Diseases of Vegetable, Ornamental and Spice Crops

Sclerotinia wilt

The disease is more important in tarai area (foot hills of H.P and Uttrakhand) of the country where it causes considerable yield losses.

Symptoms:

• Circular to elongate water-soaked lesions closer to the inflorescence appear on the branches followed by watery soft rot.
• At the point of infection, a dry, discoloured spot develops.
• As a result of tissue necrosis, the portion of the plant beyond the point of infection wilts.
• If the infection is at the base of the main stem, the entire plant wilts and if only few branches are attacked than partial wilting of the plant may take place.
• In the advance stages under cool humid conditions, the mycelium emerges out and creamy coloured compact sclerotial initials as well as matured black sclerotia of varying sizes are evident on above ground parts.
• Fruits are also attacked. The rotting of the flesh takes place and in rotting tissue large number of sclerotia are seen.

Pathogen:

• The disease is caused by Sclerotinia sclerotiorum (Lib.) de Bary.
• Mycelium in culture as well as on host surface is hyaline, cottony, branched, consisting of closely septate hyphae, and filled with dense granular protoplasm.
• Fungus germinate by two means i.e. myceliogenic and carpogenic (forming apothecia). Primary infection occurs through ascospore whereas secondary infection takes place through mycelium.
• Microconidia (spermatia) are produced on short lateral branches of the vegetative mycelium in chains.
• When food supply is exhausted and the vegetative growth ceases, the hyphae with granular protoplasm collect in small dense masses and form sclerotia.
• These sclerotia are first white in colour and then turn black.
• The sclerotia also germinate by producing stalked apothecia, which range from one to five per sclerotium when proper humidity and light conditions are provided to overwintered sclerotia.
• The best apothecial development takes place at 7-11°C and their production ceases at 16-28°C with low soil moisture.
• Apothecia are brown in colour and are round or lobate type.
• Asci are cylindrical measuring 108-153 x 4.5- 10 µm in size and each ascus contains eight ascospores, which are released in clouds.
• Fungus produces oxalic acid which kills cells in advance resulting characteristic hollow stem.

Disease cycle and epidemiology:

• The fungus survives in soil in the form of sclerotia.
• In the presence of proper humidity and light conditions, sclerotia germinate by forming apothecium which in turn forms asci and ascospores.
• The spores, upon escaping from the ascus, lodge on a susceptible host, and a new infection may originate.
• Mycelium from sclerotia is also capable of infecting eggplant.
• The pathogen can infect the susceptible host over a wide range of temperatures i.e. from 0 to 25 ^oC with an optimum at 15 to 20 ^oC.
• The fungus can tolerate wide pH range but is best adapted to an acidic substrate. Application of nitrogenous fertilizers enhances this disease.

Management:

• Collect and destroy the infected plant debris.
• Follow crop rotation with cereals i.e. paddy or maize.
• With the initiation of the disease, spray the crop with carbendazim (0.1%) or thiophanate methyl (0.1%) or combination of mancozeb (0.25%) and carbendazim (0.05%) and repeat at 10 to 14 days interval.
• Fungi like Trichoderma harzianum and T. viride (40 g/ m2 at the time of field preparation) have also been reported as antagonistic to this fungus.