Taxonomic status: tobacco brittle fracture virus belongs to Tobravirus, unclassified. The code of ICTV is 00.072.0.0 1.004.

Virus synonyms: aster ringspot virus, belladonna mosaic virus, potato cork ringspot virus, potato stem mottle virus.

Virus purification: 10 ~ 100 mg virus was purified from the diseased leaves of Cleveland tobacco system.

① purification steps of Harrison & Nixon (1959) and Robinson (1983): storing the sick juice at -20℃, melting, low-speed centrifugation, adding 10%(W/V)PEG(6000) and 2% (w/v) to the supernatant.

② Liszt-AmpBracker (1969) method: the diseased leaves were cold-treated, and 10mmol/L(pH7.4) citric acid-phosphate buffer (containing 0. 1. 1% sodium thioacetate) was added, and homogenized.

Physical and chemical characteristics of virus:

① virus particles: baculoviruses with three lengths, namely185 ~197 nm * 20 ~ 25 nm, 78 ~14 nm * 20 ~ 25 nm and 43 ~ 55 nm * 20 ~ 25 nm, respectively. The molecular weights are 7.27* 106u (long particles) and 2.95* 106u (short particles). Sedimentation coefficient 300S (long particle), 155 ~ 243s (short particle). The isoelectric point is 4 ~ 4.5. A260/A280 is 1. 15.

② nucleic acid: the content is 5%, and it is single-stranded RNA with two genomes (RNA- 1 and RNA-2). The long and short virus particles are packaged separately, and the molecular weights are about 2.2* 106u (long particles) and 0.6 *106 ~1. G ∶ A ∶ C ∶ U = 25.5 ∶ 28.9 ∶16.7 ∶ 28.8. There are some differences among different strains, such as the RNA- 1 (molecular weight 2.2* 106u) of SYM strain is 679 1. The RNA- 1 of other strains is similar in size and composition. The molecular weight of RNA-2 is 0.6 *106 ~1.4 *106u, and PSG strain has 1905 nucleotides, g: a: c: u = 22.9%: 29.4%:/kloc. There are two subgenomes: RNA- 1A and 2a, both of which have their own virus particles. RNA- 1a is a derivative of RNA- 1, and the molecular weight of RNA- 1a of several strains is 0.54 *106 u; RNA-2a is a derivative of RNA-2. The molecular weight of RNA-2a of SYM strain is 0.6* 106u, but the molecular weight of other strains is different (Robinsonet al., 1983, 1987). RNA- 1b originates from the 3' terminal region of RNA- 1, and virus particles are not assembled (Boccara et al., 1986). Sedimentation coefficient 26s (particle b), 18s (particle m) and 12s (particle t). RNA- 1 can independently replicate and systematically infect the host, carrying the gene determinant of tobacco spot type and systematic infection (cadman &; Harrison,1959; s？ Nger, 1969).RNA-2 carries the systematic disease of Amaranthus (Kurppa et al., 198 1) and the yellow leaf of tobacco (Lister &: Bracker, 1969) as well as the genetic determinant (S? nger， 1969).

③ protein: the content is 95%, the molecular weight is about 23,800 U, the molecular weight of PSG strain is 22856u(Cornelissen et al., 1986) or the molecular weight of TCM strain is 22405 U(Angenet et al., 1986), and other strains are similar in size (Mayo &: Robinson, 1975), amino acid 2 18.

Strains: There are many strains, and only a few strains have different symptoms in recognizing the host. The most unique strains are:

(1) ①PRN strain (cadman &; Harrison,1959; Harrison company. Nixon, 1959): Potato from Scotland is now a typical strain of tobacco brittle fracture virus. Its short particle length is 78 nm.

②SYM strain (Kurppa et al., 198 1): spinach from England, which caused the distortion and necrosis of amaranth system. Its short particle length is 10 1nm.

③ Oregon strain (Liszt & ampBracker, 1969): Oregon potato, USA. A variant of it leads to yellow circular spots and lines in Cleveland tobacco and heart-leaf tobacco. Its short particle length is 8 1, 90 or 100nm.

④ ④ Italy 6.(Van Hoof et al., 1966): Tobacco seedlings from Italy, which Harrison( 1973) classified as pea early blight virus (British serotype) and Robinson et al., 1987) later classified as tobacco brittle crack according to serological relationship. Its short particle length is 105nm.

⑤N5 strain: British narcissus (Harrison et al., 1983). It caused serious necrosis of Cleveland tobacco and many host deaths (Robinson et al., 1987). According to serological relationship, it has been classified as pea early blight virus (Dutch serotype). Its short particle length is 75 nanometers.

⑥PSG and TCM strains: from potato and tulip respectively (Cornelissen et al., 1986). Traditional Chinese medicine has serum relationship with pea early blight virus (Dutch serotype).

⑦NM-type isolate: Using the long particles of any of the above strains as inoculants, NM-type isolate can be obtained, which is not easy to spread through juice and easy to inoculate its nucleic acid (Sanger &: Brandenburg, 196 1). Most of them will cause more serious host necrosis (type M) than their parents and slowly and systematically infect the host (Cadman, 1962).

Others: Tobacco brittle fracture virus can be distinguished from other viruses by its special particle shape, its ability to recognize host symptoms and its ability to produce NM-type isolates. CAM strain and other South American isolates were first classified as serotype-Ⅲ of tobacco brittle fracture virus, and now they are considered as some isolates of pepper ringspot virus (Robinson &; Harrison, 1985a). Pepper ringspot virus and tobacco brittle fracture virus are only distant sera, and there is no cross reaction with RNA- 1 probe hybridization test (Robinson &: Harrison, 1985a). The difference between pea early blight virus and tobacco brittle fracture virus is that tobacco brittle fracture virus does not infect peas, does not systematically infect kidney beans, and only causes local spots of needle tip size in kidney beans. The RNA- 1 probe hybridization test of pea early blight virus and tobacco brittle fracture virus did not react. M isolates of tobacco brittle fracture virus can only be identified by serum because of their great differences in antigens, but all isolates can be identified by RNA- 1 probe hybridization test (Robinson &: Harrison,1985a; Robinson et al.,1987). the nm isolate can also be identified by RNA- 1 probe hybridization test, and the m isolate can be obtained by adding short particles or RNA-2 to the inoculum (Harrison &: Robinson, 1982).