Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales - Peronospora (The Downy Mildew)
There are a number of species of Peronospofa which occur on higher plants, all of which cause diseases known as the Downy Mildews. Most of these species occur on distinct host plants and differ from one another only in minor particulars of the size of the spores. Recent investigations have tended to split up certain of the older species. Thus the well-known form Peronospora parasitica, which is widely distributed on Cruciferous plants, has been split up into a number of separate species. One of the commonest forms of this is Peronospom cheiranthi, which occurs on wallflowers.
It lives as an obligate parasite within the tissues of its host plant and spreads by means of a branched mycelium, which consists of hyphae that ramify between the cells of the host. These hyphae do not penetrate into the host cells but only pass between them. They give off, however, lateral branches which enter the cells of the host and inside form branched or swollen structures which are called haustoria. These secrete enzymes, by the aid of which the protoplasm of the host cells is broken down and absorbed by the Fungus. Externally the Fungus causes a soft, white, downy appearance on the leaves of the host plant, often associated with twisting and stunting of the affected area.
Peronospora parasitica is commonly found on Capsella bursa-pastoris (Shepherd's Purse), frequently associated with Cystopus candidus (White Rust). Other species are known to attack many crops of economic importance, such as beet, maize, tobacco and turnips. A Fungus which, like Peronospora, only attacks a specific host is said to exhibit selective parasitism.
ASEXUAL REPRODUCTION
Peronospom reproduces both asexually and sexually. The asexual reproduction shows an advance on the condition found in Pythium, in that the reproductive bodies are specially adapted for aerial distribution. The organs are termed conidiophores, on which are borne specialized conidiospores. The conidiophores arise from the intercellular mycelium, emerge through the pores of the stomata, and become repeatedly and dichotomously branched. They may be to some extent septate. To the end of each of the branches there passes a single nucleus; a swelling occurs, and into this the nucleus passes. The swelling increases until a spherical or oval spore is formed, attached to the conidiophore by a short thin stalk or sterigma, which is water soluble, a fact which facilitates distribution of the spores in damp weather.
Only a very small proportion of the conidiospores produced by the Fungus ever come to rest on a suitable substratum, i.e., the leaf of another plant of the same species of host, under environmental conditions suitable for germination. The Fungus must therefore produce a very large number of spores on the chance that a few will find conditions where germination is possible. The presence of a water-soluble sterigma tends to ensure that spores shall only be shed in weather when the humidity is sufficiently high to render germination possible.
Under favourable conditions for germination the spore wall bursts and a germ tube is put out, which grows over the surface of the leaf until it comes to the pore of a stoma, when it immediately passes down into the leaf and forms the mycelium which, as we have already said, ramifies in the spaces
between the cells. Considerable investigation has been directed to the problem of what induces the growth of the germ tube into the stomatal opening and the growth of the young conidiophore out of it at a later stage. Chemotropism and hydrotropism have both been suggested, but the true explanation is still a matter of some uncertainty.
SEXUAL REPRODUCTION
Sexual reproduction is of an oogamous type, resembling in general that of Pythium. It takes place under conditions adverse to the Fungus and in the deeper layers of the host tissue, generally in the stem, and especially in the pith. The sex organs consist of antheridia and oogonia, and are formed terminally at the apices of either the same or of separate hypha. The oogonium is a swollen structure containing, at an early stage, a large quantity of cytoplasm and many nuclei. The antheridium is more tubular in shape, and it, too, is multinucleate.
As the oogonium swells a separation into a dense central oosphere and a less dense periplasm occurs, associated with changes in the position of the nuclei. Up to this stage they have been more or less evenly distributed through the oogonium, but with these changes in the cytoplasm one nucleus places itself centrally in the oosphere and functions as the female nucleus, while the rest pass into the peri plasm and eventually abort. The antheridium now comes into contact \yith the oogonium, and a fine fertilization tube is formed which penetrates through the periplasm and discharges a single male nucleus, together \yith a small amount of cytoplasm, into the oosphere. Conjugation of male and female nuclei takes place and a zygote is formed. Fertilization stimulates the secretion of wall material which occurs in the periplasm. The precise way in which this takes place varies in different species. In general, it appears that within the oogonial wall a very thick wall is laid down, forming the wall of an oospore, and enclosing the original oosphere. The periplasm may also contribute an exospore wall to the oospore.
The oospore is a resting spore, remaining unchanged until the decay of the host tissues, and after that it may remain in the soil for some months before germination takes place. Under favourable conditions it germinates by the bursting of the wall and the development of a germ tube. In cases where the Fungus parasitizes an annual plant the oospore is the means whereby the Fungus passes the winter months after the death of the host, and germination takes place the following spring. There is some doubt as to precisely where the reduction division occurs.
Some authorities claim that it takes place during the multiplication of nuclei in the formation of the oogonium and antheridium, while according to others it occurs in the first division of the germinated oospore, and on the whole this Control of Downy Mildew and other Mildews is generally effected by covering the plants with some substance which will kill the Fungus without damaging the host tissues. In recent years great advances have been made in the study of sprays and spraying machinery. Two chief types of sprays are employed, the one in \yhich a substance or substances are dissolved in liquid and sprayed on to the foliage with the idea of covering the whole surface of the host with a film of the solution. The second is by the use of extremely fine powders which are blown over the plant, relying on the dampness of the leaves to retain them. In the former case a great proportion of the liquid is lost, and what is known as a " spreader" is now employed to increase adhesion of the spray to the leaves. This may take the form of an oil, the whole solution being worked up into an emulsion. Where Downy Mildews are concerned the solution known as Bordeaux mixture is generally employed. It consists of a dilute solution of copper sulphate and lime, sometimes with the addition of paraffin oil.
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