Either Tilletia caries or T. foetida cause common
smut. T. caries is important in Washington, and is the
more widely spread of the two species. T. caries produces
reticulate teliospores, while T. foetida, more common
in the central and eastern United States, has smooth teliospores.
Both fungi are basidiomycetes that have many pathogenic (physiologic)
Wheat is the primary economic host, but both fungi also infect
rye and some Agropyron species.
Symptoms and Signs
Plant stunting may not be noticed. The first apparent symptoms
occur after heading. Smutted heads remain green longer than healthy
heads but are more of a gray-green. The kernels are replaced
by "smut balls" which are more rounded than normal
kernels, and which cause the glumes to spread abnormally, giving
the heads an unsymmetrical appearance (Photo 5). Since infection
occurs in the seedling stage before tillering and is systemic,
a) tillers on a plant are diseased.
Teliospores released from the smut balls during harvest either
remain in the soil or are carried on the seed. Soilborne inoculum
is an important factor to consider in the control of this disease.
The teliospores germinate in moist soil to produce a short hypha
(or promycelium) on which long, thin basidiospores (also called
primary sporidia) develop. Opposite mating types of basidiospores
fuse, and produce secondary basidiospores. Temperatures of 40
to 60F favor the production of secondary basidiospores. The basidiospores
germinate near the seed and infect the developing plant coleoptile
before it emerges from the soil (seedling infection).
The mycelium then develops in meristematic tissue of the growing
plant and is carried upward with plant growth. At flowering the
fungus colonizes the ovaries and replaces the kernel with teliospores
to produce easily ruptured bunt (smut) balls. The teliospores
are only viable in the soil for about 2 years.
Chemical. The dramatic event that brought common
smut under control was the use of the protectant fungicide, hexachlorobenzene
(HCB), as a seed treatment. This chemical, effective only against
common bunt, had great advantage over other fungicides (such
as the mercurials) because it controlled not only the teliospores
carried on the seed but those in the adjacent soil as well. Since
seedling infections resulting from soilborne inoculum constituted
the major problem, this was a major breakthrough. Other fungicides
have replaced HCB, which is no longer available.
Resistance. Even though most commercial varieties
have resistance, researchers still advise seed treatment to delay
development of pathogenic races and to prolong the effective
life of these cultivars. Plant breeders estimate that a newly
released resistant cultivar will last only 5 to 8 years before
a fungal strain develops that can attack it.
Hoffmann, J.A. 1982. Bunt of wheat. Plant Disease 66:979-986.
Other Interesting Common
of Nebraska-Lincoln, NebGuide, Bunt or Stinking Smut of Wheat
-- description and picture of grain.
University, Crop diseases -- picture and description.
Diseases * Barley
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Edited and reviewed by Ed Adams, WSU Extension Plant Pathologist
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