Fusarium
Introduction
Fusarium is a large genus of filamentous fungi, part of a group known as hyphomycetes, widely distributed in soil and associated with plants. Most species are harmless saprobes, and are relatively abundant members of the soil microbial community. Some species produce mycotoxins in cereal crops that can affect human and animal health if they enter the food chain. The main toxins produced by these Fusarium species are fumonisins and trichothecenes.
Taxonomy and Morphology
The taxonomy of the genus Fusarium is complex. The genus contains numerous species, many of which are phylogenetically distinct. The genus is traditionally classified in the family Nectriaceae, although recent molecular studies suggest that it may be more appropriately placed in its own family, the Fusariaceae. Fusarium species are characterized by the production of multicellular, banana-shaped macroconidia that are formed in clusters on monophialides. The macroconidia are typically hyaline and have a smooth wall. They range from 30-60 µm in length, and have 3-5 septa. Some species produce microconidia on polyphialides. The microconidia are usually oval or kidney shaped, and range from 8-15 µm in length.
Ecology
Fusarium species are ubiquitous in nature, and are able to cause disease in a wide range of host plants. They are commonly found in soil, and can also be found in water and air. They are capable of surviving in a wide range of environmental conditions, and are able to colonize a wide range of substrates. Some species are pathogenic on plants, causing diseases such as wilt, ear blight, and root rot. Fusarium species can also cause opportunistic infections in humans, particularly in immunocompromised individuals.
Pathogenicity and Disease
Fusarium species are among the most important fungal pathogens of plants. They cause a wide range of diseases, including root rots, wilts, and blights on many different types of crops and ornamental plants. Some of the most devastating diseases caused by Fusarium species include Fusarium wilt of banana, Fusarium wilt of tomato, and Fusarium head blight of wheat and barley. Fusarium species can also cause a range of infections in humans, particularly in immunocompromised individuals. These can range from superficial infections of the skin or nails, to more serious infections of the cornea or disseminated infections that affect multiple organs.
Mycotoxins
Some Fusarium species produce mycotoxins, toxic compounds that can contaminate crops and pose a serious health risk to humans and animals. The most important mycotoxins produced by Fusarium species are trichothecenes, zearalenone, and fumonisins. Trichothecenes are potent inhibitors of protein synthesis, and can cause a range of symptoms in humans and animals, including nausea, vomiting, diarrhea, and in severe cases, death. Zearalenone is a potent estrogenic compound, and can cause reproductive problems in animals. Fumonisins are associated with equine leukoencephalomalacia, porcine pulmonary edema, and cancer in humans.
Control and Management
Control of Fusarium diseases is difficult, and relies on a combination of cultural practices, chemical control, and the use of resistant varieties. Cultural practices include crop rotation, the use of clean seed, and the removal of infected plant material. Chemical control often involves the use of fungicides, although the efficacy of these can vary depending on the specific Fusarium species and the crop. The use of resistant varieties is the most effective method of control for many Fusarium diseases, although resistance can be difficult to breed into crops.