Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales - Cystopus candidus (Albugo candida) (The White Rust of Crucifers)
This Fungus attacks all kinds of cruciferous plants and may sometimes cause swelling and twisting of the stems, especially in young plants. The disease is easily recognizable by the white, smooth, glossy patches which gradually turn powdery and give the Fungus its common name of White Rust.
The fungal mycelium is intercellular, like that of Peronospora, and sends haustoria into the cells of the host plant; these haustoria are fairly well developed and end in button-like projections in the host cells. It is through them alone that the Fungus obtains its nourishment. It spreads rapidly within the host and may attack all parts of the plant, besides those where the white patches are apparent. Reproduction is both sexual and asexual, though there is no alternation of generations.
ASEXUAL REPRODUCTION
The asexual reproductive organs are developed just below the epidermis of the stem or leaf of the host plant. In this position short sporangiophores are produced in compact groups, forming an almost complete tissue. As growth continues a nucleus passes to the tip of each sporangiophore, and this is followed by a deepening constriction around the tip, thus cutting off a zoo sporangium with one nucleus. Immediately behind, another 200sporangium is cut off in the same way, until chains of sporangia have been formed, one above the other, from each sporangiophore. The outer layer of the wall of the sporangium becomes gelatinous, and as a result the sporangia tend to stick to one another.
The growing pressure of all these 200 sporangia cut off below the epidermis finally causes it to lift up, then to split and break up, exposing the white sporangia. It is interesting to note that during the development of the sporangia they are protected by the host epidermis, which is only ruptured when at last the distal sporangia are mature.
The zoosporangia are finally liberated and disseminated by the wind, and when conditions are favourable, that is, when the atmosphere is damp, they germinate by producing a number of zoospores (Fig. 229) in each zoosporangium. These zoospores are kidney-shaped with two laterally placed flagella. From these zoospores fresh infections of host plants are derived.
It is sometimes stated that Cystopus reproduces asexually by means of conidiospores. According to this view the sporangiophore is regarded as a conidiophore, from the tip of which conidiospores are abstricted in acropetal succession, in a way similar to that found in many Fungi, e.g, Aspergillus. It is certainly true that the method of abstriction of the zoosporangia in Cystopus is effected in much the same way. The difference lies in the method of germination of these zoosporangia. The typical conidiospore germinates by means of a germ tube and thus forms a hypha directly, one hypha being normally formed by each conidiospore. In Cystopus, as we have seen, the zoosporangium gives rise on germination to a number of zoospores, which are not only actively motile but capable of affecting a separate infection of the host tissue. It follows that the structure producing these zoospores must be regarded as a zoosporangium, comparable with that of Pythium.
From this vve may conclude that the typical conidiospore is equivalent to a
zoosporangium in which the contents do not divide into separate zoospores but germinate by a germ tube. In Cystopus we see how one stage in the process of conidiospore formation may have been evolved by abstricting these zoosporangia in chains.
SEXUAL REPRODUCTION
The sex organs are developed in the intercellular' spaces of the host tissue and very often in the deeper layers of the stem. They consist of oogonia and antheridia. The oogonium is globose and is developed from a terminal, or occasionally from an intercalary swelling of a hypha. The mature oogonium may contain as many as a hundred nuclei, together with a quantity of food material. It is finally cut off by a septum from the rest of the hypha. The antheridium is club-shaped and is developed in close association with the oogonium. It is also multinucleate, containing some twelve nuclei, and it is also separated from the supporting hypha by a septum. At this stage the nuclei in both the oogonium and antheridium simultaneously undergo one nuclear division.
At about the time when the antheridium comes into contact with the oogonium the protoplasm of the latter undergoes differentiation, leaving a zone of rather translucent periplasm surrounding the denser oosphere, which contains most of the nuclei. At the point of contact of the antheridium and the oogonium the antheridial wall becomes very thin and the oogonium pushes its way in, forming a slight protuberance, which is called the receptive spot. It appears to be functionless, for shortly afterwards there develops from the antheridium a fertilization tube, which penetrates the receptive spot and enters the oosphere. All the nuclei in the antheridium and the oosphere now undergo a second nuclear division, while the nuclei in the peri plasm disorganize. A single female nucleus passes to the centre of the oospore wall oosphere and remains there, while the others pass to the periphery and finally abort. A single nucleus from the antheridium, together with a small amount of cytoplasm, passes in through the fertilization tube and the male and female nuclei come into contact and fuse. The fertilization tube collapses and is withdrawn from the oosphere, leaving a large vacuole. The wall of the oospore, thus formed, is now laid down. It is a thick, two-Iayered structure consisting of a thin endospore and a thick, often almost spiny, exospore.
Inside the oospore the zygote nucleus begins to divide, resulting in the formation of about thirty nuclei, after which activity temporarily ceases, and the oospore may remain for a long time in this condition. Later \\"hen germination occurs the wall splits and about a hundred zoospores are liberated. It is obvious, therefore, that prior to germination further nuclear divisions must take place, and though it has never been conclusively demonstrated, it appears likely that each of the thirty nuclei found in the oospore after fusion undergoes t\\"o divisions, which involve a meiosis, so that the nuclei of the zoospores are monoploid.
The life-history of Cystopus candidus may be represented by the following diagram (Fig. 232) :-
The Fungus is not a serious disease of cultivated crucifers, for though it frequently attacks them, it is rarely that the host plant is seriously affected. The oospore can apparently retain its vitality for as much as four years. The infection of the host takes place only through the cotyledons of the seedling or through such delicate tissue as the flower buds. The parasite cannot penetrate the mature epidermis, consequently an epidemic spread of the disease among crucifers is not very likely.
Despite the fact that Cystopus was the first name given to this fungus, and therefore undoubtedly the correct one, American workers have for many years used the name Albugo. The confusion is increased by the fact that the genus is now included in a separate family of the Peronosporales under the name Albuginaceae, in which the type genus is, according to European workers, Cystopus.
