ANALYSIS OF SOIL-WATER MATRIC POTENTIAL REQUIREMENT FOR INFECTION OF TURNIP WITH PLASMODIOPHORA-BRASSICAE USING NEGATIVE-PRESSURE WATER CIRCULATION TECHNIQUE

Critical soil water matric potential for infection of turnip with Plasmodiophora brassicae was examined using the negative pressure water circulation system for the regulation of the soil water matric potential of infested soils. A series of 12 gradients of soil water matric potential ranging from -3.2 kPa to -16.5 kPa was developed and maintained with fluctuations less than a few millibars (hectopascals) for 10d after seeding. The clubroot symptoms were detected in plants in soil pots controlled at a higher soil water matric potential than -11.2 kPa. The disease was not detected in plants in soil pots controlled at a lower soil matric water potential than -12.2 kPa.