Evidence that the biometric characters of the oogonial unit of Pythium are of significant taxonomic value, allowing for a good species separation, has been presented by other workers, based on 40 species, each represented by fairly few isolates. Since broad intraspecific variation and interspecific overlapping of biometric parameters was observed in isolates of four Pythium species from Denmark, it was suspected that a such wide variability would weaken the conclusions obtained by previous biometric approaches. Species to be studied should be represented by higher numbers of isolates to reflect intraspecific variation. In the present biometric study 49 isolates were used to test the robustness of the biometric approach and to examine the taxonomic value of biometric characters from a practical perspective.

Good species separation was obtained despite intraspecific variation. The biometric parameter set for the oogonium, consisting of oogonial, oospore and ooplast diameters, supplemented with the thickness of the oospore wall or the protoplast diameter and with derived indices of volume or linear ratios of the measured parameters were all of taxonomic value for the separation of species, as shown from canonical variate plots after linear discriminant analysis. More detailed information on species separation, isolate-species or isolate–isolate associations could be obtained by separation based on the Mahalanobis distance function in the linear discriminant analysis. Both the derived volume and linear indices contributed significantly to species separation and appeared to be equally satisfactory.

The substrates used for oogonium production may influence the biometric parameters to an extent which may seriously affect the results of the biometric approach to taxonomy. These results stress the importance of standardizing substrates in morphological taxonomy and identification work.