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Stem Rot Potato disease

Thursday, September 10th, 2009

(Sclerotium rolfsii)

Stem rot, southern blight, or Sclerotium rot occurs worldwide and attacks a wide range of potatoes plants, but is a problem for potato only while growing under hot, moist conditions.

Symptoms of Stem Rot

Initially, daytime wilting and yellowing result when brownish lesions girdle the stem base just below the soil line.

A white mycelium grows on potatoes stems, tubers, or soil, often in fan-like mats, which produce small, initially white, but later brown, sclerotia, similar in appearance to mustard seed.

stem-rotIn rainy weather, the affected stem sloughs off, leaving only the vascular tissue of the xylem, which leads to stem collapse.
Potato tubers may rot in the field before harvest, in storage, or in transit. They first form a cheesy semi-firm decay, which is often invaded by soft rotting organisms. Seed tubers may decay before plant emergence.


Sclerotia are long-lived and many crops are susceptible to S. rolfsii.
Control of stem rot is difficult when conditions favour disease.

Either plow deeply to bury affected plants and crop residues or remove them from the field and burn or bury them.
Avoid plant debris around potato stems.

The pathogen is highly aerobic; therefore, deep plowing and rotation with paddy rice are effective.
Harvest  potatoes during dry weather.
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White Mold potato disease

Thursday, September 10th, 2009

(Sclerotinia sclerotiorum)
White mold disease affects potato mainly in the cool tropics and temperate zones. But it causes major damage to potato when the rotation includes susceptible vegetable crops (beans, lettuce, tomato, celery, cauliflower, cabbage). It is favoured by cool, moist weather.

Symptoms of White mold

Stem lesions develop at the soil line or near leaf axils and are slightly sunken, oval, or elongated, extending up the stem. Initially, lesions are water-soaked, later becoming tan colored, white at the center, and ringed or zonate.

white-moldAffected stems are covered with a white mycelial mat.
The central pith is destroyed and the hollow becomes filled with white mycelium that later forms hard, black, 0.5 to 1.0-cmlong sclerotia.
Tops may wilt and stalks split or break at the soil surface.
Potato tubers near the soil surface become shrunken, superficially blackened, and watery.
Cavities lined with black dead tissue later fill with white mycelium and sclerotia.

Sclerotia germinate, forming mycelial mats or small, fleshy, cup-shaped mushrooms from which air-borne spores disseminate and infect the leaves and stems of many dicotyledonous crops and weeds.


The long-lived sclerotia can be killed by flooding for about 5 weeks. Rotations with non susceptible crops, including potato only every third year, along with removal and destruction of infected plants, help reduce this disease while potatoes growing. Avoid overhead irrigation.

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Powdery Mildew Potato disease

Thursday, September 10th, 2009
Powdery mildew, a Biotrophic Fungus
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(Erysiphe cichoracearum)

Powdery mildew is widely distributed on many host plants and develops  also on potato under arid conditions with high humidity.

Symptoms of Powdery Mildew

Old infections superficially resemble late blight as leaves turn black, die, and drop from the plant. Stems may also be infected. Initially, infected potato leaves are covered with whitish spore masses resembling soil residues, dust, or spray deposits.
The disease requires high humidity, but seldom develops where water falls on the leaves as rain or sprinkler irrigation.


Where the disease is severe, biweekly applications of liquid sulfur formulations may be necessary.

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