Question
i have a fusarium oxisporum caused disease on cactus and I like to find its fsp . could you please tell me how i do it

Answer
I assume by fsp you mean formae specialis.  Most Fusarium species have another separation of species usually based on the host on which they are found.  The link below is one of those formae specialis on a barrel cactus.  If you know the exact host name you can google search for fusarium species on that host.  See this link:  

http://www.apsnet.org/online/Archive/PDCoverImages/2004v88/feb_2.asp