stems and poor or irregular growth of leaves. Roots, too, are sometimes attacked by nematodes. This disease is easily recognized by the galls that form on roots and tubers. A look through a strong magnifying glass reveals the presence of slender, minute worms in the affected tissues. Because these pests are generally spread via theit is recommended that this be disinfected with one of the commercially available soil sterilants. Plants that are heavily affected should be discarded.
Among the commonest plant pests are red spider mites. These generally occur on the underside of leaves in delicate, web-like cases, giving the leaves a mottled, dusty look. They suck the plant sap, causing the foliage to fade and turn yellow and eventually drop off. They multiply rapidly, particularly in a dry and warm environment. They are controlled by spraying with a proprietary house plant insecticide or by inverting the plant into a bucket containing an insecticidal solution made up according to the manufacturer’s instructions. However, prevention is better than cure. Ensure plant are kept in a moist, airy atmosphere. In hot dry weather spray the foliage with water frequently and set the pots in moist peat or trays of wet gravel to keep up the humidity level. If this is done red spider mite should not be a problem. Mites of the Tarsonemidae family also attack house plants causing leaves to twist and curl. These are destroyed by spraying with a suitable house plant insecticide.
Plants are also attacked by other sucking insects: white fly, aphids, scale insects, mealy bugs and spring-tails. The tiny white flies are easily detected flying round the foliage; they suck the plant sap on the underside of the leaves. The notorious aphids are found on the underside of the leaves and their presence is also revealed by sticky spots on the foliage which are in turn attacked by moulds. The spots are known as honeydew and are remnants of the plant sap that has passed through the aphids’ alimentary canals without change. Both pests may be controlled by any house plant insecticide. An alternative treatment is to hold the plant upside down,the foliage into an insecticidal solution made up according to the manufacturer’s instructions.
Scale insects are generally found clinging to stems or the axils of leaves. They are covered by a firm scale and are fairly difficult to combat. If their numbers are few then they can be scraped off with a soft piece of wood or plastic taking care not to damage the plant. Alternatively, they can be washed off with a mild soap and water solution. The same method can be used to control mealy bugs, which look like tiny wads of cotton for they are covered with a white flour- or cotton-like waxy secretion. A more effective method of dealing with mealy bug is to touch each pest with a paint brush that has been dipped in methylated spirits.
Spring-tails live in the soil where they feed on plant roots and sometimes even destroy germinating seedlings. These tiny, wingless insects are generally noticed when plants are watered, at which time they suddenly appear leaping about on the water’s surface. They are fairly easy to control by watering the soil with an insecticidal solution.
A word of warning: when using any kind of chemical preparation it is extremely important to follow carefully the manufacturer’s instructions. Many insecticides are poisonous toand warm-blooded animals (including human beings) and should not be used in the home; so make sure you obtain one of the preparations recommended for use on house plants and keep it out of the reach of children and pets.
Most cultivated plants are tolerant of the preparations mentioned in this website There are, however, exceptions, ie. plants that do not tolerate a certain spray or entire group based on a certain compound or element, and use of such preparations may harm or destroy the plant.