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Pythium abappressorium—a new species from eastern Washington

     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

A new species of Pythium isolated from wheat and apple roots in eastern Washington is described. Pythium abappressorium sp. nov. is characterized by abundant appressoria. Plerotic oospores and sporangia are formed from the appressoria and remnants of the appressoria remain attached to the base of sporangia at maturity. Smaller appressorial swellings, reminiscent of hyphal swellings, are also formed within the appressoria. Pythium abappressorium is pathogenic to wheat, causing damping-off and stunting, 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, Triticum aestivum, wheat