Increased Plant Mortality Caused by Pythium Root Rot of Poinsettia Associated with High Fertilization Rates

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Abstract
Rooted poinsettia cutting potted in peat:perlite, soil:sand:peat, peat:vermiculite, peat:vermiculite:perlite:sand, or peat:vermiculite:sand:bark ash received soluble fertilizer with each irrigation. A range of fertilizer levels from suboptimum (100 .mu.g N/g) to excessive (600 .mu.g N/g) was applied. Half of the plants in each treatment were inoculated with Pythium ultimum. Few uninouculated plants died, and few plants given the lowest level of feritlization died even when inoculated with Pythium. Mortality of inoculated plants increased rapidly as the concentration of soluble salts, associated with fertility level, increased. Plants that survived inoculation were significantly shorter than uninoculated plants at all except the highest fertilizer level.