Sheath Blight

Sheath blight first appears as oval or ellipsoidal, water-soaked lesions on the leaf sheaths near the water line. The lesions enlarge and develop a greenish-grey to straw-coloured centre with an irregular brown margin, often forming a banded pattern. Under humid conditions, web-like fungal growth and small brown resting bodies (sclerotia) can be seen, and the disease climbs onto upper sheaths and leaves in dense canopies.

Rhizoctonia solani survives between crops as sclerotia in soil and as hyphae in residue. In flooded rice, sclerotia float to the water surface and lodge against the sheaths, where they germinate and infect. The fungus then grows from plant to plant by direct contact and by producing new hyphae, spreading fastest through dense, humid canopies. New sclerotia form in infected tissue and return to the soil.

• Sclerotia in soil and residue survive between crops
• Floating sclerotia lodge against sheaths and initiate infection
• The fungus spreads by contact and new growth within the canopy
• Dense, humid canopies allow the disease to climb the plant

Rice is the principal host, and the same fungus causes banded leaf and sheath blight in maize. The disease is strongly favoured by warm temperatures with high humidity, dense plant stands, and high nitrogen fertilisation, all of which create a moist, sheltered canopy microclimate that favours infection and spread.

• Warm, humid weather with prolonged canopy wetness
• Dense plant stands and close spacing
• High nitrogen fertilisation producing lush, leafy growth
• Fields with a history of the disease and abundant sclerotia

Because sheath blight thrives in dense, over-fertilised canopies, management focuses on cultural practices that open up the canopy and moderate nitrogen, supported by tolerant varieties and attention to inoculum. There is often no fully resistant variety, so integrated tactics are important.

• Apply nitrogen in balanced, split amounts to avoid over-lush canopies
• Use appropriate plant spacing and stand density to improve air movement
• Manage water to avoid conditions that keep the canopy continuously humid
• Reduce inoculum through residue management and field sanitation
• Grow more tolerant varieties where they are available