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Proceedings of the International Plant Propagator's Society

Vol. 29


Hudson T. Hartmann and John E. Whisler

pp: 36-40

The need for removal of pathogenic organisms in soil mixes to be used for seed germination and other propagation and growing purposes is well known and accepted (1,3,5,6,7). The two general methods for doing this are: (1) by chemical treatment and (2) by heat treatment. Heat, particularly steam heat, is acknowledged to be superior to chemicals for several reasons (5). In heat pasteurization, holding the soil mix at the proper temperature for the proper length of time is crucial in obtaining the desired results. It is also important that the soil mix be moist for several days prior to the time of heat treatment to obtain satisfactory killing of pathogens and weed seeds. The ideal temperature combination is generally accepted as 140°F (60°C) for 30 minutes (5,8). Temperatures lower than this will not kill the pathogens and weed seeds. Temperatures much higher will kill non-pathogenic beneficial saprophytic microorganisms, thus creating a biological vacuum. If accidental reinoculation with pathogenic

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