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Symptoms. Common smut in corn is
caused by the fungus
Ustilago maydis (also
known as Ustilago zeae). Any actively
growing portion of the plant may become infected.1 Galls on
ears are the most dramatic in appearance; however, potential
yield losses are greatest when young seedlings are infected.
In this case, plants grow abnormally and may not produce ears
or plants die, resulting in reduced stands. Smut galls are a
combination of host and fungal tissues. Young smut galls are
firm and covered with a periderm.3 They are greenish white
silvery white in appearance as they develop.2 As they mature,
the fungal tissues begin to turn black with the development of
teliospores (Figure 1). Upon maturity the periderm ruptures and
releases the powdery black teliospores. Galls that form on
leaves usually remain small and dry out and turn hard without
Cycle. Spores overwinter in the soil and can remain viable
several years. They are spread by wind and rain.
warm, moist conditions, infection can occur on corn leaves,
tassels, silks, or ears; however, each infection is a
event. The fungus does not grow systemically in corn
Infection of seedlings
with wind and
soil, which creates wounds exposing meristematic
to the fungus. Infection of kernels on ears can occur
conditions result in poor pollination.1
drought or rainy weather that results in poor pollen
and inhibits normal pollen release. Silks that are not
remain susceptible to infection for an extended period
time. Poor pollination, caused by a drought that is followed by
rainy conditions that spread smut spores, can significantly
kernel infection. Additionally, damage from insects,
cultivation, herbicide, animals, or hail can create an entry
for the fungus and increases the likelihood of infection of
Symptoms. Head smut in corn is caused by the fungus
Sphacelotheca reiliana. The fungus infects corn plants during
the early vegetative stages and grows systemically in the plant.
Symptoms are not evident until plants reach reproductive growth
stages. Infected ears and tassels are replaced by smut sori
(spore masses) (Figure 2).
Smut sori are covered by a thin
membrane, which easily ruptures to
masses of dark brown to black
spores called teliospores.4 Smutted ears may be
do not produce silks. Infected tassels are completely or partially
covered by sori and normally do not produce pollen.
Thread-like strands of vascular bundles surrounded by black
spores are characteristic of head smut galls. These strands are
remnants of the vascular tissue of the corn plant. Individual
spikelets of the tassel may be infected, forming a shoot like
structure. Infected plants may also produce strange leafy
structures on either the ear or tassel.
Life Cycle. Although teliospores can be transmitted on the
surface of the seed, the primary source of inoculum is
teliospores in the soil. Spores may be transported into a field by
contaminated harvesting, planting, or cultivation equipment.
They can remain dormant in the soil for at least four years.
The disease is favored by low soil moisture and temperatures
between 70 and 82 0F. Head smut is more
common in clay
loam soils and in soils with nitrogen deficiency.
Although it can be difficult to determine the difference
galls of common smut and head smut once they rupture, there
are several important differences between the diseases:
Common Smut. Fungicides do not effectively control common
smut.1 Crop rotation is not a feasible option because the
fungus is widespread and remains viable in the soil for several
years. If possible, grow corn in fields with no history of common
smut. In fields with high levels of common smut, deep tillage
can bury the fungus, which might reduce the level of inoculum
available for the following year. During cultivation, avoid injury
roots, stalks, and leaves. Plant products that are less
susceptible to common smut. Excess nitrogen tends to
increase the incidence and severity of the disease, so maintain
Head Smut. Plant corn products with resistance to head smut.
Products with rapid seedling emergence may avoid infection.
Because infection occurs in the seedling stage, treating seed
with a systemic fungicide can reduce infection.
In-furrow fungicide treatments can be effective, but may not be
economically feasible in areas with only sporadic disease
incidence. As teliospores can survive for several years in the
soil, crop rotations are not effective in reducing the disease.
Where feasible, remove and burn smutted ears before the
dispersal of spores. Head smut has been reported to be more
serious when there is a lack of nitrogen, so maintain a balanced