Thanatephorus sasakii

Systematic position and pathogen type: Thanatephorus sasakii (the teleomorph of Rhizoctonia solani) is a basidiomycetous fungus classified within the Ceratobasidiaceae family. It is a necrotrophic soil-borne pathogen that persists in the environment primarily as sclerotia or mycelia embedded in crop debris, allowing it to survive for extended periods without a host.

Diseases and host crops: The fungus is predominantly known for causing Sheath Blight in rice, one of the most economically devastating diseases globally. However, it is polyphagous and affects a wide range of monocotyledonous and dicotyledonous plants, including soybeans, corn, and various weeds, which serve as alternative reservoirs for the inoculum.

Biology and life cycle: The pathogen's infection cycle begins when sclerotia float on the water surface and adhere to rice sheaths. Once environmental conditions are favorable, hyphae emerge, colonize the plant tissue, and form dense mycelial mats. New sclerotia are produced on the surface of infected lesions, which then drop back into the soil or water, preparing the fungus for the next infection cycle.

Developmental conditions and impact: Disease outbreaks are highly correlated with high humidity (above 90%) and temperatures ranging from +25°C to +30°C. The damage is severe: the pathogen obstructs nutrient transport, causing the leaves to wither and the grains to fill poorly. If left unmanaged, sheath blight can lead to complete lodging of the crop and harvest losses reaching 50% or more.

Control and management measures: Effective management relies on an integrated approach, including:

• Removal of infected crop residues to reduce primary inoculum.
• Optimizing nitrogen fertilizer application to prevent excessive canopy density.
• Use of certified seeds and resistant cultivars.
• Application of systemic fungicides from the triazole or strobilurin classes at the onset of symptoms.