Plant-pathogen interactions are an important knowledge domain within plant biotechnology, both scientifically and in economic terms. Unlike other knowledge domains within life sciences, however, semantic technologies have not been used extensively to codify it, therefore, there is a lack of axiomatic models amenable to automated integration and inference. We present the Plant-Pathogen Interactions Ontology (PPIO), a first step towards the axiomisation of plant-pathogen interactions that results in a model that encourages consistent annotation and supports both query and inference.