Biological stability of a strain of Cowpea severe mosaic virus over 20 years

Cowpea (Vigna unguiculata) is an important crop of the traditional agriculture system in the Northeast of Brazil. It can be infected by more than 20 virus species and Cowpea severe mosaic virus (CPSMV) is one of the most important pathogens that naturally infect cowpea in Brazil. Several CPSMV isolates were obtained and characterized in the Plant Virus Laboratory at the Federal University of Ceara: CPSMV-CE - the first characterized isolate of the virus obtained from cowpea in the State of Ceara; CPSMV-AL - isolated from cowpea in Alagoas; CPSMV-PE - isolated from cowpea in Pernambuco; CPSMV-PR - obtained from soybean (Glycine max) in Parana and CPSMV-CROT - isolated from Crotalaria paulinea, in Maranhao. An isolate of CPSMV with the property to infect the cv. Macaibo, a cowpea cultivar immune to most of CPSMV isolates was also biologically and serologically characterized as a new strain of the virus (CPSMV-MC). The CPSMV-MC was isolated in January 1990 and has been evaluated over 20 years by host range studies and maintenance in vivo by periodical mechanical inoculations in cowpea. The results of this periodical evaluation revealed that the biological integrity and the serological properties of CPSMV-MC were preserved over 20 years, indicating that the genetic preservation of a virus strain could occur over the years. Molecular studies involving part of the coat protein (CP) gene of CPSMV-MC and five other Brazilian CPSMV isolates indicated a high degree of conservation, with 92-100% nucleotide sequence identity among the isolates.