Thanatephorus cucumeris (Rice)

Scientific Name Thanatephorus cucumeris (Frank) Donk [teleom.], Rhizoctonia solani (Kühn), AG1-IA [anam.]
Synonyms Corticium solani (Prillieux & Delacroix) Bourdot & Galzin [syn.], Pellicularia sasakii Ito [syn.]
Common Names English: Rice sheath blight; German: Netzfäule; Spanish: Tizón de la vaina
Description Rhizoctonia solani is a member of the Agonomycetales (Deuteromycetes), sexual state Thanatephorus cucumeris (basidiomycetes).

R. solani is considered to be an aggregate species including different (host-specific) entities which may be differentiated by anastomosis grouping (AG); R. solani AG1-IA causes sheath blight of rice.

Biology

Damage

Symptoms develop on leaf sheaths and leaf blades when the rice crop reaches its full vegetative growth. Typical lesions are ellipsoid, yellow to greenish-grey, 10 - 30 mm in length. Later the margins of the spots become brown, and the centre changes to grayish-white. The size and color of the lesions depend on the age of lesions and environmental conditions. When several lesions coalesce, parts or whole leaves become blighted. Sclerotia are formed superficially on or near lesions on leaves, stems and seeds. At first sclerotia are white and change to brown to black during ripening; they are easily dislodged from plants when mature. Sheath blight symptoms may be confused with other diseases like sheath spot (Rhizoctonia oryzae) and aggravated sheath spot (R. oryzae-sativae) which exhibit similar lesions.

Lifecycle

When fields are flooded during land preparation for rice, the sclerotia float to the water surface and attach to plant surfaces. Sclerotia germinate, produce an infection cushion, and hyphae penetrate the plant tissues directly or through stomata. The mycelia rapidly grow inside the host tissues, and water-soaked lesions become visible within one to two days. Subsequently, mycelia grow inside and outside the plant tissue enabling the pathogen to spread vertically (on the infected plant) and horizontally infecting neighbouring plants.

T. cucumeris forms hyaline, multinucleate hyphae (Ø 8 - 12 µm) with a prominent dolipore septum. Young hyphae typically branch at 90 ° angles, with constrictions at the point of origin of hyphal branches. The formation of sclerotia is stimulated by a sudden change in temperature or by brief flooding. They are irregularly hemispherical, 1 - 6 mm in diameter, but may coalesce to form larger structures. The sclerotia are extremely resistant and may remain infective for up to 21 months in dry soil.

Cause

The fungus is present in most soils, surviving as actively growing mycelium, resting mycelium or sclerotia. Survival in soil is influenced by soil factors and by soil micro-organisms. The fungus spreads by soil movement with water, and on contaminated tools and plant parts. Basidiospores are only rarely produced and are considered to be of minor importance in epidemiology.

Occurrence

Additional Crop Information

Rice is a very important host plant; however, the pathogen may attack also numerous vegetable plants such as potatoes, legumes, soybeans, sorghum, barley, wheat and corn.

Control

Integrated Crop Management

The use of high-yielding varieties, high input of nitrogen fertilizer and inadequate water management promote the development of sheath blight. Disease severity is often higher in rotation systems (e.g. with wheat) than in continuous rice cropping. Simulation models for the prediction and management of sheath blight have been developed. Removal of weeds and of infected stubbles reduces the inoculum potential. In rice no significant differences in host plant resistance to T. cucumeris have been detected. Biological control is not applicable in the field.

Chemical Control

Several highly specific sheath blight fungicides are effective against T. cucumeris. Foliar sprays with validamycin A and pencycuron have been widely used. Fungicides are applied after the occurrence of first symptoms on the third to fourth leaf from the top.
Among the broad spectrum fungicides strobilurines (= QoI fungicides) such as metominostrobin show good efficacy and are partly applied systemically via nursery box granular treatment beside the normal spray treatment. Nursery box granular applications is also possible for some newer systemic representatives of the complex II inhibitors (e.g. thifluzamide and furametpyr) which are effective as well with submerged as well as with foliar treatments.
In some countries also benzimidazoles (MBC fungicides) and DMI fungicides are partly used for sheath blight control.

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