FACTORS AFFECTING THE SURVIVAL OF PLASMODIO‐PHORA BRASSICAE WOR. IN THE SOIL AND ITS ASSESSMENT BY A HOST TEST

The activity of Plasmodiophora brassicae Wor. in the soil was measured simultaneously by counts of root‐hair infection on cabbage seedlings and percentage clubbing of older plants. A linear relation was found between the logarithms of the mean number of root‐hair infections per seedling and the logarithms of the spore concentration in the inoculum. It was found that a high proportion of clubbed plants could be associated with very low average numbers of root‐hair infections on seedlings. The percentage of clubbed plants can be used for assessing quantitatively the infestation of soil by P. brassicae but only at lower levels of infestation than can counts of root‐hair infections. An effect of early application and higher levels of nutrients in increasing clubroot indicates that the nutrient status of the soil is also important, but it is not clear whether this is due to increased club growth or to a direct effect on the fungus. Experiments on the survival of resting spores showed that, in a wet acid soil, there was a rapid decline in numbers of infections to a low and nearly constant level with increasing time from inoculation. This decline could be considerably delayed by alkaline or dry soil conditions or by the addition to the soil of mustard oil (allyl isothiocyanate). This fall in activity is ascribed to spontaneous germination of resting spores, germination being prevented or delayed by such conditions as alkalinity, low soil moisture or the presence of inhibitors. In an attempt to cause biological stimulation of resting‐spore germination by growing short‐term catch crops in infested soil (re‐infestation of the soil from diseased crucifers being prevented), there were fewer infections on test cabbage seedlings after crucifers or ryegrass had been grown than after a number of other non‐crucifers. In fallow soil, a similar reduction was, however, caused by adding nutrients alone. It was confirmed that P. brassicae can infect the root hairs of a number of non‐crucifers and form zoosporangia and zoospores, but no other stages of the fungus have been seen in those plants. Possible improvement of the sampling design in experiments using the root‐hair infection‐count method is briefly discussed.