This is the VOA Special English Agriculture Report.

　　The maize streak virus causes one of the most destructive crop diseases in Africa. Not only is it economically damaging, it can also threaten food security. Maize crop failures lead to hunger in some areas. Maize streak disease also strikes parts of Asia.

　　Small insects known as leafhoppers carry the virus from field to field. Not much food can come from a field badly infected with the virus. The maize plants cannot produce a healthy cob. The disease gets its name from the streaks of yellow and white that discolor the leaves.

　　There are many less harmful kinds of maize streak virus. These cause minor infections in crops like sugarcane and wheat. But only one kind causes the most severe form of maize streak disease.

　　Researchers have been working to understand how this form of the virus got started and spread. They studied eighty versions of maize streak virus. They found that the severe kind may have formed from a recombination of less harmful grass viruses. In the process of recombination, the parent viruses exchanged a gene.

　　The study compared the genome of the maize streak virus with those of ten related but less harmful viruses. A genome contains all the information about an organism, including information carried from its parent organisms.

　　The most severe virus may have formed more than a century ago, when two mostly harmless viruses that infect wild grass combined.

　　Darren Martin of the University of Cape Town in South Africa led the research. He says the researchers found that every maize streak virus that severely affects maize came from an ancestral virus. That ancestral virus was the recombinant result of the two comparatively harmless viruses.

　　But the virus they formed was stronger. It could infect more plants than its parents. It could live through winters in wild grasses better than its relations. The researchers believe that this quality also made the severe form of maize streak virus spread faster.

　　Scientists from several parts of the world took part in the study, which appeared in the Journal of General Virology. The research continues. The goal is to find more biological information that could help lead to a cure.

　　And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. For more reports -- with transcripts, MP3s and podcasts -- go to voaspecialenglish.com. I'm Steve Ember.