False-negative isolations or absence of lesions may cause mis-diagnosis ofdiseased plants infected with Phytophthora cinnamomi

In a series of growth cabinet. glasshouse and field experiments, tissue sam ples from living clonal Eucalyptus marginata (jarrah) were incubated immedi ately after sampling on agar (NARPH) selective for Phytophthora. Phytophtho ra cinnamomi was recovered 3-6 months after inoculation from 50% of samples with lesions and 30% of symptomless samples. However. up to 11% of samples with and without lesions and from which P. cinnamomi was not initially iso lated contained viable pathogen. This was shown by removing tissue which ha d not produced any growth of P. cinnamomi on NARPH plates, cutting it into smaller sections, washing in sterile deionised water repeatedly for 9 days, and replating. Plating stem or bark tissue directly onto NARPH produced fa lse-negative results for nine P. cinnamomi isolates and six jarrah clones. The behaviour of the pathogen indicates that it could be present as dormant structures. such as chlamydospores, that need to be induced to germinate. Alternatively, fungistatic compounds in the tissue needed to be removed to allow the pathogen to grow. These results have important implications for d isease diagnosis and management, disease-free certification and quarantine clearance.