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Infected sheath (IRRI)
Rhizoctonia solani Kunh (anamorph), Thanatephorus cucumeris (Frank) Donk (teleomorph)
Bigger lesions (Suparyono, RIR)
The disease is easily distinguished by the irregular lesions, which are initially water-soaked to greenish gray and later becomes grayish white with brown margin. These lesions are usually found on the leaf sheaths near the waterline and on the leaves. The disease can be confirmed by the presence of sclerotia. Sclerotia and mycelia may be produced on the lesions. Sclerotia are compact masses of mycelia, which are irregular, hemispherical, flattened at the bottom, white when young, and turn brown or dark brown when mature.
Lesions on the stem are sometimes confused with those caused by stem rot. Lesions on the stem resulting from stemborer infestation can be sometimes confused with sheath blight lesions.
The disease is soilborne. It usually starts at the base of the plant near the water level. Later, the symptoms are observed on the upper leaf sheath and on the leaf blade. The disease usually infects the plant at late tillering or early internode elongation growth stages. Disease may spread from one hill to another through leaf-to-leaf or leaf-to-sheath contacts.
It is commonly assumed that the critical factors for disease development are relative humidity and temperature. Relative humidity ranging from 96 to 100% and temperature ranging from 28-32°C have been reported to favor the disease. High supply of nitrogen fertilizer, and growing of high-yielding, high-tillering, nitrogen-responsive improved varieties favor the development of the disease. High leaf wetness and high frequency of tissue contacts among plants also favor the disease.
The pathogen can be spread through irrigation water and by movement of soil and infected crop residues during land preparation.
The young mycelium of the fungus is colorless. With age, it turns yellowish to brown and measures 8-12 µm in diameter with infrequent septations. There are three types of mycelium produced: runner hyphae, lobate hyphae, and monilioid cells. The runner hyphae have thick, parallel walls and spread rapidly over the sheath and leaf surfaces of the rice plant. The runner hyphae give rise to lobate hyphae or appressoria. Monilioid cells are short, broad cells involved in the formation of sclerotia.
Sclerotia consist of compact masses of mycelia. They are irregular, hemispherical, flattened at the bottom, white when young, and turn brown or dark brown when older. Individual sclerotia are 1-6 mm in diameter. They may unite to form a larger mass. Large sclerotia are significantly more virulent than smaller ones.
Aside from the rice plant, the disease survives in citrus, cruciferous vegetables, legumes, cucurbits, Arachis hypogaea L. (groundnut), Capsicum annum L. (chilli), Daucus carota L. (carrot), Glycine max (soyabean), Gossypium sp. (cotton), Hordeum vulgare (barley), Lactuca sativa L. (lettuce), Lycopersicon esculentum Mill. (tomato), Sorghum bicolor (sorghum), Triticum sp. (wheat), Tulipa sp. (tulips), and Zea mays L. (maize).
The sclerotia germinate and initiate infection once they get in contact with the rice plant.
The fungus penetrates through the cuticle or the stomatal slit. Infection pegs are formed from each lobe of the lobate appressorium of infection cushion. The mycelium grows from the outer surface of the sheath going through the sheath edge and finally through the inner surface. Primary lesions are formed while the mycelium grows rapidly on the surface of the plant tissue and inside its tissue. It proceeds upwards and laterally to initiate formation of the secondary lesions.
The disease starts during the maximum growth stage of the rice crop. Under favorable conditions, the disease increases as the plant grows older. The damage caused by the disease depends on the infection of the plants at plant growth stages.
Sheath blight is considered to be an important disease next to rice blast.
Rice sheath blight is an increasing concern for rice production especially in intensified production systems. In Japan, the disease caused a yield loss of as high as 20% and affected about 120,000-190,000 hectares. A yield loss of 25% was reported if the flag leaves are infected. In the United States, a yield loss of 50% was reported when susceptible cultivars were planted. Studies at IRRI showed that sheath blight causes a yield loss of 6% in tropical Asia.
Seeding rate or plant spacing should be optimized to avoid closer plant spacing or dense crop growth which favors the horizontal spread of the disease. Need-based or real-time application of nitrogen fertilizer is recommended in fields known to have a high amount of inoculum.
Sanitation, specifically removing of weeds, can help control sheath blight considering that the pathogen also attacks weeds which are commonly found in rice fields. Removal of infected stubbles or crop residues from the field is also recommended to reduce the amount of inoculum for the succeeding cropping season.
Spraying infected plants with fungicides, such as benomyl and iprodione, and antibiotics, such as validamycin and polyoxin, is effective against the disease.
Suparyono, JLA Catindig, NP Castilla, and FA Elazequi