Phytophthora cinnamomi causes Phytophthora root rot (PRR) in avocado (Persea americana), an
important disease that causes severe economic losses to the avocado industry globally. To date,no PRR-resistant avocado rootstock variety has been discovered, although certain rootstock varieties have been shown to be more tolerant than others. In this study we developed an accurate, low cost assay for in planta quantification of P. cinnamomi to evaluate disease tolerance. A nested real time PCR assay was developed to sensitively detect pathogen DNA in plant tissues. Root samples from a highly tolerant (Dusa®) and less tolerant (R0.12) rootstock were collected at 0, 3, 7, 14 and 21 days after inoculation with P. cinnamomi and used for pathogen quantification. Nested primers developed in this study were specific and sensitive and could detect P. cinnamomi in root tissues. The amount of P. cinnamomi quantified in roots was
significantly higher in the less tolerant R0.12 plants when compared to the highly tolerant Dusa® plants at all time points. This study has confirmed the known status of disease tolerance of Dusa® and R0.12 avocado rootstocks in a quantitative manner and provides a reliable molecular tool to assist with industry breeding programs for the selection of PRR-resistant avocado rootstock varieties.