Some effects of Plasmodiophora brassicae Woron. on the growth of the young cabbage plant

Abstract

When clubroot galls developed on cabbage inoculated with Plasmodiophora brassicae Woron., the distribution of dry matter in the plant was altered. As soon as clubbed roots were visible the stem: root ratio diminished with time until the clubs rotted. After clubs appeared the increase in root weight was nearly all from growth of the clubroot gall and the rate of growth of tops correspondingly diminished. Succulence of the shoot, water content per unit dry weight, was unaffected. Increase in total leaf area closely paralleled that of total dry weight. It was slower in diseased plants and attained a lower limit than in healthy ones. From the 35 th day after inoculation, onwards, diseased plants had fewer and smaller leaves and their leaves opened more slowly, one new leaf unfolding every 9 days on diseased plants against one every 4 days on healthy plants. Leaves of diseased plants were both smaller and thinner than those in corresponding positions on healthy plants. The relationship between the absorbing system on the one hand, and the assimilating and transpiring system on the other, was greatly altered; the dry weight of fibrous root per unit leaf area was decreased after infection from 1.20 to 0.79 mg./cm.2 20 days after inoculation and from 1.64 to 0.38 mg./cm.2 82 days after inoculation. Though infection consistently lowered net assimilation rate, on the average by about 15 per cent, of the control value, this decrease was never statistically significant. Increases in the number of spores in the inoculum progressively decreased the dry weights of tops and fibrous roots, and sometimes increased the dry weight of clubs. Delaying inoculation increased the initial growth rate of the galls, and sometimes their final size. The extent to which plants were damaged depended more on how long they had been infected than on age at inoculation.