Biological suppression of seedborne Fusarium spp. during cold stratification of Douglas fir seeds

Fusarium spp, are important soil- and seedborne pathogens of Douglas fir (P seudotsuga menziesii) in conifer seedling nurseries. We investigated the ef fects of culture media and cold stratification on isolation of Fusarium spp . from three Douglas fir seedlots and tested whether the numbers and specie s mixtures found on stratified seeds could cause disease. Nearly all cold-s tratified seeds plated on three semiselective culture media yielded Fusariu m spp., including F. avenaceum, F. culmorum, F. lateritium, F. moniliforme, F. poae, F. proliferatum, F. sambucinum, F. solani, and F. tricinctum. Spe cies composition did not differ significantly among the media. Isolation of Fusarium spp. from seeds plated on Komada's medium (pH 6.8) at various sta ges of imbibition and cold stratification progressively increased from 10 t o 22% to 65 to 100%. When stratified seeds were planted in conditions condu cive to disease development, however, little disease attributable to Fusari um spp. resulted. A subsequent study was conducted to determine whether a b iological control agent applied during imbibition could reduce the prolifer ation of Fusarium spp. during stratification. Unstratified Douglas fir seed s were imbibed for 24 h in a suspension of Pseudomonas chlororaphis isolate RD31-3A, a rifampicin-resistant fluorescent pseudomonad with previously de monstrated biocontrol activity against F. oxysporum. This treatment reduced the proliferation of Fusarium spp. during cold stratification without sign ificantly 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 imbibi tion in live bacteria. Seed imbibition in bacterial suspensions may be an e ffective means to deliver biological control agents to Douglas fir seeds.