Symptoms are distributed in Henan, Shandong, Yunnan, Guizhou, Hubei, Anhui, and Taiwan. From the seedling stage to the flowering period can occur. In the early stage of disease, the edges of the leaves are yellow, and the midribs extend backward. The severe veins turn yellow and the lateral veins remain dark green and reticular. Only the midribs of the leaves are elongated to form narrow sword blades. The growth of the top of the plant was inhibited, showing dwarf or arborescence, and the roots often became thicker and shorter. The lower leaves of the plant sometimes turn yellow.

The pathogen Bacillus cereus Frankland and Frankland called Bacillus subtilis. Rod-shaped bacteria, spores, Gram-positive, size 3.0-5.01.0-1.2 (um). Produce smooth or rough cells on the medium. The smooth bacterial flagella have multiple roots and can be active; rough bacteria can not move much. After stimulating tobacco with the culture fluid of the bacteria, typical flag leaf symptoms can be produced. The bacteria in the soil in the rhizosphere of the disease plant a large number. Recent studies have found that isoleucine is the direct cause of flag leaf symptoms in the disease, and the accumulation of isoleucine and toxins secreted by bacteria disrupt the host's normal nitrogen metabolism.

Transmission routes and pathogenic conditions Pathogenic bacteria can survive in the soil for long periods of time and generally do not cause disease. Only when the bacteria secrete toxins to destroy the normal metabolism of the host and cause isoleucine accumulation to reach a certain amount can cause tobacco to form flag leaf symptoms. Most people believe that soils are susceptible to moisture, poor ventilation, poor drainage or soil salinization and nitrogen deficiency. The soil temperature is high, and the incidence is above 35°C. Soil temperature below 21 °C symptoms are not obvious. Poor soil structure, rough land preparation, poor drainage, or early hatchery tobacco fields are prone to disease.

Prevention methods (1) Increase organic fertilizer, improve soil, improve soil physicochemical properties, increase soil drainage capacity, prevent stagnant water in tobacco fields, meet the need for tobacco growth, and nitrogen fertilizer application after the onset can reduce symptoms. (2) Strengthen field management. Drought irrigation and top dressing in a timely manner. (3) Applying 2kg of sulfur per 667m2 in autumn tillage will reduce the incidence of the disease in the next year.


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