Effect of ethylene on root growth, ectomycorrhiza formation, and Fusarium infection of Douglas-fir

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Abstract
Roots of 2 mo. old Douglas-fir [P. menziesii] seedlings were exposed to 6 ethylene concentrations, ranging from 0.006 (soil ambient control) 0.5 ppm, by adding ethephon, an ethylene-releasing compound, as a soil drench to the root zone. After 2 mo. exposure, 0.01-0.05 ppm had stimulated, 0.05-0.15 ppm had no efect, and concentrations > 0.15 ppm had inhibited lateral root formation. Root dry weight increased and shoot dry weight decreased as the ethylene concentration was increased. Ectomycorrhiza formation on 4 mo. old seedlings inoculated with Hebeloma crustuliniforme was not increased by exposure of the root zone to 0.1 ppm ethylene for 3 mo., whether or not the seedling roots had been previously exposed to ethylene for 2 mo. before inoculation. By contrast, when 2 mo. old seedlings preinoculated with F. oxysporum f. sp. pini were exposed to 0.1 ppm ethylene for 2 mo., disease incidence was significantly increased. The presence of ethylene in the root zone may have a different effect on root infection by pathogenic fungi from that by mycorrhizal fungi.