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Potting mixes prepared with Canadian sphagnum peats varied in suppressiveness to Pythium root rot of poinsettia caused by Pythium ultimum. Root rot and population development of P. ultimum were highest in the most decomposed, dark (H 4 on the von Post decomposition scale) peat, intermediate in a slightly decomposed (H 3 ) peat, and suppressed in the least decomposed light (H 2 ) peat. Microbial activity, based on the rate of hydrolysis of fluorescein diacetate (FDA), was highest in the suppressive and lowest in the conducive potting mixes (...)
Michael J. Boehm (1992) studied this question.