Hydrolysis of fluorescein diacetate in sphagnum peat container media for predicting suppressiveness to damping-off caused by Pythium ultimum

Microbial activity, based on the rate of hydrolysis of fluorescein diacetate (PDA), accurately predicted supprcssivcness to Pythium damping-off of cucumber in media containing light-colored. relatively nondecomposed Canadian sphagnum peat. Suppressiveness was directly related to the quantity of FDA hydrolysed. Organic matter in container media adsorbed fluorescein. Adsorption was directly related to peat decomposition level. In media prepared with dark, decomposed sphagnum peats, that are H ₃-H ₄ or higher on the von Post scale, adsorption of fluorescein was strong enough to interfere with the assay. Since such decomposed peats were consistently conducive, the procedure did not yield false readings even here.