Inoculum potential of Pythium arrhenomanes in soils from three crop rotation experiments

Soil from crop rotation experiments at three different sites was tested by using bait plants and a most probable number (MPN) method to estimate the inoculum potential of Pythium arrhenomanes in terms of the density of infective units. Recorded values varied between 0 and 85 infective units/kg soil. In soil from one of the sites the inoculum densities following barley monoculture were significantly higher than those from crop rotations, whether sampled in the spring or autumn. At the other two sites, which had consistently lower inoculum densities, small effects of rotation treatment were also recorded when they were sampled in the autumn. However, the results did not support the tentative conclusions of earlier studies, that inoculum of P. arrhenomanes could be a major cause of yield reduction in barley monoculture. The MPN method cannot be recommended for predicting expected disease levels of P. arrhenomanes , unless the infective unit can be carefully defined and its relation to yield loss reliably established.