Biotic Diseases

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Biotic Diseases

• Fungal diseases

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Powdery Scab of Potatoes
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Causative organism Spongospora subterranea
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Symptoms The disease causes severe blemishes
(2-20 mm diameter) on the tuber. These blemishes
contain dark-colored spore balls.
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Symptoms Powdery scab is more prevalent in cool,
damp growing seasons. The disease causes severe
blemishes that spread beneath the epidermis and
erupt when the tuber emerges. In severe cases,
the tubers are malformed and covered with
unattractive secondary growths. Deep cracks could
appear.
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Wart disease of potatoes
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Causative organism Synchytrium endobioticum
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Symptoms Tubers, stolons and occasionally leaf
stalks affected, but not roots. Tubers may bear
cauliflower-like tumors at the eyes or whole
tuber may become shapeless warted mass. May
progress in store even from minute warts not seen
at lifting. Organism in soil, may remain
infective for more than 30 years. Spread on
implements and in dung from stock fed with
infected tubers. More severe in wet seasons.
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Club root of crucifers
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Causative organism Plasmodiophora brassica
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Symptoms The first symptom usually observed
is a wilting of the foliage especially on hot
days. Wilting is the result of impaired root
function which reduces the capacity of the plant
to take up water. The diseased roots are also
less able to absorb nutrients and so the foliage
may appear yellowish and the plant may be
stunted. Although these above-ground symptoms
could be due to other causes, an abnormal
enlargement of the roots is diagnostic for this
disease. The fungus penetrates the roots directly
or through wounds and induces the root cells to
enlarge. The resulting enlarged roots appear
spindle-shaped, knobby, spherical, or
club-shaped. All roots including the underground
stem may be affected .
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White Rust of Crucifers
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Causative organism Albugo candida
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Symptoms The symptoms appear as shiny white
postulations on the leaf and stem due to the
formation of sori containing conidiophores.
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Late Blight of Potatoes
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Causative organism Phytophthora infestans
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Symptoms The disease strikes down plants like
a hard frost in summer. It spreads so fast. It
reduces the foliage to a putrid mass in a few
days, and the tubers are affected to various
degrees in a similar way, although they do not
rot as rapidly. Wilting of young leaves are the
first symptoms. Leaf margins begin to turn brown
and entire leaves will die.
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Peach leaf curl
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Causative organism Taphrina deformans
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Symptoms Taphrina deformans can infect leaves,
fruits, and young twigs. Infected leaves become
distorted, puckered, and thickened, initially
with a distinct reddish or purple coloration. As
infection progresses, affected leaves turn gray
with a powdery appearance as a result of the
production of fungal spores on the leaf surface.
Shortly thereafter these leaves turn yellow or
brown and drop. Fruit, which becomes infected,
tends to drop shortly after infection occurs.
Infected twigs are swollen and stunted, usually
with deformed leaves at their tips.