inoculum

1. The material that initiates disease in a previously uninfected plant. Under artificial conditions this may be a suspension of spores, which is sprayed onto the plant. In nature the inoculum is  carried  to  a  healthy plant  from  its source,   e.g.   an  infected  plant,   by  an agent, such as wind or an insect.

2.  In microbiology or tissue-culture work, the cells that are introduced into a sterile medium to initiate a culture.