Efficacy of Chemical and Biological Agents to Suppress Fusarium and Pythium Damping-Off of Container-Grown Douglas-fir Seedlings

Douglas-fir seedlings are susceptible to Fusarium or Pythium damping-off that currently is controlled by pre-plant soil fumigation in bareroot nurseries and steam pasteurization or chemical drenches of soilless media in container nurseries. However, because few chemical or biological agents are registered for use on conifer seedlings, we tested several on greenhouse-grown seedlings and found that over-seed applications of Cleary's 3336, Strike, Compass, Compass + Strike, Cygnus, Endura, Medallion, Medallion + Strike, Thiram, and Enzone effectively suppressed pre-emergence damping-off by Fusarium oxysporum, but only Cleary's 3336WP and Medallion + Strike also reduced post-emergence damping-off. Compass, Medallion, and Thiram reduced post-emergence damping-off, but not to a statistically significant level. Pre-emergence damping-off by Pythium irregulare was reduced only by Ranman, but post-emergence damping-off was reduced by Thiram, Hurricane, Ranman, and Subdue MAXX. Over-seed drenches of biological control agents Companion, Kodiak, Subtilex, Taegro, Primastop, SoilGard, Actinovate, Mycostop, RootShield, and Green-Releaf were ineffective in suppressing either Fusarium or Pythium diseases, and combining several agents with chemicals did not improve efficacy. These results provide data in support of registration of some of the effective chemicals, but no biological control agents, for the control of conifer seedling damping-off. Accepted for publication 14 January 2008. Published 17 March 2008.