Common Diseases

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Interior plants are occasionally affected by diseases.

The effects of an infection may vary from a cosmetic disfigurement to a life-threatening condition that might spread to other plants. Fortunately, the occurrence of plant diseases is rare in most interior landscape situations.

Definition of disease

A plant may be said to be diseased when there is a harmful deviation from normal functioning of physiological processes. (Federation of British Plant Pathologists, 1973). This definition is very wide ranging and includes conditions such as those caused by environmental factors or nutritional disorders. For the purposes of this Information Leaflet, disease relates only to those conditions caused by pathogens infecting plants.

A pathogen is defined as a parasite that causes a disease. A parasite is defined as an organism or virus living in or on another living organism (the host) from which it takes materials it requires for growth without conferring any benefit in return. Infection is defined as the establishment of a parasitic relationship. All the important diseases infecting interior landscape plants are usually caused by infection by species of fungi, bacteria or viruses, although other organisms can also be said to be disease causing.

Identifying plant diseases

Plant diseases can be identified by examining the symptoms of the plant. However, many diseases show similar symptoms, but it is usually possible to narrow down the possibilities to a small number. Laboratory cultures may be necessary to confirm the correct identity of the disease. The following table describes some of the commonest symptoms of plant diseases.

Anthracnose

AnthracnoseLiterally means ‘little coal’ and describes small, black spots found on leaves and shoots.

Example: Anthracnose symptoms on Dieffenbachia.

Blight

BlightThe rapid killing of leaves, stems and flowers.

Example: Phytophthora leaf blight of taro (Colocasia esculenta)

Blotch

BlotchLarge, irregular shaped spots and blots on leaves, shoots and stems.

Example: Leaf blotches on Syngonium podophyllum

Canker

CankerNecrotic, sometimes sunken, lesion on the stem or branch of a plant.

Example: Citrus canker.

Chlorosis

ChlorosisYellowing of normally green tissue by the destruction of chlorophyll. Usually the result of nutritional or environmental disorders but may be caused by some viruses.

Example: Chlorosis of a leaf due to insufficient levels of iron.

Dieback

DiebackProgressive destruction of shoots and branches, starting at the tips. Dieback may be caused by physical damage, such as from pruning, or may sometimes be caused by acid rain, heavy metal pollution or imported pathogens.

Example: Phomopsis dieback.

Exudates

ExudatesLiquid or gummy discharge from a lesion, often stems and petioles.

Example: Gliocladium blight of palms often produces a gummy exudate.

Gall

GallLeaf galls are fairly common on trees and shrubs. A gall is actually plant tissue that has developed as the result of feeding or other activity of insects or mites.

Example: Oak Apple gall.

Leaf spot

Leaf spotA self-limiting lesion on a leaf. Can refer to any of a large number of fungal, bacterial, or viral plant diseases which cause leaves to develop discoloured spots.

Example: Leaf spot caused by a fungal disease.

Mildew

MildewA fungal disease in which the mycelium and spores are visible as a white or grey growth on the surface of plant tissues.

Example: Powdery mildew caused by a fungus.

Rot

RotSoftening, discolouration and disintegration of soft tissue, often accompanied by foul odours.

Example: Soft rot of Dracaena deremensis ‘Janet Craig Compacta’ caused by bacteria.

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