Spore load, light intensity and plant nutrition as factors influencing the incidence of club root of brassicae

Abstract

Light intensity markedly affects the relationship between spore load and number of plants developing club root (or their disease index). A direct relationship exists at low light intensities up to a high level of spore load, but at a high light intensity this relationship exists only at fairly low spore loads. At any spore load within the range 10 1 –10 7 spores/g. soil, the disease index for plants was lowest at the lowest light intensity employed. At high spore loads the number of diseased plants is not influenced by light intensity, but at low spore loads the highest number of diseased plants is associated with the highest light intensity. In composts with different spore loads, the number of diseased plants was shown to be uninfluenced by the supply of nitrogen, potassium or phosphorus. Plants well supplied with nitrogen had a lower disease index than had those not receiving more nitrogen than was adequate for very slow growth. This nitrogen effect was usually more marked as the fertilizer dosage increased. The bearing of these findings on the use of a pot test for evaluation of spore loads of P. brassicae in soils is discussed.