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Biological Suppression of Seedborne Fusarium spp. During Cold Stratification of Douglas Fir Seeds

Fusarium spp. are important soil- and seedborne pathogens of Douglas fir (Pseudotsuga menziesii) in conifer seedling nurseries. We investigated the effects of culture media and cold stratification on isolation of Fusarium spp. from three Douglas fir seedlots and tested whether the numbers and species mixtures found on stratified seeds could cause disease. Nearly all cold-stratified seeds plated on three semiselective culture media yielded Fusarium spp., including F. avenaceum, F. culmorum, F. lateritium, F. moniliforme, F. poae, F. proliferatum, F. sambucinum, F. solani, and F. tricinctum. Species composition did not differ significantly among the media. Isolation of Fusarium spp. from seeds plated on Komada's medium (pH 6.8) at various stages of imbibition and cold stratification progressively increased from 10 to 22% to 65 to 100%. When stratified seeds were planted in conditions conducive to disease development, however, little disease attributable to Fusarium spp. resulted. A subsequent study was conducted to determine whether a biological control agent applied during imbibition could reduce the proliferation of Fusarium spp. during stratification. Unstratified Douglas fir seeds were imbibed for 24 h in a suspension of Pseudomonas chlororaphis isolate RD31-3A, a rifampicin-resistant fluorescent pseudomonad with previously demonstrated biocontrol activity against F. oxysporum. This treatment reduced the proliferation of Fusarium spp. during cold stratification without significantly affecting subsequent seed germination. The greatest reduction in poststratification populations of seedborne Fusarium spp. was achieved when preimbibition treatment with hydrogen peroxide was followed by seed imbibition in live bacteria. Seed imbibition in bacterial suspensions may be an effective means to deliver biological control agents to Douglas fir seeds.