Phylum Thallophyta - The Algae - Phycomycetes - Chytridiales - Synchytrium endobioticum (Wart Disease)
This Fungus is a very important plant pathogen, causing the well-known Wart Disease of potatoes, which may cause very great loss of the tubers either in the ground or later in storage. The disease is common in most parts of the world where potatoes are grown commercially. In comparatively recent years a number of varieties have been produced which are immune to the disease, and such varieties should alone be planted in infected soils. So far as the British Isles are concerned the West of England is more liable to attacks of Wart Disease than either Scotland or the Eastern Counties.
The Fungus responsible for this disease has a relatively simple life-history, though it is considerably more complex than that of Rhizophidium, in fact Synchytrium endobioticum may be regarded as a member of a specialized sideline from an evolutionary view-point.
In describing the life-history it is desirable to start with the zoospore, which enters a cell of the young potato tuber at or about soil level. Once inside, this naked body passes to the base of the host cell and there forms a wall around itself. It rounds off and enlarges greatly, and finally forms a thick, two-layered wall, and is called the prosorus. The nucleus also increases very greatly in size and gives off stainable chromatin material into the cytoplasm. A pore now appears in the outer layer of the wall, and the contents migrate into a sac formed by the extrusion of the inner wall layer. During the migration of the protoplast the nucleus begins to divide until some thirty-two nuclei have been formed, when planes of cleavage appear and walls are laid down, cutting the body into some four or five zoosporangia, each enclosed in a separate wall, and the whole lying inside the original inner membrane.
In these zoosporangia further nuclear division goes on until about three hundred nuclei have been produced in each sporangium. Around these nuclei zoospores are differentiated. Thus from the original zoospore which entered the host cell some 1,500 zoospores are developed.
The mature sorus, which is the name given to the group of sporangia, now absorbs water and swells, rupturing the wall of the host cell and forcing out the separate sporangia, which thus come to lie near the surface of the potato. Through slits, or sometimes through definite projections of the wall, termed papillae, the zoospores escape.
REPRODUCTION
The behaviour of these zoospores depends upon the environmental conditions. If they are liberated from the sporangium immediately they are formed, they function as asexual zoospores and give rise to fresh sori in the way already described. These zoospores are spherical bodies with a very long apical flagellum. When they reach the host cell the flagellum shortens and contracts until finally it is absorbed and the body enters the host tissue as a naked mass. On the other hand, if as a result of lack of sufficient moisture, the zoospores are retained within the sporangium for a longer time, they appear to get smaller in size, and on liberation they then function as gametes, fusing in pairs to produce biflagellate zygotes. The zygote also enters the host tissue after the retraction of the flagella and sinks to the bottom of the host cell.
This body then enlarges in size, forming a resting sporangium, which is characterized by having a thick wall composed of three layers. The inner two are derived from the tissues of the parasite, but the third is obtained from material originating from the disorganization of the host cell. The contents of the resting sporangium divide up into zoospores, which are finally liberated by the swelling of the outer layer and the cracking of the inner layers of the wall. These zoospores are oval in shape with a basal nucleus and a single long apical flagellum. It is during the formation of these zoospores that meiosis occurs. They germinate to produce sori in the tissues of the host plant.
In order to distinguish more clearly between the sporangia produced in the sori and those with thick walls produced from the zygotes, the former are sometimes referred to as summer sporangia and the others as the winter sporangia.
WART FORMATION
The formation of wart-galls in the host tissue is brought about in the following way. A cell invaded by a zoospore enlarges as the parasite grows, and at the same time the adjacent cells elongate and divide, thus forming a raised rosette of cells around the infected one. Later the cells of the rosette may be secondarily infected by zoospores originating from the sporangia formed in the cell of primary infection. These cells then undergo changes of a similar kind, so that in a short while a mass of gall tissue develops. Cells invaded by the zygotes appear to behave differently. They are stimulated to tangential cell division, and since the zygote lies at the base of the cell the thick-walled sporangium, when formed, is buried in the wart produced.
