Chapter 23

Greenhouse Disease Management

Typical Plant Disease Symptoms

A plant symptom can be defined as a visible or detectable abnormality on or in a plant arising from a disease or disorder.  Most of the symptoms of plant diseases are visible and are caused by biotic and/or abiotic factors. Plant diseases are classified as “abiotic,” or diseases that are non-infectious and include damage from air pollution, nutritional deficiencies or toxicities, and grow under less than optimal conditions. Symptoms are usually the results of a morphological change, alteration or damage to plant tissue and/or cells due to an interference of the plant’s metabolism. Most plant diseases—around 85 percent—are caused by fungal or fungal-like organisms. However, other serious diseases of crops are caused by viral and bacterial organisms. Certain nematodes also cause plant disease. The following is a description of some common symptom of plant diseases known to cause damage to greenhouse crops.

Leaf Spots

Leaf spots are localized infections of leaves. Most are caused by fungi or bacteria, but some are caused by hail, insects, pesticide applications or drought stress. Many fungal spot diseases require free moisture on the leaf surface to germinate and develop. Spots caused by fungi tend to be round in outline, while those of bacteria are often angular.


Rusts are diseases caused by fungi and are named for the yellow to reddish spore masses they form on plant surfaces. Rust fungi have multiple spore stages and may require more than one host to complete their lifecycle. The rust-colored pustules break through the surface of leaf and stem tissue. The “rust” is easily rubbed off with your finger.


Cankers are localized dead areas on twigs, branches and trunks. Hail, sunscald, pruning wounds, damage from improper staking and maintenance, as well as infectious agents, may cause cankers. Cankers caused by disease organisms appear as sunken areas on branches and trunks. The edge of the canker often, but not always, shows a thickened area or margin. Sometimes the bark within the sunken area will split or tear as it dries out.

Root Rots

Root rots are difficult to diagnose because the affected portion of the plant is underground. Look at the entire root system of container plants and portions of an established system in order to diagnose root rot. When a root system deteriorates as a result of root rot damage, above-ground symptoms may include dieback, wilting, small leaves, dead leaves and increased seed production.

Viral Diseases

Viral diseases cause changes in plant growth or coloration, and may kill plants. Common symptoms include stunting, mottling, mosaic patterns, lack of or reduced flowering, chlorosis, or changes in the normal development of leaves and buds.

Other Diseases

Other diseases include blights, scabs, powdery mildew, smuts, galls, and storage rots (commonly found on stored bulbs, corms, rhizomes or tubers).

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