STUDIES ON THE LIFE HISTORY OF THE CLUB ROOT ORGANISM, PLASMODIOPHORA BRASSICAE

Abstract

In the present studies on Plasmodiophora Brassicae Wor., attention was focussed principally on the development of the zoosporangium and the formation and discharge of the zoospores.Resting spores, released from thoroughly clubbed tissue by the action of bacteria and fungi, germinated well in tap water at room temperature in 1 to 10 days. Germination was hastened by exposure to frost.The zoospores from the resting spores are biflagellate and heterokont, and vary in size. When such a zoospore comes in contact with root hairs or epidermal root cells of cruciferous seedlings, it settles down as an amoeba and penetrates the cell wall to form a young thallus within the host cell. The thallus may or may not become considerably enlarged before the protoplasm undergoes cleavage and an irregular cluster of zoosporangia is formed. Each zoosporangium contains four to eight zoospores, which, at maturity, are discharged from the root hair if free moisture is present; otherwise the fungal protoplasm disintegrates. At ordinary field temperatures, from four to six days are required from infection of the host to the discharge of zoospores from the zoosporangium. Discharge takes place through small openings at the point of contact of the zoosporangia with the cell wall of the root hair. These zoospores when discharged are not more than one-half the average diameter of the zoospores from germinating resting spores.Controlled temperature, studies have shown that temperatures most favourable for the growth of the host are also most favourable for infection. The optimum temperature for infection was about 70° F., the minimum between 54° and 57° F., and the maximum above 92° F. Only two to two and one-half days were needed for the process.