pH and nutritional effects on infection by Plasmodiophora brassicae Wor. and on clubroot symptoms
Clubroot (casual agent Plasmodiophora brassicae Woronin,) is a major disease of crucifers, liming is the traditionally applied control method. This project aimed to identify effects of calcium and pH, in isolation, on host invasion and pathogen development. Nitrogen and boron were investigated as potential control agents in order to develop an integrated nutrient control treatment effective at high inoculum pressures. Chinese cabbage seedlings Brassica oleracea ssp. pekinensis cv. Granaat,) were grown in acid washed sand or peat: sand composts containing various concentrations of nutrients and P. brassicae propagules. pH was varied and concentrations of calcium, nitrogen and boron were raised for specific periods of the host parasite cycle. Assessments of infection number and sporogenesis in the root hair were made 10-16 days after inoculation. Clubbing symptoms, host dry weights and mineral contents of root and leat tissue were assessed at 30 days. Infection was reduced by separate increases of pH to 7.2 and calcium to 55mel-1. Sporogenesis of root hair infections was inhibited by pH 7.2, 55mel-1 calcium and 6.4ppm boron. Suppression of clubbing by pH 7.2, 55mel-1 calcium and 60mel-1 nitrate was effected 0-14 days after inoculation, applications of such inhibitory treatments later than 14 days were ineffective. In contrast 6.4ppm boron applied over 0-14 and 14-30 day periods actively reduced clubbing. Nitrate, boron pH treatments controlling clubroot were non-phytotoxic. Control due to pH7.2 may in part be attributed to enhanced uptake of calcium into host plants at higher pH. Suppressive nutrients applied at pH7.2 and calcium applied with boron showed synergistically enhanced control of clubroot when compared with single treatments. A non-phytotoxic nutrient containing intermediate levels of calcium (30mel-1), nitrate (30mel-1) and boron (6.4ppm) at pH7.2 reduced disease index from 100% to 3% in plants subject to heavy inoculum pressures. An integrated control treatment for clubroot was thus demonstrated.