The avocado (Persea americana Mill.) is an economically important crop worldwide. The most important disease of avocado is Phytophthora root rot, which is caused by Phytophthora cinnamomi Rands. Currently, phosphonate trunk injections provide satisfactory disease control; however, the possibility of reduced sensitivity and eventually resistance to this fungicide is lurking on the horizon. Furthermore, consumer demands for “organic” fruit has been increasing over the past decade, emphasising the need to use root rot-resistant rootstocks. Due to a lack of understanding of the interaction between these two organisms, screening for specific resistant mechanisms is not possible and consequently only partially resistant rootstocks are currently commercially available. The aim of this thesis was therefore to address this need by investigating phenotypic traits in avocado rootstocks that could play a role in resistance against P. cinnamomi.