Often when we discuss plant diseases, we talk about fruiting bodies. Sometimes this causes quizzical looks from the audience or blank stares. Every once in a while I am asked, “What is a fruiting body?”
Plant diseases may be noninfectious or infectious. The infectious diseases are those caused by a pathogen, also known as a disease-causing agent. The major pathogen groups that cause plant disease include fungi, bacteria, viruses, phytoplasmas, and nematodes.
Of these pathogens, only fungi form fruiting bodies and spores that are used in diagnosis of plant disease. When we talk about fruiting bodies, we are referring only to fungal pathogens. Keep in mind that the vegetative body of a fungus is made up of threadlike hyphae. Hyphae are recognizable as the threadlike filaments in a mold or the white threads of mycelium seen on rotting wood. Usually, we cannot identify a specific fungus by the hyphae alone. Spores are the reproductive unit of fungi, analogous to plant seeds. Spores are often the dusty, colored part of a fungus that is easily moved in the wind or in water. Fruiting bodies are fungal structures that contain spores. They come in many sizes, shapes, and colors, all of which aid in identification of the specific fungus.
Here is some information that is helpful but not necessary to fruiting body identification. Fungal pathogens often have an imperfect stage (also called an anomorph) and sometimes a perfect stage (teleomorph) as well. This really confuses things because a disease might be named for either stage. For instance, oak wilt is caused by Ceratocystis (perfect stage); but we identify its imperfect stage, Chalara, in the lab. The point is not to confuse but to alert you to the fact that more than one fungal name may be associated with a single disease. For example, powdery mildew forms white, powdery spores called Oidium (imperfect stage). It also forms a resting stage that is a perfect stage of the fungus, such as Erisyphe. You can see this as the black, pinhead-sized structures on a leaf with white, powdery growth of powdery mildew.
How do we identify fruiting bodies? It is not necessary to know the fungal names or even whether the stage is imperfect or perfect. The first step to disease and fruiting body identification is to use reference books that describe the disease on your host plant. When the fruiting body is listed (as an example) as a pycnidium or an acervulus, pathologists know what to look for on the plant.
The most significant structures in fungal ID are spores, fruiting bodies, and sometimes mycelium. Often these structures can be seen on a dry plant sample. More often, however, the sample must be placed in a moisture chamber overnight to encourage formation of these structures. Laboratory samples for fungal identification are often incubated in moisture chambers for 24 to 36 hours. After that, they become a moldy mess and of little value in diagnosis. In the lab, we use a dissecting microscope to identify fruiting bodies, but you can use a hand lens. Spores need to be observed with a dissecting microscope.