Influence of Isolation method on mecovery of Pythium species from forest nursery soils in Oregon and Washington

Authors: Weiland, Gerald - Jerry

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2010
Publication Date: 4/1/2011
Citation: Weiland, G.E. 2011. Influence of isolation method on recovery of Pythium species from forest nursery soils in Oregon and Washington. Plant Disease. 95:547-553.

Interpretive Summary: Pythium species are common soilborne pathogens that kill conifer seedlings in the Pacific Northwest (PNW). Despite the prevalence and importance of these pathogens, relatively little is known about the identity of Pythium species causing problems in Washington and Oregon. Several species have been reported from the PNW, but the majority of studies do not identify the Pythium species that were isolated from nursery soils. As a consequence, many Pythium species have probably been overlooked. In an attempt to assess a greater variety of Pythium species associated with forest nurseries, field surveys were conducted at three forest nurseries in 2008 using three methods. Pythium species were isolated by placing soil on petri plates containing a semiselective agar medium and by baiting soil samples with rhododendron leaf disks or Douglas-fir needles. One hundred Pythium cultures were randomly selected from each method and nursery and identified by DNA sequence analysis. A total of 19 Pythium species were identified. Each nursery was associated different Pythium species and each method affected which species were isolated. Pythium dissotocum, P. irregulare, and P. ‘vipa’ were the most frequently isolated species.

Technical Abstract: Pythium species are common damping off pathogens causing stunting, chlorosis, and death of conifer seedlings in the Pacific Northwest (PNW). Despite the prevalence and importance of these pathogens in forest nurseries, relatively little is known about the identity of Pythium species associated with forest nursery soils in Washington (WA) and Oregon (OR). A limited number of studies report P. aphanidermatum, P. irregulare, P. mamillatum, and P. ultimum as the predominant species in the PNW, but most studies have not reported Pythium species identity. In an attempt to identify a diverse range of Pythium species associated with forest nursery soils, field surveys were conducted at three forest nurseries (2 in OR, 1 in WA) in 2008 using three methods. Pythium species were isolated by plating soil onto a semiselective medium, PARP, and by baiting with rhododendron leaf disks and split Douglas-fir needles. One hundred isolates were randomly selected from each method and nursery and identified on the basis of ITS sequence. A total of 19 Pythium species were identified from the survey. Species richness and abundance were strongly influenced by both nursery and method. Each nursery was associated with a different predominate Pythium species (P. dissotocum, P. irregulare, and P. 'vipa').