Temporal colonization of macadamia tree roots by Phytophthora cinnamomi based on a newly developed quantitative real-time PCR assay

Phytophthora cinnamomi is an important soilborne pathogen causing root rot and stem cankers on macadamia in South Africa and other countries. It is unknown whether a temporal pattern exists in the colonization of macadamia tree roots by P. cinnamomi. To investigate this, three macadamia orchards were studied bimonthly for 2 years. The study also examined whether two types of feeder roots contained varying levels of P. cinnamomi DNA. A new sensitive P. cinnamomi-specific quantitative real-time PCR (qPCR) assay was used to conduct the assessments. Only symptomatic trees were studied in two of the orchards (ZC and ZD), while symptomatic and asymptomatic trees were assessed in the third orchard (ZA). In the ZC and ZD orchards, white non-suberized feeder roots from a new root flush had higher levels of P. cinnamomi DNA than suberized feeder roots. In orchard ZD, autumn and winter yielded the highest P. cinnamomi root quantities in suberized roots. The symptomatic and asymptomatic trees in orchard ZA generally contained higher P. cinnamomi root quantities in winter than in summer. However, in orchard ZC, no significant temporal pattern was evident, although a trend was noticeable towards higher P. cinnamomi root colonization in autumn, winter and spring. Principal component analyses supported autumn and winter as the months yielding the highest P. cinnamomi root colonization levels. The study has contributed to a better understanding of the temporal nature of P. cinnamomi root colonization, and feeder root types that are optimal for qPCR quantifying the pathogen in macadamia.