Like in humans, plant diseases can be caused by a number of different types of pathogens, including bacteria, viruses, protozoa and parasitic plants, although about 85% of plant diseases are caused by fungi.

Plant pathology, also known as phytopathology, is the study of these diseases. It is important to know exactly what is causing the disease in the plant so that an appropriate treatment, where possible, can be chosen, and to prevent it spreading. Effective disease management is crucial in the agricultural world for food security and to maximize crop efficiency, reducing land and water usage, a significant concern with the increase of the global population. Many plant diseases are highly contagious and must be controlled so they do not spread across entire crops, countries or even continents, such as in the case of Dutch Elm Disease.

Plant diseases reduce crop yield in many ways, frequently by reducing the photosynthetic area of leaves. They can also act as parasites, depriving the affected plant of essential nutrients and/or water – this is the method of fungal pathogens. A similar effect is observed when disease affects root growth, for which the catastrophic effects can be seen in the rapid decline of many banana species.

Signs of fungal infection include leaf rust, stem rust and sclerotinia (white mold).

Bacterial infection symptoms include (often difficult to see): leaf spot with yellow halo, fruit spot, canker, crown gall, shepherd's crook stem ends on woody plants.

Viral infections include mosaic leaf pattern, crinkled leaves, yellowed leaves, plant stunting.