Distribution of Phytophthora cinnamomi at different spatial scales: When can a negative result be considered positively?

In October 1999, patches of dead and dying trees were identified in rainforest vegetation throughout the Tully Falls area in north Queensland, Australia. Previous incidents of patch death in the region had been attributed to Phytophthora cinnamomi . The distribution of P. cinnamomi was assessed by testing for its presence in seven sites displaying signs of dieback and seven sites that appeared healthy. Each site was a circular quadrat, 20-m radius (total area = 1256.6 m(2) ). Within each quadrat, two perpendicular line transects were constructed. A single soil sample (250 g) was taken at the centre point and at 1-m intervals along each transect. All soil samples were tested for the presence of P. cinnamomi using a combination of lupin baiting, subsequent culturing and microscopic identification. Of the 1134 samples, 783 recorded positive responses. The mean number of positive responses was not significantly greater in patch death sites than in control sites, suggesting that at this scale of resolution the distribution of P. cinnamomi was uniform. However, at spatial scales of 1-m intervals across transects the distribution of P. cinnamomi was random.