Rice stem rot

Initial symptoms appear as small, dark, irregular spots on the leaf sheaths that are in contact with the water surface in the rice paddy.

As the disease progresses, the infection penetrates the stem, causing internal decay and necrosis of the leaf sheaths and the internodes.

A diagnostic feature is the presence of dark, shiny sclerotia embedded within the decayed tissues of the stem when split open.

Affected plants often exhibit lodging (falling over) because the structural support of the stems is severely weakened by the fungal colonization.

In later stages, the panicles may become blank or poorly filled, and the entire plant may show premature yellowing and death due to vascular blockage.

The causal agent of rice stem rot is the fungus Sclerotium oryzae (anamorph of Magnaporthe salvinii). It is characterized by the production of numerous small, black, spherical sclerotia.

These sclerotia serve as the primary inoculum, capable of surviving for several years in soil or within infected plant debris until conditions become favorable.

When rice paddies are flooded, sclerotia float on the water surface and come into contact with the rice culms, initiating the infection process at the base of the plant.

Beyond rice, this pathogen has a broad host range, affecting crops such as peanut, papaya, poinsettia, common bean, potato, and wild rice (Zizania).

Once inside the host tissue, the fungus secretes enzymes that degrade cell walls, leading to the collapse of the structural integrity of the rice stem.

The development of stem rot is highly dependent on environmental conditions, particularly the presence of standing water in the fields for extended periods.

Temperatures ranging from 25°C to 30°C are optimal for the rapid germination of sclerotia and the subsequent spread of fungal mycelium.

High nitrogen application creates lush, dense canopies that increase humidity around the base of the plant, favoring the infection process.

Poor water management, such as stagnant water in the fields, provides a continuous pathway for the movement of inoculum between plants.

Continuous cropping of rice on the same land allows for the buildup of a significant population of sclerotia in the topsoil, increasing disease pressure over time.

Stem rot causes severe reductions in grain yield by interfering with the plant's ability to transport nutrients to the developing panicles.

Lodging leads to increased harvesting costs and significant losses as the grain may germinate or rot on the ground if not harvested immediately.

The quality of the harvested rice is significantly degraded, as infected plants produce light, chalky, and often broken grains with low milling yields.

Premature plant death forces a reduction in the grain-filling period, leading to lighter grains and overall lower biomass production for the field.

The persistent nature of the pathogen in the soil results in long-term economic impacts, requiring farmers to implement intensive crop management strategies.

Effective management begins with strict adherence to crop rotation, which prevents the continuous buildup of the pathogen in the soil profile.

Adjusting irrigation schedules by drying the fields at specific growth stages helps to reduce the survival of the fungus and limits its ability to spread.

Deep plowing after harvest is essential to bury crop residues deep in the soil, preventing the sclerotia from floating to the surface in the next season.

• Using resistant or tolerant rice varieties to naturally limit the impact of the pathogen.
• Applying balanced fertilization, focusing on adequate potassium to strengthen plant tissues.
• Utilizing targeted fungicide applications during the early stages of disease appearance.

Proper field sanitation, including the removal of weeds and debris, is crucial for reducing the overwintering inoculum and ensuring a healthy crop development.