Publication no. D-2002-0409-01R

Frequency, Virulence, and Metalaxyl Sensitivity of Pythium spp. Isolated from Apple Roots Under Conventional and Organic Production Systems. Mark Mazzola, USDA-ARS, Tree Fruit Research Laboratory, Wenatchee, WA, 98801; Preston K. Andrews and John P. Reganold, Department of Horticulture & Landscape Architecture, Washington State University, Pullman 99164; and C. André Lévesque, Eastern Cereal and Oilseed Research Centre, Agriculture & Agri-Food Canada, Ottawa, ON. Plant Dis. 86:669-675. Accepted for publication 12 February 2002. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2002.

Studies were conducted to characterize the community of Pythium spp. associated with apple in Washington state. Seventeen different species were identified among the isolates of Pythium recovered from apple roots at six orchard sites. Commonly isolated species included P. intermedium, P. irregulare, P. heterothallicum, P. sylvaticum, and Pythium MM1 (aff. macrosporum), a previously undescribed species. The population of Pythium spp. at any given site typically was dominated by a single pathogenic species. Species composition of Pythium populations in adjacent conventional and organic blocks in two orchards did not differ; however, the relative recovery of Pythium spp. from apple roots was consistently lower in the organically managed systems. In greenhouse studies, the associations formed between apple seedlings and Pythium spp. ranged from pathogenic to growth-promoting. Precolonization of apple seedling roots with any one of three nonpathogenic isolates tested—isolate 584 of Pythium MM1, isolate 1-12 of Pythium MM3 (aff. oedochilum), and isolate 1-19 of Pythium MM5 (aff. vexans)—was found to provide biological control of root rot caused by P. sylvaticum and P. ultimum. Metalaxyl sensitivity varied among species of Pythium, with several nonpathogenic species exhibiting the highest levels of insensitivity to this fungicide. Additional keywords: biological control, rhizosphere.