Factors Affecting Suppression of Pythium Damping-Off in Container Media Amended with Composts

Abstract

A cucumber seedling bioassay was developed to determine suppressiveness of container media to damping-off caused by Pythium ultimum. Container media amended with samples of composted municipal sludge, removed from the low-temperature edge of 4-mo or older compost piles, were suppressive to Pythium damping-off. Those amended with samples from the high temperature (> 60 C) center of the same piles were conducive. The suppression was biological in nature because heating (60 C, 5 days) destroyed the suppression, incorporation of small volumes (10% v/v) of suppressive compost into conducive container media restored suppression, and addition of antagonists to conducive container media induced suppression. Conductive and suppressive container media responded differentially to increasing inoculum densities of Pythium. Estimated ED50 values based on logarithm-probability transformation were 268, 30, 880, and 855 mg of inoculum of Pythium per liter of conducive composted municipal sludge, heat-treated composted hardwood bark and suppressive composted municipal sludge, and composted hardwood bark container media, respectively. In the absence of cucumber plants, the population of P. ultimum declined at a similar rate in conducive and suppressive container media over a period of 40 days. However, in the presence of cucumber plants, buildup of populations of P. ultimum was prevented in suppressive but not in conducive container media.