False smut / Lakshmi disease

Causal organism: Ustilaginoidea virens (Sexual stage: Villosiclava virens)

Symptoms:

• The fungus transforms individual ovaries/grains into greenish spore balls of velvety appearance.
• Few to several spikelets in a panicle are affected.

Pathogen:

• Sclerotia present in soil germinate and produce ascospores. They serve as primary source of inoculum.
• Chlamydospores are formed in the infected grains as spore balls which are spherical to elliptical, warty and olivaceous.
• Smut spores are air borne and responsible for secondary spread.

• Rainfall and cloudy weather during flowering and maturity, high soil nitrogen.
• Some of the green spore balls develop one to four sclerotia in the center.
• These sclerotia overwinter in the field and produce stalked stromata the following summer or autumn.
• In temperate regions, the fungus survives the winter by means of sclerotia as well as chlamydospores.
• The primary infections are initiated mainly by the ascospores produced from the sclerotia.
•  Chlamydospores play an important role in secondary infection, which is a major part of the disease cycle.

Management:

Cultural methods:

• Destruction of straw and stubble from infected plants.
• Avoid field activities when the plants are wet.
• At the time of harvesting, diseased plants should be removed and destroyed so that sclerotia do not fall in the field.
• This will reduce primary inoculum for the next crop.
•  Field bunds and irrigation channels should be kept clean to eliminate alternate hosts.
• Excess application of nitrogenous fertilizer should be avoided.
• Regular monitoring of disease incidence during rabi season is very essential.

Preventive methods

• Use of disease-free seeds that are selected from healthy crop.
• Split application of nitrogen is recommended.
• Removal and proper disposal of infected plant debris.

Chemical methods:

• Seed treatment with carbendazim 2.0g/kg of seeds.
• Spraying of Hexaconazole @ 100ml or Chlorothalonil 100ml or copper oxychloride @ 250g or Propiconazole @ 100 ml/ac at boot leaf and milky stages.

Source of information:

• AICRIP rice, TNAU, Coimbatore, Rice diseases- online resource, IRRI, Phillippines.

Acknowledgements:

• IRRI, Phillippines