Sclerotinia rot of rice
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September 05 05:14:50, 2025
Rice sclerotium stalk rot is primarily caused by *P. ovale* and *P. sclerotiorum*. These two pathogens, either individually or in combination, are also referred to as "sclerotium neck" or "stem rot." They are commonly known as rice brown neck disease, rice blastomycosis, rice gray sclerotia, and similar terms, collectively called rice sclerotinia or stem rot. The disease occurs throughout all rice-growing regions in China, but its prevalence varies by region. In the southern part of the Yangtze River Basin, *Sclerotium rolfsii* and *Sclerotium microsclerotium* are more prevalent. The symptoms of these two pathogens are similar, affecting the lower leaf sheaths and stems of rice plants.
Initially, small brown spots appear on the leaf sheaths near the water surface, gradually expanding into black longitudinal necrotic lines and large black spots. A mold layer often forms inside the infected sheath, containing mycelium. In contrast, *Sclerotinia* does not produce mycelium, and the black lines are shallower. As the lesions expand, the base of the stem turns black and soft, becoming grayish or reddish-brown and eventually rotting. The stalks may peel away, revealing pale white mycelium and dark brown sclerotia inside. If the panicle neck is infected, the ear may dry up.
In the case of brown sclerotinia disease, the leaf sheaths turn yellow and die without forming clear lesions. When the booting stage is affected, young ears may fail to emerge. Later, spherical black micro-sclerotia form in the sheath tissue. For gray sclerotinia, reddish-brown spots appear on the flag leaf sheath, forming long streaks. This type generally does not cause lodging, but gray-brown granular sclerotia develop on the lesion surface and within the tissue.
Another pathogen, *Helminthosporium sigmoideum var. irregulare*, was previously classified under *Aspergillus oryzae*, a fungus belonging to the subphylum *Aspergillus spp.* It is a variant of *Escherichia coli* and lacks a sexual stage. Conidial stems form on diseased tissues or on the surface of sclerotia, either singly or in clusters. The conidia are spindle-shaped, curved, or "S"-shaped, with 3–4 septa, measuring 50–748–12 μm. They have tendril-like filaments on the top cells. Sclerotia are about 0.15 mm in size, dark olive in color.
Other species include *H. sigmoideum Cav.*, which is similar to *N. sigmoidea*, and has characteristics of *Magnaporthe salvinii*. It is known as *Ascosphere* fungus, but it has not been found in China yet. Spore stems are dark brown, unbranched, with crescent-shaped conidia, ranging from 41–63 × 11–15 μm, with 0–4 septa. More than three septa are common, with the central two cells dark brown and the ends pale. Sclerotia are spherical, about 0.25 mm in diameter, with two layers—outer black, inner brown.
*Sclerotium oryzae-sativae Saw* is another species, known as the brown fungus. Its sclerotia are small, spherical, oval, or cylindrical, dark brown, 0.3–2 mm in size, with a rough surface and no distinction between inner and outer layers. *Sclerotium hydrophilum Sacc.* is a bacterium that forms spherical, brown to black sclerotia, 0.25–0.68 mm in size, with two layers, and is commonly found in leaf sheath groups, though less frequently.
The transmission of the disease mainly involves *Pediococcus sclerotiorum* and *S. sclerotiorum*. Sclerotia survive for many years in rice straw or soil. During irrigation, they float in water and adhere to the base of the field or leaf sheaths. Under suitable conditions (around 17°C), the sclerotia germinate and send hyphae into the leaf sheaths, forming new sclerotia in the stem and sheath. Sometimes, a light gray mold layer appears on the lesion surface, which contains the pathogen’s conidia. These conidia can be spread through air or insects, but the main infection route is short-distance re-infection.
The number of sclerotia directly affects the incidence of the disease the following year. The temperature range for pathogen development is 11–35°C, with an optimal range of 25–30°C. Rainy days with limited sunlight are unfavorable for the disease. Poorly drained fields with excessive or delayed irrigation during the mid-season increase disease severity. Overuse of nitrogen fertilizer, late planting, or weak rice plants can also contribute to the problem. Single-season rice is more susceptible than early rice. Tall varieties are more resistant than short ones, and japonica rice tends to be more resistant than indica rice. Disease is more likely to occur after heading, especially when insect damage creates wounds.