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.