The virulence of Coniophora puteana BAM Ebw. 15, the obligatory test fungus on softwood in tests according to EN 113, is known to be rather inconstant at least in some laboratories. The mass losses of untreated Scots pine sapwood blocks in an EN 113 test may range from below 20% to up to more than 50%. Possible reasons for these differences as well as the impact of a low virulence on the toxic values of a preservative under test were discussed and a round robin test with subcultures of C. puteana Ebw.15 and of Poria placenta FPRL 280 of different virulence and origin was carried out. Subcultures of both test fungi considered as either virulent or weak were distributed by there laboratories of origin. All test specimens originated from one laboratory. They were treated with a CCB salt by another laboratory and distributed to the partners. The results show that the virulence of identical subcultures may differ remarkable most probably depending on the culture conditions of the laboratories involved.In the case of P. placenta differences in virulences did not cause different toxic values in the round robin test. In the case of C.puteana, however, different toxic values were obtained. Furthermore, this fungus showed an increased sensitive behaviour to varying test conditions. A virulence <40% of C. puteana generally reduced the toxic value at least by one concentration.

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