Pythium Species Associated with Forest Tree Nurseries of Oregon and Washington

Authors: Weiland, Gerald - Jerry

Submitted to: International Union Of Forestry Research Organization
Publication Type: Proceedings
Publication Acceptance Date: 1/28/2010
Publication Date: 10/7/2010
Citation: Weiland, G.E. 2010. Pythium species associated with forest tree nurseries of Oregon and Washington. Proceedings of the 7th Meeting of the International Union Of Forestry Research Organization Working Party 7.03.04 Diseases and Insects in Forest Nurseries. p. 104-108.

Interpretive Summary: Pythium species are soilborne plant pathogens that can cause root rot and death of conifer seedlings in forest tree nurseries. Traditionally, disease management has relied on fumigation with methyl bromide. However, fumigant use is facing increased regulatory pressure and nursery managers are interested in alternative methods for managing soilborne diseases. Field trials were established in 2008 at three nurseries (2 in OR, 1 in WA) to: 1) evaluate the impact of lower application rates of fumigants on soil populations of Pythium; and 2) identify Pythium species associated with damping off of douglas-fir seedlings. Six fumigant treatments (including a conventional methyl bromide treatment and a nonfumigated control) were tested at each nurseries and soil populations of Pythium were analyzed before and after fumigation. Pythium isolates were recovered using three different methods and then identified on the basis of DNA sequence. Prior to fumigation, Pythium populations averaged 40-45 cultures/gram soil in nurseries A and B, and 19 cultures/gram soil in nursery C. All fumigant treatments reduced Pythium populations by at least 86% and populations were similar 7 months later. Of the 450 isolates identified to date, 42% are P. irregulare, 27% are P. dissotocum, 10% are P. macrosporum, and the remaining 21% are a mixture of 12 different Pythium species. Preliminary evidence shows that the isolation method affects Pythium species recovery and finds that most Pythium species are pathogenic to douglas-fir seedlings.

Technical Abstract: Pythium species are one of several pathogen genera responsible for damping off of conifer seedlings in forest tree nurseries. Field trials were established in 2008 at three nurseries (2 in OR, 1 in WA) to: 1) evaluate the impact of lower application rates of fumigants on Pythium soil populations; and 2) identify Pythium species associated with damping off. Six fumigant treatments (including a conventional methyl bromide treatment and a nonfumigated control) were applied according to a randomized complete block design with four blocks at each nursery. Soil samples were collected before and after fumigation and Pythium populations were assessed by baiting with rhododendron leaves and douglasfir needles and by dilution plating onto PARP, a semiselective medium for pythiaceous species. Isolates were identified based on their ITS sequence. Prior to fumigation, populations averaged 40-45 CFU/g soil at nurseries A and B and 19 CFU/g soil at nursery C. All fumigant treatments reduced soil populations by at least 86% and populations were similar 7 months after fumigation. Of the 450 isolates identified to date, 42% are P. irregulare, 27% are P. dissotocum, 10% are P. macrosporum, and the remaining 21% is composed of 12 different Pythium species. Preliminary evidence shows that the isolation method affects Pythium species recovery and finds that most of the isolated Pythium species are pathogenic to douglas-fir seedlings.