When a plant (marine or terrestrial) senses a threat, it reacts through a process called basal resistance. Basically, it fortifies the cell wall to make it more resistant. This is a response to both pathogens and non-pathogenic threats. If the pathogen can still make it through, then the plant undergoes 'hypersensitive response' otherwise known as cell suicide. The cell infected with the pathogen and the cells around the infected cells will die. The plant purposely kills a bit of itself in order to keep the whole plant alive. If the pathogen still manages to sneak past that defense, the plant has one of two options. The first option works only on viruses and is called RNA silencing. The plant literally eats the RNA of the virus so there is nothing left to infect the plant. However, since not all pathogens are viruses, the plant can also undergo 'systematic acquired resistance' (SAR). SAR causes the plant to become more immune to all diseases for a long period of time. Scientists have figured out how to generate SAR in agricultural plants so that they will be resistant to diseases.
A plant goes through a very different process when it senses herbivory. Bugs and other herbivores release saliva when they eat plants. This saliva triggers a response in the plants to release 'volatile organic compounds' (VOC). The VOCs can do one of two things. They can either repel the herbivores through making the plant bitter tasting or cause it to have a bad odor. The other way VOCs protect against herbivory is by attracting predators of herbivores. The predators will then eat the herbivores and the plant will not be eaten anymore (hopefully).
Although in my experiment I will not be able to measure VOC levels or whether the eelgrass is exhibiting SAR, it is helpful to know some of the reasons why the eelgrass did not get infected. This information can also help direct future research questions about the mechanisms of infection for eelgrass.