Fungous diseases are those caused by parasitic fungi, and with them are grouped the very similar bacterial diseases.

Parasitic fungi are mostly microscopic. They invade higher plants and grow in their tissues (cells), which they kill and then absorb the contents for food. They penetrate and grow in the plant cells by means of fine fungal threads (hyphae), and spread from plant to plant by means of spores (the equivalent of seeds in higher plants). These spores are formed at the ends of special threads, often inside special fruit bodies, and are produced in enormous numbers. When released, they are carried by wind currents or water (by splashing) to healthy plants, where they alight, germinate, grow into the tissue and spread the disease.

Most fungous parasites spend winter on the plant or in the soil by forming a thick-walled structure which is resistant to adverse weather.

These fungous parasites are of two types. The first which includes the powdery mildews (common on many plants such as roses, delphiniums, lilacs and zinnias) produces a whitish growth on the surface of the leaves, stems, and petals. This growth is made of fungal threads and spores, which cover the leaf surface and feed by sending down a kind of sucker (haustorium) into the surface cells (epidermis) to absorb nourishment. In the second type, the parasite grows down deeply into the internal tissues, sending up threads to produce spores at the surface. The first type is easy to check; unfortunately most fungous diseases belong to the second.