Stem rot of rice

The fungus Helminthosporium sigmoideum, also known as the perfect stage Magnaporthe salvinii, is the causal agent of stem rot in rice. This disease is a major constraint in rice production worldwide, significantly affecting the vascular integrity of the plant stem.

Rice is the primary host for this pathogen. The disease is most prevalent in intensive rice-growing regions where flooded conditions and warm temperatures persist throughout the growing season, facilitating the survival and transmission of the fungal sclerotia.

Symptoms typically appear during the tillering stage. Initial signs include dark, irregular lesions on the leaf sheaths near the waterline. As the disease progresses, the mycelium penetrates the stem, causing tissue necrosis and the formation of tiny, black, carbon-like sclerotia inside the stem pith, which weakens the stalk structurally.

Environmental conditions play a critical role in the development of stem rot. High humidity levels combined with temperatures between 25°C and 30°C favor rapid fungal spread. The sclerotia, which act as the primary inoculum, survive in the soil and on crop debris, floating to the water surface when paddies are irrigated to initiate new infections.

The economic impact is severe due to lodging, reduced grain filling, and increased percentages of empty or chalky grains. Management strategies involve a combination of approaches:

• planting resistant or tolerant varieties
• optimizing nitrogen fertilization to avoid excessive vegetative growth
• proper drainage management during the cropping season
• applying fungicides during the early stages of disease progression