Fusarium Head Blight (FHB), Scab, Pink Mold or White Heads

Gibberella zeae (asexual Fusarium graminearum), F. avenaceum, F. culmorum, F. poae


Biology

This disease affects all grain crops, especially those grown in areas with humid summers. The causal fungi may also incite different diseases on other plant parts. Therefore, head blight may appear with root rot or leaf infections or be the precursor of future seedling blight infections. Cereal planted after corn seem to be particularly prone to this disease because corn residues produce very large quantities of the head blight fungi.

The fungi overwinter in crop residues and wild or cultivated grasses. Seedlings may become infected at emergence. Spores are first produced on stem infections at the base of the plant. These spores are spread by rain or wind to infect flower parts, glumes or other portions of the head. This fungus is favoured by prolonged warm, moist weather during anthesis (flowering) of the cereal crop.

The disease is most conspicuous on wheat, and less so on barley, oats, rye and triticale. Head blight may be recognized by premature bleaching of one or all of the spikelets in the head. Infected spikelets are often sterile. The seed in the spikelet stalk above the point of infection may not develop. Diseased wheat heads have small dark spots and fungus growth with an orange to pinkish tinge. Infected grain can be lightly to severely shrivelled and lighter in weight with individual kernels usually whitish in colour.

In decreasing order of susceptibility, fusarium head blight affects durum wheat, Canadian prairie spring wheat, hard red spring wheat, barley, oats, rye and triticale. Yields are reduced by floret failure and poor seed filling. However, a more important factor of this disease is the contamination of the grain by mycotoxins. Mycotoxins such as deoxynivalenol (DON) are poisonous, especially to non-ruminant animals. DON commonly occurs in soft white winter wheat grown in eastern Canada. Wheat is acceptable for human consumption only when it is virtually free of mycotoxins such as DON. Mycotoxins reduce quality and marketability of crops such as malting barley. These losses outweigh yield losses. This disease may be more severe on semi-dwarf and durum wheats than on hard red spring wheat cultivars. Although all four of the fungi listed above can cause grain damage, only F. graminearum generally produces the problem mycotoxin. F. culmorum may also be an occasional toxin producer in irrigated wheat. Therefore it is important to identify the species of fusarium attacking the grain head.

On the prairies DON commonly occurs in Manitoba grain and recently eastern Saskatchewan grain. There are no records of DON in Alberta cereals except for some instances in soft white spring wheats under irrigation. The scab or head blight phase of this disease is fairly common in some years but DON is normally absent.