Development of a Nested Quantitative Real-Time PCR for in planta Monitoring of Phytophthora cinnamomi in Persea americana

Phytophthora cinnamomi is an important oomycete which causes severe economic losses in many important tree species worldwide. It is the causal agent of Phytophthora Root Rot (PRR), one of the most devastating diseases of avocado. Control strategies for PRR rely on the use of tolerant avocado rootstocks combined with phosphite trunk injections. However, breeding for disease resistance is time consuming and often subjective as it is based on visual assessment of disease incidence and severity. In this study we developed a P. cinnamomi specific low cost assay for the in planta quantification of pathogen growth in avocado roots. This was done by making use of a nested PCR combined with real-time qPCR to enhance the sensitivity of the detection of pathogen DNA from within the plant tissues. Two rootstocks, Dusa® (highly tolerant) and R0.12 (susceptible) were infected with P. cinnamomi, root samples were collected at 0, 3, 7, 14 and 21 days, and DNA was extracted. This technique allowed the detection of P. cinnamomi in root tissue before the onset of visual disease symptoms. Results showed that the amount of P. cinnamomi in the less tolerant rootstock R0.12 was significantly higher when compared to the highly tolerant Dusa®. This assay may provide a reliable tool to assist in the screening and selection of PRR tolerant avocado rootstocks.