Diseases start when a fungal spore, bacterial organism, or virus finds a point of entry into the plant. Maintaining good standards of hygiene, and keeping plants healthy and well-fed, helps to reduce the chances of infection.

Anthracnose

Fungi causing sunken, discolored spots on the leaf’s surface.

Remedy: Spray immediately with a general fungicide.

Blackleg

Cuttings turn black at the base, the top becomes discolored and dies. Caused by a fungus, soil- or water-borne, and often carried on dirty tools.

Remedy: Discard cuttings. Keep propagation equipment very clean.

Damping-off

Seedlings wilt over and die, spreads rapidly to affect whole patches. Caused by various fungi.

Remedy: Always use clean seed trays and sterile soil mix.

Gray mold (Botrytis)

Covers the affected part of the plant with gray fluff. It usually enters via a wound and can thrive on living or dead tissue.

Remedy: Remove infected areas by cutting back to healthy tissue. Clear away all dead leaves and flowers immediately. Spray with general fungicide.

Mildew

All living parts of the plant are susceptible to this white powdery fungus.

Remedy: Remove infected areas by cutting back to healthy tissue.

Rot

Usually attacks the stem of the plant, making it turn wet and slimy. Often associated with overwatering.

Remedy: Parts of the plant may be salvageable for cuttings; otherwise, it should be discarded.

Rust

Forms red-brown pustules, normally on leaf surfaces. Spreads rapidly. Affects certain plants such as geraniums.

Remedy: Spray with proprietary cop­per-based chemical.

Sooty mold A black fungus, which grows on the honeydew excreted by many insects. Looks unsightly and interferes with photosynthesis. Remedy Wash with a soft cloth and soapy water. Identify and treat the pest.

Virus

Causes changes in plant cells, resulting in yellowing or mottling of leaves, and distortion of shoots. Often spread by sap-sucking insects.

Remedy: There is no cure.