Biotic Diseases (cont.)

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Biotic Diseases (cont.)

• Fungal Diseases (cont.)

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Ergotism
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Claviceps purpurea on Sea Couch
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Syptoms Claviceps purpurea is commonly
found on grains of rye or sometimes on other
grasses such as quackgrass. The fungus infects
the flowers when they're young. It induces the
cells to divide (hyperplasia) and to enlarge
(hypertrophy), creating the relatively large
brown sclerotia. These sclerotia are hard resting
structures that allow the fungus to survive
adverse conditions, such as winter and
desiccation. In the life cycle of this
organism, the sclerotia fall to the ground and
overwinter, germinating in the spring to produce
a stroma that contains perithecia, which produces
spores.
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It's really not a devastating parasite to
the plant. You might think its main detriment is
that it replaces one of the grains of the plant,
thus reducing yield. However it also draws
nutrients away from the other uninfected grains
so that they become stunted, thus reducing yield
quite a bit more. But its worst problem is when
the sclerotia inadvertently get mixed in with the
grains and are incorporated into foods, thus
causing a devastating and sometimes deadly
syndrome called ergotism in humans and other
animals. Ergotism is caused by the
chemicals in the fungus called ergot. Consumption
of foods contaminated with ergot and ergot
derivatives may cause vomiting, diarrhea,
hallucinations, and may lead to gangrene in
serious cases.
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Perithecial head
Stroma
Stalk
Germinating sclerotium
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L.S. in Perithecial head
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Causative organism Claviceps purpurea
perithecium
asci
L.S. in perithecial head showing perithecia
containing asci
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Covered Smut
Barley heads infected with covered smut
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Symptoms Losses from the Covered smut
disease are rare because the environments are not
conducive. In addition, seed treatments have
reduced incidence of the disease. However,
because heads infected with covered smut are
harvested with healthy grain, losses can arise
from lowered grade due to smutty grain.
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Causative organism Ustilago hordei
Chlamidospores of Ustilago hordei
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Loose Smut
Loose smut of wheat caused by Ustilago tritici
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Barley Loose Smut
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Loose smut is most obvious just after the plant
has headed. Diseased plants produce blackened
heads among a field of green heads. The spikelets
of colonized heads become a mass of olive-black
spores that have a characteristic "dead fish"
odor. The fungus that causes loose smut survives
as dormant mycelia (fungal threads) within the
embryo of an infested seed. When the seed
germinates, the fungus resumes growth along with
the shoot apex. As the juvenile head develops
within the stem the fungus colonizes the seed
primordia (tissue that would become a seed within
a head). When the head emerges from the boot,
instead of flowering and releasing pollen, it
releases the olive-black spores that can be
wind-blown to "healthy" flowering heads. The
spore germinates on the stigma (female receptive
portion of flower) of a healthy head and
colonizes the developing seed embryo. The
colonized seed appears healthy but carries the
dormant smut fungus within to start the cycle
over again with the planting of the seed.
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Beginning of the sporulation of Ustilago
syntherismae on the rachis of Digitaria
sanguinalis racemes.
Digitaria sanguinalis loose smut
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Spikelets are replaced by a blackish-brown spore
mass
Digitaria sanguinalis loose smut
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Spores of Ustilago syntherismae
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Scanning electron micrograph of Ustilago
syntherismaeon Digitaria sanguinalis showing
globose echinulate spores
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Flag smut
Wheat plant with severe flag smut infection
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Dark striping of leaves caused by flag smut
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'Cracking' of stem caused by flag smut
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Symptoms Affected plants are severely stunted.
The ears fail to emerge, remaining within the
boot. Plants show long dark grey to black streaks
on the leaf blades and leaf sheaths. The streaks
eventually erupt, giving the leaves a ragged
appearance and exposing the black teliospores
which are then dispersed, giving the plants the
appearance of being covered in soot.
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Causal organism  Urocystis agropyri
Sterile cells
Fertile Chlamidospores (1 to 4)
Chlamidospores forming spore balls
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Wheat stem rust
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Berberis leaf infected with Puccinia graminis
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SYMPTOMS Uredinia generally appear as oval
lesions on leaf sheaths, true stem, and spike.
Uredinia can appear on the leaves if other
diseases have not killed them. Uredinia are brick
red in color and can be seen to rupture the host
epidermis, on the leaves uredinia generally
penetrate to sporulate on both surfaces. Infected
areas are rough to the touch. Stem rust is
favored by hot days 25-30 C, mild nights 15-20 C
with adequate moisture for night time dews. Wind
can effectively disperse urediniospores over
great distances. Rain is necessary for effective
deposition of uredinospore involved in regional
spore transport. Disease dispersal through
aeciospores from Berberis vulgaris is currently
rare, but historically it was an important source
of inoculum. Mycelium or uredinia on volunteer
wheat, are the most important source of inoculum
in tropical and subtropical climates.
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Urediniospores and aeciospore germinate when
in contact with free water. Infection is by
penetration through the stoma. Penetration
requires at least a low light intensity.
Stem rust can survive as teliospores during
winter when aeciospores are a major source of
inoculum. It generally survives as mycelium or
uredinia on volunteer wheat during the non-wheat
growing season. Uredinospore can be spread by
wind into disease-free areas. Sporulating
uredinia are active in tropical and some
subtropical areas throughout the winter.
Occasional dormant mycelium may survive beneath
the snow pack in more northern temperate regions.
Urediniospores and aeciospores are wind
borne. Teliospores remain with the straw.
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T. S. in infected wheat stem showing uredosorus
with uredospores of Puccinia graminis
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T. S. in infected wheat stem showing teleutosorus
with teleutospores of Puccinia graminis
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T.S. in infected Berberis leaf showing pycnidium
with pycniospores of Puccinia graminis
T.S. in infected Berberis leaf showing aecidial
cup with aeciospores of Puccinia graminis