Biotic Diseases . Fungal diseases. Powdery Scab of Potatoes. Causative organism : Spongospora subterranea. Symptoms The disease causes severe blemishes (2-20 mm diameter) on the tuber. These blemishes contain dark-colored spore balls. Symptoms
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The disease causes severe blemishes (2-20 mm diameter) on the tuber. These blemishes contain dark-colored spore balls.
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.
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.
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.
The symptoms appear as shiny white postulations on the leaf and stem due to the formation of sori containing conidiophores.
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.
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.