80 Mycologia, 95(1), 2003, pp. 80–86.  2003 by The Mycological Society of America, Lawrence, KS 66044-8897 Pythium abappressorium—a new species from eastern Washington Timothy C. Paulitz 1 Karen Adams USDA-ARS, Root Disease and Biocontrol Laboratory, 365 Johnson Hall, Washington State University, Pullman, Washington 99164-6430 Mark Mazzola USDA ARS Tree Fruit Research Lab, 1104 N Western Ave, Wenatchee, Washington 98801-1230 Abstract: A new species of Pythium isolated from wheat and apple roots in eastern Washington is de- scribed. Pythium abappressorium sp. nov. is character- ized by abundant appressoria. Plerotic oospores and sporangia are formed from the appressoria and rem- nants of the appressoria remain attached to the base of sporangia at maturity. Smaller appressorial swell- ings, reminiscent of hyphal swellings, are also formed within the appressoria. Pythium abappressorium is pathogenic to wheat, causing damping-off and stunt- ing, but is not pathogenic to apples. The fungus can grow in the temperature range 5 to 30 C, with an optimum of 20 C. The sequence of the ITS1 region of the rDNA did not match the sequences from a worldwide collection of over 1200 isolates, including types and neotypes, suggesting that this species has not been previously described. Key Words: apple, appressoria, biological control agent, damping-off, Malus domestica, root rot, Triti- cum aestivum, wheat INTRODUCTION Over two million acres of dryland wheat and barley are grown in eastern Washington. These areas are characterized by deep wind-deposited loess soils, and range in rainfall from 22 to 65 cm per year. The most easterly part of Washington was formerly a grassland prairie on rolling hills. As of the late 1980s, 19 species of Pythium had been reported to be pathogenic on wheat and 10 on barley in North America (Farr et al 1989). Pythium root rot has been demonstrated to stunt plants and reduce yields in the Pacific North- west, based on plots treated with metalaxyl, which is Accepted for publication June 27, 2002. 1 Corresponding author, Email: paulitz@wsu.edu specific for Oomycetes (Cook et al 1987). Cham- swarng and Cook (1985) isolated ten species of Pyth- ium and two unidentified Pythium isolates from fields around Pullman, Washington and Moscow, Idaho, and showed that all were pathogenic. Despite the im- portance of Pythium in cereal production, little is known about which species are predominant, both in terms of populations and pathogenicity, across this or any wheat-growing region. Pythium spp. are diffi- cult to key out, especially heterothallic isolates or those that are completely asexual, because these spe- cies do not form sexual structures in culture that are required for identification. However, recent molecu- lar techniques such as PCR and sequencing may be useful for ascertaining the genetic diversity of Pyth- ium spp. in wheat and barley. In the summer of 2000, over 80 sites across Eastern Washington and North- ern Idaho were sampled for Pythium spp, as part of a larger project to describe the species and popula- tion diversity of this fungal genus. The ITS 1 region of the rDNA was amplified by PCR and sequenced for a subsample of this collection, and compared to a sequence database of type species from a worldwide collection (C. A. Le ´vesque, unpubl). One set of iso- lates represented a taxon widely distributed in most sites and lacking sequence homology and morpho- logical similarity to known species in the keys of van der Plaats-Niterink (1981) or Dick (1990). These iso- lates also resembled unknown isolates found by Maz- zola et al (2002) in apple orchards in the Wenatchee area of central Washington. These latter isolates were recovered from the roots of apple at three of six or- chards surveyed in central Washington. All isolates of this undescribed species exhibited moderate to high levels of resistance to the fungicide metalaxyl regard- less of orchard source. Application of certain isolates to roots stimulated growth of apple in sterile soil and reduced root infection when seedlings were planted in soil infested with pathogenic Pythium spp. includ- ing P. ultimum Trow and P. sylvaticum Campbell & Hendrix. Based on the unique morphological char- acteristics and ITS sequence, a new species is de- scribed. MATERIALS AND METHODS Isolate collection and maintenance. In July and August, 2000, soil and root samples were collected from wheat and