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Pathogen Biology

Fusarium graminearum is an ascomycete, producing sexual spores in a sac known as an ascus (plural asci). The asexual state of the fungus produces spores called macroconidia, and the sexual state produces spores called ascospores.

Asexual reproduction

The anamorph (asexual state) of the fungus causing FHB is Fusarium graminearum. Macroconidia (asexual spores) are derived from conidium-producing cells called phialides (Figure 8). The phialides are clustered together in cushion-shaped masses known as sporodochia. The macroconidia are hyaline, canoe-shaped spores usually with five or more septa.

Figure 8

Sexual reproduction

The teleomorph (sexual state) of the fungus is Gibberella zeae. The genus Gibberella belongs in the Hypocreaceae, a family chracterized by brightly colored perithecia that frequently form in stromata (somatic structures on which fruiting bodies develop). The perithecia of G. zeae are a dark blue in color at maturity (Figure 9). Ascospores (sexual spores) form within sacs called asci, and are forcibly discharged from the perithecium through a single small opening known as an ostiole (Figure 10). The ascospores are hyaline to light brown in color, and slightly curved with rounded ends (Figure 11).


Figure 9	Figure 10	Figure11

The majority of the isolates of F. graminearum are homothallic, meaning that they are able to sexually reproduce without a partner. Heterothallic isolates, which require a compatible partner for sexual reproduction, are less common. Laboratory studies, however, have demonstrated that many of the homothallic isolates have the ability to outcross with other compatible isolates. The degree to which this occurs in the field under natural conditions is unclear.

Mycotoxins

Many Fusarium species (including F. graminearum) produce mycotoxins-- fungal chemicals that are harmful to animals. These chemicals may operate in nature to disable plant defense mechanisms or to defend the fungus against other microorganisms. The major toxin produced by F. graminearum in association with FHB in wheat and barley is deoxynivalenol (DON). DON is sometimes called vomitoxin because of its deleterious effects on the digestive system of swine and other monogastric animals. Humans consuming flour made from wheat contaminated with DON will often demonstrate symptoms of nausea, fever, headaches, and vomiting.

DON contamination is measured in parts per million (ppm). DON levels in FHB-infected wheat are frequently quite high (>20 ppm). The USDA recommends that DON levels in human foods not exceed 1 ppm. However, individual grain buyers may have lower tolerances of DON in purchased grain. Higher levels of DON are permitted in feed for poultry and ruminant animals.