Phyllum Thallophyta - The Algae - Ascomycetes - Erysiphales
The Erysiphales are Ascomycetes which are characterized by an abundant mycelium,
which is often dark coloured. The sex organs are enclosed within a tissue termed the perithecium, which is without an ostiole. The asci are formed together in a group from a regular layer in the base of the perithecium.
Many of them cause the White or Powdery Mildews or Blights on flowering plants, where they live as parasites on the leaves. They are almost cosmopolitan in distribution, but have been most commonly found in Europe and North America.
Consider two examples of this order, Sphaerotheca humuli and Erysiphe polygoni because of the marked difference in the methods of sexual reproduction.
Sphaerotheca humuli (The Hop Mildew)
This Fungus is the cause of Hop Mildew or Mould and is one of the most serious diseases of the Hop. It chiefly affects the leaves and the young strobili, rendering the latter useless. The intensity of the disease varies from season to season and is best controlled by dusting the hop bines with finely powdered sulphur.
The life-cycle begins with the germination of either the asexual conidiospore or the sexual ascospore upon the surface of the host plant. A series of hyphae are formed which spread over the surface of the leaf, producing a white web-like coating with uninucleate cells. From these hyphae slender branches penetrate the cuticle and grow down into the epidermal cells, forming ramifying branches within the cells by which food is absorbed. These special cells are termed haustoria, and are the only part of the Fungus which enters into the host plant. Penetration by the haustoria is secured in a peculiar manner. The mycelial hyphae send out short branches which apply themselves very closely to the epidermis of the host and spread out into an adhesive disc called an appressorium. From the centre of this disc appears a new and extremely fine outgrowth which pushes through the cuticle like a needle-point. On entering the cell this outgrowth enlarges into a normal-sized hypha and branches freely.
ASEXUAL REPRODUCTION
During the summer months reproduction is asexual by means of conidiospores. The conidiophores are short, upright, unbranched hyphae which abstrict from their upper ends single chains of large oval conidiospores.
SEXUAL REPRODUCTION
Sexual reproduction in Sphaerotheca hu.muli occurs in the late summer by the formation of perithecia, which are spherical structures 0.05 to 0.3 mm. in diameter and furnished with simple appendages. The perithecium is formed by a mass of hyphae which aggregate into a tiny ball. It is white at first, but as it matures it turns clear yellow, and finally brown.
The sexual organs arise as lateral branches from the mycelium. The ascogonium arises from a cell which divides to form a stalk cell and an oogonial cell, the latter being uninucleate. The antheridium is smaller and is also borne on a narrow stalk cell. It also contains a single nucleus. Antheridium and ascogonium stand side by side in close apposition. The wall between the two sex organs breaks down, and the male nucleus enters the ascogonium and unites with the female nucleus. Nuclear division follows and the zygote cell divides into a row of cells, the penultimate cell of \yhich contains two nuclei. This row of cells is equivalent to an ascogenous hypha. From the penultimate cell the ascus is produced. \Yhile this is going on, hyphae form around the developing sex organs, producing the wall of the perithecium. The two nuclei in each ascus then fuse, and immediately the zygote nucleus divides into four, during which reduction occurs. This is followed by a further mitotic division which, according to some workers, is also reductional. Thus eight nuclei are formed in each ascus and around them walls are formed producing the eight ascospores.
It will be seen from this account that there may be two fusions, one in the ascogonium itself and a second in the ascogenous hypha, and hence, to bring the chromosome number back to the monoploid, two reduction divisions are necessary. According to many workers a nuclear fusion does not occur in the ascogonium, but there is merely an association of nuclei, and they consider that the only true fusion occurs in the ascogenous hypha. If this is the case there is only one reduction division, namely the first division of the nucleus in the ascus.
During development the wall of the perithecium becomes differentiated into inner and outer layers, which can be easily distinguished. The inner layer is made up of thin-walled cells, rich in protoplasm. It forms a packing round the developing asci and supplies them with food. The function of the outer layer is for protection. Its cells have scanty contents and their walls undergo a change analogous to lignification.
From special cells of this layer the characteristic filamentous appendages are differentiated. These appendages consist of long, single-celled outgrowths which show various degrees of complexity in different genera. Their shapes are quite characteristic and are chiefly used as a basis of the separation of the genera.
The life-cycle of Sphaerotheca humuli is represented in the following diagram.
In the classification of the Powdery Mildews we have a clear and simple example of the principles of plant classification, and it may be worth while to mention it briefly here. The genera fall naturally into two groups; firstly those with only one ascus in each perithecium, and secondly those in \yhich more than one ascus is produced. In the former section there are two genera:
Sphaerotheca, in which the appendages are long and hyphal-like, and Podosphaera, in which the ends of the appendages are repeatedly and dichotomously branched. In the second group are four common genera: Erysiphe, in which the appendages are like those of Sphaerotheca; Microsphaera, in \yhich they closely resemble Podosphaera; Uncinula, in which the tips of the appendages curl inwards; and, finally, Phyllactinia, in which they are straight and stiff with a swelling at the base. The following diagram brings out these points clearly.
Erysiphe polygoni (The Pea Mildew)
This species causes a Mildew of Peas and also of Swedes and Turnips.
In the former case it may attack the pods as well as the leaves. Swedes and Turnips are often attacked in a dry summer to such an extent that the leaves are turned completely white by the mycelium.
The Fungus spreads oyer the surface of the host plant, sending down fine haustoria into the epidermal cells where they spread out forming absorptive organs. During the summer months reproduction is by means of conidiophores which abstrict conidiospores in chains. These are entirely similar to those in Sphaerotheca both in their development and form, and we need not consider them here. In fact it is only in the method of formation of the sexual reproductive organs that this species displays an interesting and important difference.
SEXUAL REPRODUCTION
Initially there is a conjugation between short branches of two adjacent hyphae, with the result that a fusion cell is formed. There is no evidence that any nuclear fusion occurs, though the nucleus of one hypha passes into the other. This stage is followed by formation of additional cells, which are budded off the fusion cell, or central cell. The cross walls of these cells are not closed, and the whole structure forms a coenocyte or maze, as it is sometimes called. Meanwhile, additional tissue grows up surrounding this maze and forming the inner and possibly the outer wall of the perithecium.
After a while the cells of the maze break down to form a cavity in the centre of the perithecium, while by the formation of additional septa the cells of the perithecium, which were originally multinucleate, become progressively uninucleate.
From near the base of the perithecium there then grow up into the cavity, several cells each of which is binucleate. Their nuclei fuse, and in this way a number of asci are produced. The fusion nuclei divide meiotically, followed by a second and third division to form the eight nuclei of the ascospores, around each of which the spore wall is ultimately formed.
When mature the whole perithecium is blown away by the wind, and may become attached to a fresh leaf by the appendages. Here the wall of the perithecium breaks down and the asci burst, liberating th.e ascospores, which germinate, forming fresh hyphae. It is generally found that the ascospores are not formed until the spring, since the perithecium oyerwinters on the ground among dead grass, development stopping in the autumn at the formation of the asci.
BIOLOGIC RACES
Recent work has shown that in Erysiphe graminis there are several biologic races, that is to say that different strains exist, which are morphologically indistinguishable but among which it can be shown that the strain \yhich attacks one species of grass will not infect any other grass. Similarly, cultivated cereals each have their own particular strain. A point of some interest is the existence of bridging species among the host plants of the biologic strains of E. graminis. Thus the strain which infects Bromus racemosus will not attack B. commutatus directly, but it will attack B. hordeaceus, and an infection from the latter species will attack B. commutatus. No explanation is forthcoming as to what advantage the fungus deriyes from this very specialized restriction of possible host plants. We shall, however, meet with the same specialization in Puccinia graminis.
Similar biologic races occur in the types we have described. In Sphaerotheca humuli there is a biologic form which occurs on the cultivated strawberry, while in the United States there is a form which attacks roses. In Erysiphe polygoni the form which attacks Peas will not affect either Swedes or Turnips, while only the race on the latter plant is able to attack other species of Brassica, especially Kale. In England, however, this latter form rarely produces perithecia.
