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Evolution and InnovationSave Biodiversity...Rise

Sclerotinia rot

Infected lettuce

Note white mould and black sclerotes on lettuce head

Infected plants

Note bleached stems (left); black sclerotes - 2 to 10 mm long - inside stem (right)

Infected potato tubers

Sclerotinia rot. External symptoms (left); internal symptoms (right)

General information

Fungi of the genus Sclerotinia attack a wide range of vegetable, fruit and field crops. Two species,S. sclerotiorum and S. minor, cause important diseases in vegetables. Both species have wide host ranges. Sclerotinia sclerotiorum occurs on more than 400 different hosts, including numerous cultivated vegetables. Sclerotinia minor occurs on more than 90 hosts, including peanut, lettuce and potato. Diseases caused by S. sclerotiorum are known by several names, including white mould or nest of bean, drop of lettuce and sclerotinia rot of cabbage, carrot, celery, potato and tomato.

Cause

The fungi Sclerotinia sclerotiorum and Sclerotinia minor.

Symptoms

The two species of Sclerotinia cause similar symptoms. These include water-soaked rotting of leaves, stems or fruit, with a white, fluffy fungal mycelium covering affected areas. Compact resting bodies or sclerotia of the fungus soon develop on the rotted tissues. The sclerotia are white at first, but later become black and hard. The sclerotia are often found inside the stem (e.g. of tomato plants if you break the stem open).

Sclerotia of S. sclerotiorum are large and elongated (2-20 mm long and 3-7 mm wide), whereas those ofS. minor are smaller and irregular to roughly spherical (0.5-2 mm in diameter).

Stem infection by Sclerotinia causes a pale or dark brown lesion, often quickly covered by white cottony patches of fungus. Leaves may show little evidence of attack, but the fungus can rapidly rot the stem, causing collapse of the plant.

Leaves and petioles of plants such as celery and lettuce rapidly wilt and die following infection, with total collapse of the plant occurring as the fungus spreads through the stem.

How does it spread?

Sclerotinia species have a wide range of hosts among crop, weed and ornamental species. Sclerotia formed in infected plants can survive in the soil for many years.

Sclerotinia sclerotiorum

During moist weather, sclerotia of S. sclerotiorum near the soil surface germinate and produce slender stalks that terminate in small, cup-shaped structures called apothecia. Ascospores produced in the apothecia discharge into the air and are carried by wind.

Ascospores germinate when they land on senescing plant parts such as old blossoms and leaves, which provide a readily available food source. The fungus then multiplies and rapidly infects adjacent healthy tissue. Ascospore germination occurs in free water from dew, rain and fog or sprinkler irrigation.

Sclerotinia minor

A senescing food base is not required for S. minor infection. Instead, sclerotia germinate to produce fungal mycelium that infects plants directly.

Cool, wet weather favours sclerotinia rots. Disease can occur at temperatures ranging between 4-30°C, with temperatures slightly below 20°C being optimal for infection.

Crops affected

A wide range of vegetable, fruit and field crops.

Control options

Rotate susceptible crops with resistant ones such as summer and winter cereals, onion and sweetpotato. Plough in diseased crops immediately after harvest.

Apply the recommended fungicides. Correct timing and good penetration of foliage are essential for effective control.

Seek advice for disease management in specific crops.

Chemical registrations and permits

Check the indian Pesticides and Veterinary Medicines Authority chemical database and permit database for chemicals registered or approved under permit to treat this disease on the target crop in your state or location. Always read the label and observe withholding periods.

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