Classification of Erysiphe (With Diagram)| Fungi

In this article we will discuss about the classification of erysiphe.

Family Erysiphaceae:

The family is very well-known on account of its abundance everywhere, its simpli­city of structure, and its possession of typical ascigerous and conidial stages. Members of the family are easily recognizable since they form a coating of white conidia, coni­diophores and mycelium upon the surface of the host producing a powdery appearance by infecting leaves (Fig. 240A), stem, flowers and fruits.

They are largely known as powdery mildews. The powdery mildews, except Erysiphe graminis which infects grasses, are usually limited to dicotyledonous angiosperms. They produce powdery mildew disease.

Except perhaps the powdery mildew of grape-vine caused by Uncinula necator, the powdery mildews induce very mild infection and cause little damage to the hosts. Most of the genera have a limited host range with a large number of physiological races.

Some of the important members of the Erysiphaceae are: Erysiphe graminis is widespread on wheat, oat, barley, rye and grasses; Erysiphe cichoracearum seriously attacks cucurbits, Sphaerotheca humuli causes powdery mildew of hops and Sphaerotheca pannosa attacks roses; Phyllactinia corylea has world-wide distribution and has very wide host range including plants of economic importance.

Geographically the Erysiphaceae are widely distributed, practically of world distribution, but they are more abundknt in the temperate zones than elsewhere. Working with Sphaerotheca humuli, Harper (1895) traced for the first time the development of antheridia and ascogonia, and the cytological features that accompany nuclear fusion and nuclear division during the develop­ment of asci and ascospores.

The mycelium of the Erysiphaceae is usually hyaline and is branched, septate and its cells are uninucleate. The mycelium is hyaline when .young and greyish-brown when old. It is entirely external to the host tissues except for that of Phyllactinia. The mycelium fastens to the host epidermal cells by haustoria.

The haustoria may be grouped in three general categories:

(i) Those arising directly from the lower surface of the mycelium;

(ii) Those arising at the side of the mycelium as small semicircular pro­cesses; and

(iii) Arising from more or less deeply-lobed lateral swellings of the mycelium. In most species the haustoria are bulbous and uninucleate (Fig. 240C). In certain species of Erysiphe they are ellipsoid, with digitate processes at the ends (Fig. 240F).

In Phyllactinia corylea some hyphae of the mycelium remain external and do not develop any haustoria, and the rest of the hyphal branches enter into the host through the stomata. When they come in contact with the mesophyll cells they send haustoria in them to obtain-nourishment from the host cells (Fig. 240E).

This is how the entire mycelium receives nutrition. Again in Uncinula salicis the haustoria may even pene­trate the cells of the hypodermis (Fig. 240D). Some members of this family perennate as mycelium.

A few days after infection, several of the hyphae grow erect, perpendicular to the host surface. These are the conidiophores, each of which cuts off cells one after the other. The cells are short, at first cylindrical, then become barrel-shaped and finally, when ready for dissemination become elliptical and are then known as conidia.

The conidia arise in basipetal succession on simple un-branched, scattered conidiophores. They are produced in chains (Fig. 240G to M).

The conidia are hyaline smooth and thin-walled, and one-celled (Fig. 240N). They vary greatly in size with nutrition and other conditions. The conidia are short-lived summer spores. Conidial production goes on at a quick rate and they are produced in profuse quantity which is responsible for giving the powdery appearance on the infected plant parts.

The name powdery mildew very appropriately describes the white, mealy appearance of affected plant parts like leaves, stems, flowers, and fruits.

The whiteness being imparted by the pro­fuse growth of external mycelium and conidia. With favourable conditions each conidium germinates by germ tube which ultimately gives rise to new mycelium.

Sexual reproduction occurs at the end of summer, when conidial production slows down and eventually ceases. The white powdery appearance of the host surface now changes to greyish or brown shade, and the hyphae prepare to form ascocarps which are cleistothecia. The cleistothecia are large enough to be visible to be naked eye as black dots on the infected host surface (Fig. 240B).

Sexual reproduction involves the production of recognizable ascogonium and antheridium, at their point of contact, a passage is developed by the dissolution of walls. The antheridial nucleus passes through this passage into the ascogonium where plasmogamy takes place which is followed by the development of ascogenous hyphae and ultimately asci and ascospores are developed within the cleistothecium.

Some species are homothallic, others heterothallic.

The cleistothecium is a small more or less spherical structure bearing appen­dages which are commonly simple, curled or branched at the tip, in a manner charac­teristic of each genus. The function of these appendages is rather dubious, but their structure furnishes a useful taxonomic character. They serve by hygroscopic move­ments to aid in the distribution of the fungus.

The cleistothecium contains one to several asci which arise in one or more tufts from the base.

The asoi are globose to ovoid, and may have short’ stalk. The genera Podosphaera and Sphaerotheca produce only one ascus in each cleistothecium. The asci of the Erysiphaceae bear two to eight ascospores. Members of the Erysiphaceae survive the winter as ascospores in the asci developed in the cleistothecium, and in the spring both cleistothecium and asci absorb water and swell.

The cleistothecium cracks open, and the asci discharge the ascospores which on germination on a suitable host produce new mycelia. The origin and structure of cleistothecia were, for the first time, traced and described by De Bary in 1863.

In Sphaerotheca humuli sexual organs originate at the points where two hyphae cross or come into contact with each other. Each of the adjoining hyphae sends out an upright branch; the one enlarges and becomes club-shaped forms the ascogonium: the other remains more slender is the antheridium, both are cut off by septa.

The antheridium comes into close contact with the ascogoniym, and grows up with it. The antheri­dium soon overtops the ascogonium and bends over its apex.

Both the organs are uninucleate structures. The ascogonium enlarges and the antheridium lengthens. The antheridial nucleus divides into two daughter nuclei and simultaneously a septum appears which divides the antheridium into an uninucleate stalk cell and an uninucleate terminal cell which is the antheridium proper.

At the point of contact of the antheri­dium and ascogonium, the common walls dissolve resulting an opening through which the contents of the antheridium pass into the ascogonium.

This is followed by plasmo­gamy between the protoplasm of antheridium and that of ascogonium. Harper (1895) believed that karyogamy of the antheridial nad ascogonial nuclei takes place in the ascogonium. But researches of Bergmann (1941) reveal that there is no such nuclear fusion in the ascogonium. The two nuclei form h dikaryon in the ascogonium and undergo mitotic division there.

The ascogonium then elongates and becomes divided by transverse septa into a 3- to 5-celled structure, whose binucleate penultimate cell becomes the ascus mother cell. The ascus mother cell enlarges to develop into an ascus. Its two nuclei fuse to form a diploid nucleus which by three divisions, of which one is reductional, produces eight haploid nuclei, around each of which an ascospore is developed.

While the ascus development is going on, the hyphae next below the ascogonium send out several branches which grow rapidly around the ascogonium, completely envelope it forming a thick wall, the peridium.

From the inner cells of the peridium thus formed, short branches filled with very dense protoplasm grow inwards and apply themselves closely to the ascus, probably supplying it with food. The cells supplying nourishment to the ascus are the nurse cells. The outer cells of the peri­dium become thick-walled, and form a dense protective layer.

Some of its superficial cells grow out into long filamentous appendages. The cleistothecium is subglobose and bears mycelioid appendages. At first its colour is white but turns black at matu­rity. They ascus absorbs water, swells up, and bursts the cleistothecium, whereupon its own membrane dehisces at the top, and the ascospores are violently shot into the air.

Nature and stages of development of cleistothecium, ascus and ascospores have been indicated in Figure 191.

In Erysiphe, sexual organs are constricted from the erect branch-tips of two adjacent but distinct hyphae. The ascogonium is large, ovoid in shape, while the antheridium is smaller and more slender, but each contains one nucleus only (Fig. 241A & B).

The tips of these organs come together and plasmogamy takes place in the ascogonium when the antheridial protoplasm comes in contact with that of the ascogonium through an opening that is developed by the dissolution of walls at the point of contact of the antheridium and the ascogonium (Fig. 241C). The male and female nuclei then divide mitotically.

The ascogonium develops to a multinucleate tube-like structure which divides simultaneously into several cells, the sub terminal one of which is binu­cleate (Fig. 241 D to F). This binucleate cell gives rise to one to several ascogenous hyphae, the cells of which give rise to asci. Karyogamy and the development of ascospores take place in a process similar to that of Sphaerotheca humuli.

By the time the asci were being developed, sterile branches have arisen from the base of the asco­gonium surrounding it, closely intertwining to form the cleistothecial peridium. The outer surface of the peridium eventually becomes dark-brown, and develops long filamentous mycelioid appendages.

The inner layer of tissue forms a source of food for the asci which are developing in the cleistothecium (Fig. 241G). The cleistothe­cium bears more than one asci, in each of which 2 to 8 ascospores are developed. The asci and ascospores are liberated out by the rupture of the cleistothecial wall (Fig. 241H to J). Life cycle of Erysiphe is presented in Figure 242.

The family Erysiphaceae is subdivided into six genera depending on the nature of appendages and the number of asci in the cleistothecium.

The genera are:

Sphaerotheca with mycelioid appendages and one ascus in the cleistothecium (Fig. 243B), Erysiphe bears mycelioid appendages having several asci in the cleistothecium (Fig. 243A), Podosphaera possesses appendages having tips dichotomously branched and one ascus in the cleistothecium (Fig. 243E), Phyllactinia having appendages with bulbous base and several asci in the cleistothecium (Fig. 243D), Microsphaera possesses appendage tips dichotomously branched and several asci in the cleistothecium (Fig. 243F), and in Uncinula appendage tips are coiled and cleistothecium bears several asci (Fig. 243C).

Some Indian species of Erysiphaceae:

Erysiphe acaciae Blumer; E. communis Wallr. ex. Fr.; E. galeopsidis DC.; E. graminis DC.; E. polygoni DC.; Microsphaera alni ( Wallr.) Salm.; Phyllactinia guttata (Fr.) Lev.; P. heterophragmitis Ramakr.; P. leucotricha (Ellis and Everh.) Salm.; P. terminaliae Ramakr.; P. tridactyla (Wallr.) De Bary; Sphaerotheca euphorbiae (Cast). Salm.; S. macularis (Wallr. ex Fr.) Cooke; S. pannosa (Wallr.) Lev.; Unicinula necator (Schw.) Burr.; U. salicis (DC.) Wint.; U. tectonae Salm.