Disease:Collar rot and web blight

Rhizoctonia solani

Thanatephorus cucumeris - Teleomorph

This is an important disease noticed at all stages of crop growth. The pathogen causes seed rot, seedling blight, collar rot, root rot and web blight among which, the collar rot and web blight symptoms are more prevalent.

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  • Collarrot image1
  • Collarrot image2

Symptoms

  • The collar rot is manifested as reddish brown water soaked lesions at the base of the stem in contact with soil.
  • Under favorable condition, the lesions extend and cause girdling of the stem.
  • The stem at the collar region becomes sunken and rotten.
  • Thick mycelium and brown sclerotia are noticed in the affected portion at later stages.
  • The foliage above the infected stem shows yellowing.
  • Defoliation sets in, after lesion formation.
  • If the conditions are unfavorable, the lesion does not expand further, but the plant remains weak and stunted.
  • Web blight is noticed on the foliage. Initially the symptoms are seen as small necrotic lesions, 5-10 mm wide, with brown centers and pale green margins.
  • The leaves show water soaking and rotting.
  • On the rotten areas white cobwebby mycelial growth is noticed.
  • The adjacent leaves are seen webbed together by the mycelium.
  • Sclerotia are also noticed in the affected areas.

Epidemiology

The pathogen survives in infected crop debris in soil. Sclerotia can remain in soil for several years. A large number of weed species are known to be sources of survival and primary inoculum of the pathogen. High soil moisture and relative humidity are important for disease development, colonization and survival. Survival is affected by soil temperature. Total inorganic N and ammoniacal N are positively correlated with infection.

Procedure for Observation

Assess the number of plants affected by observing 25 pits at random.

ETL

  • Mere presence

    • Control Measures

  • Cultural practices like shallow sowing, use of raised beds, deep ploughing and rotation with cereal crops help in decreasing disease incidence and severity.
  • When cowpea is rotated with rice, the severity of disease may be augmented Mulching is effective in suppressing web blight.
  • Application of organic amendments like neem cake.
  • Collection of seed from healthy plants.
  • Roguing of severely infected plants and drenching the soil around (spot drenching) with copper oxychloride or mancozeb @ 2-4 g/l or carbendazim 1g/L .Seed treatment with carbendazim 1g/kg seed.