A New Heterothallic Pythium from the United States and Canada

SUMMARY

Pythium heterothallicum is described as a new species. The species designation “heterothallicum” was chosen to emphasize the absence of homothallic isolates, in contrast to P. sylvaticum in which isolates with varying degrees of homothallism occur. P. heterothallicum will not mate with any of the tester strains of P. sylvaticum found to date. It differs morphologically from P. sylvaticum in possessing a larger number of antheridial branches and antheridia. It also grows much slower at 25 C, averaging only 1.5 cm of radial growth in 24 hr in contrast to P. sylvaticum which averages 2.6 cm. P. heterothallicum will not grow at 35 C while P. sylvaticum makes slight growth.