Fungi and Plant Disease

This FAQ will help you understand what fungi are and how some of them cause important plant diseases that reduce crop yields.

What is a disease?	A disease is a condition of being sick from a particular cause. Both plants and animals can contract diseases (but not the same diseases). Some diseases are fatal, but most are not.
What is a pathogen?	A pathogen is an organism (living thing) that causes disease in other organisms, which are its hosts. Humans can be hosts to bacterial pathogens that cause diseases like strep throat and pneumonia, for example. We are also host to many kinds of bacteria that aren’t pathogens, including the enormous number of bacteria living in our lower digestive tract. Plants, too, can be hosts to many kinds of fungi and other pathogens, as well as to many kinds of fungi that are helpful or even necessary to the plant’s health.
What kinds of pathogens are there?	The bacteria, viruses, and fungi all include pathogens among their members, although most bacteria and fungi are not pathogens. That is, the majority of bacteria and fungi don’t cause disease in other organisms, but at least a few bacteria, most viruses, and a few fungi do cause disease in one or more kinds of host.
What is a fungus?	The fungi are a group of organisms that get their nutrition (“food”) by absorbing it directly from their environment into their cells as opposed to, for example, what people do, which is get our nutrition from foods that we ingest (swallow). The cells of fungi differ in significant ways from those of bacteria, animals, and plants. Fungi are more closely related to us than either they or we are to plants. Familiar fungi include mushrooms, molds (seen on rotting fruit and other foods), and yeasts such as the one used to make bread rise and in producing alcoholic beverages.
What does the body of a fungus look like?	Some fungi, including yeasts, consist of a single cell, but most are multicellular—they consist of many cells. The body of a multicellular fungus is composed of rapidly growing individual tube-shaped filaments called hyphae. These tiny filaments are in intimate contact with their environment, making it easy for them to absorb nutrients. The body of a fungus, comprising all its hyphae, is called a mycelium.
How big are fungi?	Yeasts are microscopic—you could fit several hundred of them, side by side, along the diameter of a pinhead—as are individual hyphae. In some fungi, hyphae may mass together to form fleshy structures such as mushrooms. There are even fungi with filamentous bodies that extend for miles! Some of these form mushroom-like structures that pop up on tree trunks far apart from one another, but you can’t see most of the filamentous body that extends through the soil between the trees. Photos taken from airplanes show the shape of the giant fungus by revealing the damage it has done to plants.
How does a fungus grow?	Hyphae grow at their tips. The mycelium branches out in all directions and spreads out over the surface on which it is growing.
How do fungi spread?	Fungi spread by means of spores—tiny reproductive cells that may travel great distances borne by wind, water, or animals. There are several kinds of fungal spores. Two types of spores are commonly the most important in the spread of a fungal pathogen during a crop’s growing season. One of these types is called a conidium (plural conidia). Conidia form as naked (unenclosed) spores at the tips of hyphae. The other type, called a sporangiospore, forms inside walled structures called sporangia, each containing multiple spores.
How do fungi affect plants?	Hyphae of some fungi invade plant leaves (or other body parts). Once inside, the hyphae form a mycelium. Some hyphae produce haustoria. These are branching projections that absorb—steal—nutrients from plant cells. The haustoria don’t actually break through the membranes surrounding the cells. Instead, they simply press into the cells, with the membrane fitting them like a glove. Picture yourself pushing your fingers (the “haustoria”) into a large balloon (the “cell”) without popping it. Nutrients pass through the plant cell’s membrane and the membrane of the haustorium and support the growth of the fungus. By robbing the plant of nutrients, pathogenic fungi weaken the plants and reduce crop yield.
Are fungal diseases of crops common?	Fungi are by far the most important plant pathogens, causing crop losses amounting to billions of dollars. In 1935, a single kind of fungus was responsible for the loss of about one-fourth of the entire wheat crop in Canada and the United States. Bacteria and viruses are less important as plant pathogens. In contrast, most human diseases are caused by bacteria or viruses.