Diseases of Vegetable, Ornamental and Spice Crops

Exercise 10: DISEASES CAUSED BY SCLEROTIA FORMING FUNGI

a) Stalk rot of cauliflower/ white rot of pea/ wilt of brinjal (Causal organism Sclerotinia sclerotiorum (Lib.) de Bary)

Kingdom : Fungi
Phyllum : Ascomycota
Class : Discomycetes
Order : Helotiales
Family : Sclerotiniaceae
Genus : Sclerotinia
Species : sclerotiorum

Symptoms: Leaves lose their bright green shine and droop down. Stalk rots from inside, become hollow and is filled with black sclerotia. Curds loose compactness followed by development of characteristic rot, which generally start, from the center. Water-soaked spots also appear on the bolting shoots later exhibiting white silvery appearance followed by complete wilting. There is no pod formation in diseased plants.

Mycelium: White, cottony, branched, consisting of closely septate hyphae.

Micro conidia: Micro-conidia are produced on short lateral branches of the vegetative mycelium in chains.

Sclerotia: Sclerotia are first white in colour, then turn brown to black and are round to irregular in shape.

Carpogenic germination: The sclerotia also germinate by producing stalked apothecia, which range from one to five per sclerotium. Apothecia are brown in colour and are round or lobate type. Asci are cylindrical. Each ascus contains eight ascospores. Ascospores are elliptical (Fig. 1).

Fig. 1: Carpogenic germination of Sclerotinia sclerotiorum

b) Black Scurf of potato (Causal organism Rhizoctonia solani Kuhn)

Kingdom : Fungi
Phyllum : Basidiomycota
Class : Basidiomycetes
Order : Tulasnellales
Family : Ceratobasidiaceae
Genus : Thanatephorus (Ana= Rhizoctonia solani)
Species : cucumeris

Symptoms: The disease appears in two phases i.e. stem canker and scurf. In stem canker phase, the growing tips of sprouts show browning. Sunken, circular or elongated brown necrotic spots may also be observed on the mature sprouts at the point of infection near mother tubers. Severely infected sprouts are killed. In scurf phase, black crust appears on the surface of tubers due to formation of sclerotia of the fungus. These sclerotia are normally found on the skin and do not cause damage to the tuber inside. Dry rot symptoms on tubers are also seen if the pathogen enters through wounds.

Mycelium: Septate, brownish with dolipore septum. The lateral branches of the mycelium are constricted at branching point and possess septum near the junction (Fig. 1).

Sclerotia: Sclerotia are small, brown to black and unlike other fungal sclerotia are undifferentiated into rind and medullae (Fig. 1).

Sexual reproduction: The basidiospores appear on sterigmata arising from barrel shaped clavate basidia and are usually 4 in number. These are ellipsoid to oblong and flattened on one side.

Fig. 2: Mycelium and sclerotia of Rhizoctonia solani

c) Root rot and web blight/ root rot of fenugreek (Rhizoctonia solani Kuhn)

Kingdom : Fungi
Phyllum : Ascomycota/ Basidiomycota
Class/ order : Deuteromycetes/ Ceratobasidiales
Genus :Rhizoctonia/Thanatephorus
Species : solani / cucumeris

Symptoms: Appear in two phases.

Root rot: Symptoms appear on roots and stem as reddish brown sunken cankers near the soil line, leading to partial or complete rotting of the root system.

Web blight: Small, circular, water soaked, tan coloured spots appear on leaves. Dark brown, more or less circular, slightly zonated and definitely sunken spots appear on pods which in moist weather covered with hyphae and sclerotia of the fungus. Tan brown to reddish brown discoloration is observed on infected seed.

Mycelium: Hyphae hyaline when young and brown at maturity. Mycelium branched at acute angles with dolipore septum.

Sclerotia: Small, smooth sclerotia measuring 1 to 2.5 mm in dia., and brown in colour.

Sexual reproduction: The basidiospores appear on sterigmata arising from barrel shaped clavate basidia and are usually 4 in number. These are ellipsoid to oblong and flattened on one side.