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False smut in paddy

Rice false smut is a panicle disease caused by the fungus Ustilaginoidea virens. The pathogen replaces individual rice grains with yellow-green spore balls (smut balls) that gradually enlarge and turn olive-black at maturity.

Identification and symptoms

The diagnostic sign is the spore ball itself - initially yellow-green and velvety, eventually olive-black and powdery - occurring scattered across panicles rather than uniformly. A single panicle usually carries only a few smut balls, but contamination of grain with the spore mass affects head rice recovery, market quality and food safety because the pathogen produces mycotoxins (ustiloxins).

Host crops and life cycle

The fungus infects rice spikelets at booting and flowering. Inoculum survives on smut balls left on the soil and on contaminated seed. Continuous wet weather, dense canopies, late luxury nitrogen and prolonged dew at heading favour outbreaks. Susceptibility differs strongly by variety and many modern high-yielding lines are more vulnerable than older landraces.

Damage and economic impact

Beyond direct grain loss, the disease lowers head rice recovery and grade because broken and discoloured grains have to be screened out. Mycotoxin contamination is an additional food-safety and trade concern, and infected lots are often rejected by seed companies.

Management

  • Variety: avoid known highly susceptible lines in disease-prone tracts.
  • Cultural: balanced nitrogen, avoid late top-dressing close to flowering (Paddy Panicle Stage Management); destroy smut balls in stubble.
  • Chemical: two sprays of propiconazole (Tilt 25 EC) - first at 10% panicle emergence (booting/heading), second at 50% panicle emergence - have shown 70-74% disease reduction in field trials. Azoxystrobin and hexaconazole are alternative actives.
  • Resistance management: field-resistant isolates of U. virens to propiconazole have been documented, so FRAC-group rotation across seasons is recommended.

See also: Paddy Blast Disease, Bacterial Leaf Blight Paddy, Brown Spot Paddy, Sheath Blight Paddy, Paddy Panicle Stage Management.

References

  1. Evaluation of fungicides to manage rice false smut. Heliyon (PMC).
  2. Management of False Smut Disease of Rice: A Review. IntechOpen.