SUSTENANCE OF MICROBIAL ACTIVITY IN POTTING MIXES AND ITS IMPACT ON SEVERITY OF PYTHIUM ROOT-ROT OF POINSETTIA

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. In mixes prepared with the slightly (H-3) and most decomposed (H-4) peats, microbial activity declined with time, whereas the population density of P. ultimum increased, and root rot developed. In a mix prepared with the least decomposed (H-2) peat and in a composted pine bark-amended mix, microbial activity and suppression were sustained throughout the growth period. Populations of P. ultimum and Pythium root rot were suppressed as long as the rate of hydrolysis of FDA was sustained above a level of 3.2-mu-g min-1 g-1 dry weight potting mix. Suppression was negated by heating, suggesting that the effect was biological in origin. Both the microflora in and the microbial carrying capacity of the peat contributed to sustained suppression.