Biology Reference
In-Depth Information
blisters or pustules of spores break out on scales
or leaves of young plants. Many plants die; others
survive and have black or brown smut pustules on
the cured bulbs. Plants are stunted but not rotten,
although smut may be followed by secondary rot
organisms.
The spores can live in soil for years, but infec-
tion is possible only in young plants from the
second day after seed germination until the seed-
ling is in first leaf, a period of 10 to 15 days. The
spore is able to penetrate the onion through root
and cotyledon but cannot enter a true leaf. After
entrance it spreads through the seedling until it
reaches the leaves to form fruiting pustules just
below the epidermis. When this ruptures, spores
are dropped, to be disseminated by running water
and tools, on feet of persons and animals, and on
roots of transplanted vegetables. Onion smut is
confined to states with cool summers, optimum
soil temperature for infection being 72
F.
Urocystis tritici
Flag Smut
of wheat. Plants are
dwarfed with twisted leaf blades; sheaths are
marked with grayish-black stripes; diseased tis-
sues dry up and are shredded. Infected plants
rarely produce heads.
Ustilago hordei
Covered Smut
of barley. Heads
are converted into hard, black, smutted masses,
enclosed within thin membranes.
Ustilago kolleri
Covered Smut
of oats. Spore
balls remain intact within glumes until threshing,
when spores are distributed over surface of seed,
ready to infect young seedlings.
Ustilago maydis (
U. segetum
).
Corn Smut
,
Boil
Smut
, general on corn but most destructive to
sweet corn. The average annual loss is 3 to 5 %
but it can be 100 % in any one field. The fungus
was described in Europe in 1754 and not reported
here before 1822, but it may be native along with
its host. There are many physiological races, and
smut resistance is likely to be correlated with lack
of vigor, so that it has been hard to breed desir-
able resistant varieties.
Any plant parts aboveground may be
attacked - stalks, prop roots, leaves, tassels,
husks, and ears (see
Fig. 2
). Large boils are
formed, at first covered with a greenish white
membrane, said to be good eating when boiled
or fried. Later the membrane breaks and releases
myriads of dark chlamydospores. The plant
is often distorted. Infections are local; each boil
is formed where a spore lands, and there is
no systemic growth through the plant. The fungus
is not seed-borne, and germinating seedlings
are not affected. Chlamydospores winter in soil,
corn debris, and manure. They produce sporidia,
which may bud to form secondary sporidia,
and these are carried by wind and other agencies
to corn plants, which are 1 to 3 feet high.
Mycelium from spores of two sexes is needed
for active development. Spores formed in
the first boils provide inoculum for secondary
infection of ears.
Corn smut thrives in warm weather, optimum
temperature for spore germination being 80
to
92
F. Heaviest infection occurs when scant rain-
fall in early stages of growth is followed by
moderate rainfall as corn approaches maturity.
Vigorous plants are most susceptible, but may
escape the most serious effects because of their
rapid growth. Spores retain viability 5 to 7 years.
They remain viable in passage through an
animal into manure, but are killed by the acids
in silage.
Ustilago
Ustilaginaceae. Sori in various host parts; spore
masses powdery to agglutinated; usually dark
brown to black, in some species yellow to purple
without a peridium. Spores single, not united in
balls (see
Fig. 1
).
Ustilago avenae
Nigra Loose Smut
, general on
barley.
Ustilago avenae (including
U. perennans
).
Black Loose Smut
on oats and some grasses.
Individual flowers in panicle are largely replaced
by a spore mass. The young seedling is diseased
from the seed, and the fungus grows systemically
in the plant.
Ustilago buchloes
Stripe Smut
on grass.
Ustilago bullata
Head Smut
on many grasses.
Ustilago esculenta
Smut
on wild rice.
Ustilago heufleri
Erythronium Smut
. Large
dusty pustules lead to cracking and dying of
leaves of dogtooth violet.
