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Cotton bacterial blight (Xanthomonas citri pv. malvacearum)

Cotton bacterial blight, also called angular leaf spot or blackarm, is a seed-borne bacterial disease caused by Xanthomonas citri pv. malvacearum (formerly X. campestris pv. malvacearum). It is one of the oldest documented diseases of cotton in India and historically caused yield losses of 30-35% before the widespread adoption of seed acid-delinting.

Identification and symptoms

Symptoms occur on all aerial plant parts and progress through several phases. On leaves, small water-soaked lesions enlarge between veins to form angular black-edged spots; on stems and petioles, elongated black cankers ("blackarm") girdle the tissue and cause shoot dieback; on bolls, round water-soaked lesions develop into sunken black spots and predispose the boll to internal rot. Severe infections lead to premature defoliation.

Host crops and life cycle

The pathogen is largely seed-borne, surviving on the lint and inside the seed coat between seasons. Secondary spread occurs through rain splash, irrigation water and farm implements, and is favoured by warm humid weather. Crop debris is a less important secondary inoculum source.

Damage and economic impact

In the decades before seed treatment became routine, bacterial blight inflicted 30-35% yield losses in major Indian cotton tracts. Boll-stage infections directly degrade lint quality.

Management

Because transmission is primarily through infected seed, acid-delinting of seed (concentrated sulphuric acid) is the single most effective control and is now standard for Indian cotton seed. This is supplemented by use of resistant or tolerant cultivars and by foliar sprays of copper oxychloride combined with streptocyclin (streptomycin sulphate plus tetracycline) at early symptom appearance. Bacillus-based biocontrol agents are an active research area. Field hygiene, balanced fertilisation and removal of crop debris reduce secondary spread.

See also Bt Cotton Bg Ii, Sucking Pest Complex Cotton, Cotton First 30 Days Management.

References

  1. Bacterial blight: Xanthomonas campestris pv. malvacearum. TNAU Agritech Portal.
  2. Bacterial blight of cotton in India. Plant Pathology, 2024.