ABSTRACT Phytopathology 86, 763-768.

The proliferation of Phytophthora cinnaniomi in avocado roots was quantified with a sensitive, species-specific DNA probe. The probe was isolated from a library of genomic DNA by identifying colonies that strongly hybridized to DNA of P. cinnamomi but not to DNA from other species of Phytophthora. The characterization of five such clones indicated that each detected the same repetitive sequence that was present in genomic DNA at about 12,000 copies perhaploid genome. Cross-hybridizing sequences were absent from DNA of other Oomycetes, bacteria, ascommycetes, basidiomycetes, or plants. The probes detected as little as 5 pg or P. cinnamomi DNA in dot- and slot-blot assays. The P. cinnamomi probe and a probe for avocado DNA were used to quantitate the growth of the pathogen in roots. The extent of colonization, judged by measuring the relative amounts of pathogen and host DNA, increased over time with increasing amounts of inoculum.