What are the control methods for rice kernel smut?

Symptoms Also known as smut, rice dark smut, black kernel smut, etc. It is distributed in the Yangtze River Basin and areas to the south in China, mainly occurring from the flowering stage to the milk-ripe stage of rice, and only infecting the grains. Each panicle may have 1, several, or even dozens of infected grains, with symptoms generally appearing when the rice is near maturity. Infected rice grains turn a dirty green or dirty yellow color, containing black powdery material inside. At maturity, the abdomen splits open, exposing the black powder. Between the inner and outer glumes of the diseased grain there is a black tongue-shaped protrusion, and a black liquid often oozes out, contaminating the surface of the grain. When the diseased grain is opened, the seed can be seen to be partially or entirely transformed into black powdery material, which is the chlamydospore of the pathogen.

Pathogen Tilletia barclayana (Bref.) Sacc. et Syd., known as the pencil grass smut fungus, belongs to the Basidiomycotina fungi. Synonyms: Neovossia horrida (Tak.) Padw. et Azmat Kahn. The spore sori form in the host ovary, enclosed by the glumes, with some spikelets destroyed, producing black powder. Chlamydospores are spherical to broadly oval or elliptical, black, measuring 25-32 x 23-30 (μm), with densely covered tooth-like protrusions on the surface, colorless to nearly colorless, with pointed tips and polygonal slightly curved bases, height 2.5-4 (μm); basidiospores are filamentous, colorless and non-septate, measuring 38-55 x 1.8 (μm); secondary sporidia are sausage-shaped, measuring 10-14 x 2 (μm).

Transmission routes and disease conditions The pathogen overwinters as chlamydospores in seeds and in soil. Seed-borne infection entering rice fields with sowing and soil-borne infection are the main sources of inoculum. The chlamydospores of this fungus have strong resistance, surviving 1 year under natural conditions, 3 years on stored seeds, and still surviving after soaking in constant-temperature water at 55°C for 10 minutes. After passing through the digestive tract of poultry and livestock, the pathogen can still germinate. The fungus requires a dormancy period of more than 5 months. When the temperature is above 20°C, with high humidity and good ventilation and light, the chlamydospores germinate, producing basidiospores and secondary sporidia. These are spread by air currents to rice panicles at the heading and flowering stage, invading the floral organs or young seeds, and reproducing within the grains to produce chlamydospores. The period from booting to heading and flowering of rice, and in hybrid rice seed production fields where the flowering periods of the parental lines do not coincide well, result in higher disease incidence rates and more severe disease. In addition, excessive rain or high humidity and excessive application of nitrogen fertilizer can also aggravate the disease. Among different combinations in hybrid seed production, there exists a phenomenon where the inner and outer glumes of the female parent ultimately cannot close, called glume opening. Combinations with high glume-opening rates, such as Xianyou 63, with a glume-opening rate as high as 30%-40%, have high disease incidence. There is considerable variation in disease incidence among varieties. In the first-year seed production field of Shanyou 63, the diseased panicle rate reached 92%, and the diseased grain rate was 25.19%.

Control methods (1) Implement quarantine to strictly prevent the introduction of infected rice seeds into disease-free areas. (2) Pay attention to clarifying whether soil-borne or seed-borne infection is the primary factor in local old seed production fields. In areas where seed-borne infection is predominant, seed selection with 10% salt water must be carried out before sowing to remove diseased grains, followed by seed disinfection. For disinfection methods, refer to rice blast. (3) Implement crop rotation of 2 years or more. In diseased areas, poultry and livestock manure should be composted and decomposed before application to prevent transmission through soil and manure. (4) Strengthen cultivation management, avoid excessive or one-sided application of nitrogen fertilizer. In seed production fields, regulate the uniformity of seedling emergence through transplanting density and seedling age to ensure flowering periods coincide. Spraying gibberellin in the late booting stage can reduce disease incidence. (5) In hybrid seed production fields or fields planted with susceptible varieties, in areas or years with severe disease, spray once each at the late peak of full flowering and at the beginning of heading with 14% Heiluijing

suspension concentrate at 2400 times dilution or Miehei No. 1 suspension concentrate at 250 times dilution. In mild disease years, spraying once at the late peak of full flowering is sufficient. In addition, 30% Baike emulsifiable concentrate at 75 ml per 667 sq m, or 50% wettable powder at 50 g, 40% Miebingwei suspension concentrate at 200 ml, or 25% triadimefon wettable powder at 50 g can be selected, mixed with 50 L of water for spraying. When using triadimefon, avoid the flowering period and apply in the afternoon to prevent phytotoxicity. Furthermore, spraying 25% propiconazole emulsifiable concentrate at 2000 times dilution during the panicle stage of rice can effectively control this disease and also simultaneously control sheath blight, rice false smut, and sheath rot. (6) Select resistant varieties. In hybrid rice combinations, choose varieties with closed glumes to reduce disease incidence.