Rice stem rot

Taxonomy and nature of the pathogen: Sclerotium oryzae belongs to the Fungi kingdom, genus Sclerotium. It represents the asexual (anamorphic) stage of the fungus known as Magnaporthe salvinii. It is a major fungal pathogen that specifically targets rice, causing significant yield losses in warm, high-humidity rice-growing regions globally.

Diseases and host plants: The primary disease caused by this pathogen is rice stem rot. It manifests as a necrotic lesion on the leaf sheaths near the water level of the paddy field. As the infection progresses, the pathogen invades the stem, weakening the plant structure, which leads to lodging, stunted growth, and significantly reduced grain weight and quality.

Biology and lifecycle: The pathogen overwinters as sclerotia in the soil or in infected plant debris left after harvest. When the field is flooded for the new crop, these sclerotia float on the water surface and come into contact with the base of the rice plants. They germinate and penetrate the epidermal tissues, starting the primary infection cycle that spreads through the field.

Environmental conditions: The disease is favored by high temperatures, typically between 25°C and 30°C, and high humidity levels inside the canopy. Stagnant water in the paddies facilitates the dissemination of the fungal propagules. Dense planting densities and poor water management significantly increase the severity of the outbreak within a single season.

Control measures: Management of stem rot relies on an integrated approach to reduce the inoculum levels and protect the plants:

• Planting resistant rice cultivars.
• Proper water management, including periodic draining of the fields.
• Burning or incorporating crop residues to reduce the number of sclerotia.
• Avoiding excessive nitrogen application, which makes tissues more susceptible.
• Application of approved systemic fungicides during the early disease stages.