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Tea blister blight (Exobasidium vexans) Photo: placeholder pending image-fill pass

Tea blister blight (Exobasidium vexans)

Blister blight, caused by the obligate basidiomycete Exobasidium vexans Massee, is the most important foliar disease of tea in south India and the Darjeeling hills. It attacks the young harvestable shoots — the very leaves that become made tea — and routinely causes 30-50% crop losses in untreated estates during the south-west monsoon.

Identification and symptoms

Infection starts as small translucent pinhead spots on the upper surface of young, soft leaves. The spots expand into circular concave lesions on the upper side and convex, velvety, white sporulating "blisters" on the underside. Mature blisters burst, leaving necrotic centres; severely affected shoots are unfit for plucking and shoots flush poorly.

Hosts and lifecycle

E. vexans is restricted to Camellia sinensis and a few related Camellia species. Basidiospores are wind-dispersed, germinate at 18-25 °C in the presence of free water, and complete a cycle in 11-28 days depending on temperature. The pathogen survives between flushes as resting mycelium in dormant buds. Disease is severe in shaded, high-rainfall, low-temperature pockets — i.e. Nilgiris, Anamallais, Wayanad and Darjeeling — and far less common in the hot plains of Assam and Dooars.

Damage and economic impact

UPASI surveys place average south-India losses at 17-43% of crop where no protection is applied, with the Nilgiris worst affected. In Darjeeling, the disease coincides with the second-flush window and depresses the high-value muscatel crop.

Management

  • Forecasting: UPASI's sunshine-hour forecast model triggers sprays when sunshine drops below 3-4 hours per day for several consecutive days.
  • Chemical: protective sprays of copper oxychloride 210 g + nickel chloride 35 g in 40 L water per ha, alternated with systemic hexaconazole 5 EC 200 mL/ha or propiconazole 25 EC, at 7-10 day intervals through the monsoon. The Tea Board's Plant Protection Code lists permitted molecules and pre-harvest intervals to keep MRLs below export limits.
  • Cultural: harder pruning, shade thinning, wider plant spacing and removal of "jat" seedlings in favour of resistant clones reduce humidity in the canopy.
  • Host resistance: TRI 2025, TRI 2026 (Sri Lanka) and several Tocklai TV clones rate moderately tolerant; complete resistance is not available.

See also: Tea Camellia Sinensis Assamica, Tea Camellia Sinensis China Type, Tea Grey Blight Pestalotiopsis, Tea Mosquito Bug Helopeltis Tea, Tea Tv 1 Tocklai Vegetative Clone.

References

  1. Blister blight of tea — UPASI Tea Research Foundation. https://www.upasitearesearch.org/
  2. Tea Board India — Pests and Diseases Advisory. https://www.teaboard.gov.in/