Potential Transferability of Verticillium dahliae to Susceptible Hosts When Using Ornamental Bare-root Tree Wood Waste for a Potential Alternative Substrate

Ornamental and fruit tree growers in the Pacific Northwest commonly have surplus biomass generated from grading and inventory management. The wood waste has historically been burned whole or chipped before incorporating into production fields. Due to new and unique economic constraints, nurseries are now considering new uses for wood waste. The use of hammer-milled, culled bare-root trees as a substrate component in nursery containers is one new potential use, however there is concern of the spread of Verticillium dahliae.  A study was conducted to determine if susceptible plants, eggplant (Solanum melongena var. esculentum) 'Black Beauty', grown in a soilless substrate containing symptomatic trees become infected with V. dahliae.  Eggplant was grown for 6 weeks in nine soilless substrates comprised of combinations of zero, 50% and 100% (v/v) autoclaved and non-autoclaved Douglas fir bark (DFB) and shade tree (ST).  Verticillium inoculum was provided via verticillium-containing processed ST or a suspension drench of 107 conidia/ml.  Presence of V. dahliae was confirmed via culture isolation prior to experiment initiation from chipped symptomatic trees collected from a Oregon nursery. At experiment termination, eggplant stem and leaf tissue from each treatment were plated in PDA media to isolate the pathogen and verify its presence in the plant tissue. Disease presence did not decrease height or shoot weight except when V. dahliae was inoculated via a conidia drench. When shoot growth was visually rated for disease severity, the conidia drench inoculated treatment was rated as having greater disease symptoms than the autoclaved equivalent. There was no difference in disease severity between autoclaved and non-autoclaved within the same substrate type. Streaking of the stem vascular tissue was observed in 10% to 60% of plants in treatments containing non-autoclaved ST, whereas vascular discoloration was not observed in treatments without ST or in autoclaved ST.  V. dahliae was recovered in culture from stem or leaf isolates collected at experiment harvest for each of the substrate treatments containing either 50 or 100% non-autoclaved ST.  Symptomatic shade trees should not be utilized as a soilless substrate because of the potential transmission of V. dahliae to susceptible host material if not properly sterilized before use.