Morphological features and classification of fungi of the order Erysiphales

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Representatives of the group mainly form perithecia, less often cleistothecia. Among this group, there are orders whose representatives cause dangerous plant diseases.

Order Erysiphales, or Powdery Mildew Fungi (Erysiphales)

In the developmental cycle, they form a cleistothecium and conidial sporulation. All species are obligate parasites causing diseases known as powdery mildew. Representatives of the order Erysiphales include the causal agent of powdery mildew of cucurbits — Sphaerotheca fuliginea, and many others. The classification into genera within the order is based on the morphology of the mycelial appendages of the cleistothecium and the number of asci in the fruit bodies.

The following features are characteristic of powdery mildew fungi:

* mycelium is multicellular, develops superficially on the parasitized tissues; only haustoria penetrate into the cells; infection occurs over a wide range of relative humidity values. The promycelium of the germinating spore penetrates directly through healthy tissues. Symptoms of diseases include a white powdery bloom on young aerial organs or only on leaves (upper and lower parts of the blade). The bloom consists of superficially located mycelium and conidial sporulation in the form of oval unicellular conidia joined in chains. Conidia cause re-infection of plants. Over time, cleistothecia form on the mycelium in the form of dark brown or black dots, visible to the naked eye. Asci with ascospores form within the cleistothecia. They ensure the survival of the pathogen under unfavorable conditions. Conidium ® o _ I 10800 0 SA A SH O o in G eH in ad 1 U 2% | ‚ sse-AD eh |7\ yy |) 5 em omalida G 5 \\ | Phialides. -Y C Prophialides p: ‹ n —= 7 Conidiophore p. 5. a Supporting /]: cell cell ud and | / Genus Erysiphe. There are many asci in the cleistothecium; mycelial appendages are simple. The most well-known representative is one of the powdery mildew pathogens of cucurbits — Erysiphe cichoracearum Fig. 43. MYCELIUM Genus Leveillula — leveillula. Two types of mycelium: (1 AND CONIDIA primary — endophytic, with suckers penetrating into Sphaerotheca. adjacent cells deep into the substrate tissue, and secondary — (ectophytic, without suckers, forming thick white felt on the surface of the substrate. Conidiophores protruding from stomata emerge singly or in groups from the endophytic mycelium in the form of a thin colorless hypha, with a single, apical, colorless, elongated-elliptical conidium. [] The presence of intracellular (endophytic) mycelium in the ® fungi of this genus allows them to survive in hot conditions and [7 [) at low air humidity. The conidial stage of the fungi [) develops until the end of the growing season of plants, and only in autumn 7] do cleistothecia appear. They have a rounded shape with a x d [) strongly depressed apex and numerous appendages \ D located in the lower part of the fruit body. There are many asci, U ke. 2 in them, two spores usually develop. Species of this genus cause extremely harmful ga diseases — powdery mildew of certain crops, including E E R tomato. \

Genus Uncinula — uncinula. The conidial stage of this fungus is called Oidium. Mycelium is ectophytic, with suckers. Conidiophores are simple, colorless, in the form of very short branches of mycelium. Conidia are barrel-shaped, elliptical, or almost cylindrical, forming in chains. The species U. necator causes powdery mildew of grapes.

The order Erysiphales also includes the genera Podosphaera, Microsphaera, etc.

Pyrenomycetes also include the order Hypocreales, which causes fusarium wilt of plants (in the ascus stage).

Group of orders Discomycetes Form an open fruit body — an apothecium. It can be cup-shaped, saucer-shaped, or funnel-shaped, sessile or stalked. The conidial stage, as a rule, is absent. The most important is the order

Genus Sclerotinia [syn.: Sclerotinia] sclerotinia. The conidial stage is absent. The most well-known is the fungus S. sclerotiorum, causing sclerotiniosis, or white rot of cucumber and a number of other crops. The affected tissue becomes watery, then covered with a white cottony bloom of mycelium. Gradually, it settles, densifies, and large dark sclerotia are formed, in the form of which the fungi survive. From them, later, either mycelium or an apothecium (fruit body) grows. Cylindrical asci with unicellular colorless ascospores form on the apothecia. { C \ Subclass Loculoascomycetidae 2 In representatives of this subclass, a true fruit body is absent. and № Asci form in special cavities — locules, which, in their turn, are located in mycelial formations — pseudothecia. Many loculoascomycetes, in terms of the degree of parasitism, are classified as facultative I saprotrophs. Usually, the ascus stage is finally formed on m || [% dead plant debris, but the conidial |} v. | stage can also persist, through which secondary infection of growing plants occurs. \ || 7 In greenhouses, the pathogen of cucumber scab, Cladosporium cucumerinum, occasionally causes damage. u || ‹ | \ Ue-nita cucumerum. GV \ ь 7 yyy Ii rei OOO sclerotiorum

A feature of basidiomycetes is the formation of basidia and basidiospores upon the completion of the sexual process, which is based on heterothallism. The structure of the fungi, their way of life, and the nature of the damage are diverse. Among them are saprotrophs, hemiparasites, and obligate parasites. Based on the type of basidia and the place of their formation, three subclasses are distinguished: Holobasidiomycetes (or Homobasidiomycetes), Heterobasidiomycetes, and Teliomycetes. Only representatives of the last subclass cause damage in greenhouse conditions.

The subclass Teliomycetes includes two orders: Ustilaginales and Uredinales. The symptoms of diseases called rust can be varied, but most often they are pustules of a rusty or yellowish-brown color. Rust fungi have a complex development cycle. The full cycle consists of three stages and five spore-bearing phases: I - spring, or aecial stage. In spring, after the plant is infected by heterothallic basidiospores, a haploid mycelium of its sexual sign (+ or -) develops. Spermogonia are formed on this mycelium.

The resulting spermia with different sexual signs fuse and form a dikaryotic mycelium. Aecia are then formed on it, and aeciospores within them.

II — summer, or uredostage, when urediniospores develop in uredinia. During the growing season, several generations of urediniospores develop, which repeatedly re-infect the plants.

Development stages and classification of rust fungi

Morphology of the teliostage and biological cycles of rust fungi

III — autumn-winter, or teliostage, during which teliospores are formed in teliopustules; these germinate to form basidia with basidiospores. They usually have a dark color, which makes the teliopustules appear dark brown or almost black.

Rust fungi overwinter as teliospores. After overwintering, each cell of the teliospore germinates into a basidium, on which four basidiospores are formed.

According to their development cycle, fungi are divided into the following groups:

• In some fungi (autoecious), all stages occur on the same plant.
• In others (heteroecious), the cycle of development necessarily involves a change of host plants. The plant on which the spring (aecial) stage develops is called the intermediate host. The summer and winter stages develop on the primary host.

Genus Puccinia. Pathogens of this genus infect some flower crops, for example, Puccinia horiana — white rust of chrysanthemum.

Genus Phragmidium. In greenhouses, a disease such as rust of roses is sometimes encountered, caused by

Genus Uromyces. The most well-known disease is rust of carnation, caused by Uromyces caryophyllinus.

Characteristics of the class Fungi Imperfecti

The class Fungi Imperfecti, or Deuteromycetes, is the most representative group of fungi. The class unites fungi with multicellular mycelium that develops only to the haploid stage. Conidial sporulation performs the functions of preserving and spreading the fungi.

Some fungi develop a sexual stage, represented by ascosporic or basidial sporulation, but it does not play a significant role. Some species of imperfect fungi do not even have conidial sporulation; they develop as sterile mycelium.

The vast majority of pathogens in this class belong to hemiparasites (facultative parasites and facultative saprotrophs) that cause rots, leaf spots, wilting, coatings, ulcers, etc.

According to the type of conidial sporulation, deuteromycetes are divided into the following orders:

• Hyphomycetes;
• Melanconiales;
• Sphaeropsidales, or Pycnidiales;
• Fungi that do not form sporulation are classified into the order Mycelia Sterilia.

Sporulation develops directly on the mycelium forming on the surface of the affected plants and has the appearance of a coating. The order includes a great many genera, the division of which is based on the morphology of conidiophores and conidia (branching, septation, shape, color, etc.).

Below is brief information about the pathogens of fungal diseases belonging to this order.

Genus Alternaria — alternaria. Within this genus, there are both parasites and saprotrophs. A characteristic feature of the morphology of this genus is simple, unbranched brown conidiophores with a long chain of club-shaped conidia with transverse and longitudinal septa and a concave neck; however, many pathogenic species in the parasitic state on plants form conidia not in long chains, but in groups of 1-2. Based on this trait, fungi were previously divided into two genera: Macrosporium — parasitic species, and Alternaria — more often saprotrophs. In new systems, the former has been excluded from the nomenclature. However, there is no clarity in the names of diseases; in some publications, "macrosporiosis" remains, in others "alternariosis". Several species are common in greenhouses. The most well-known disease is tomato alternariosis, caused by Alternaria solani. Genus Botrytis — botrytis. Saprotrophs or parasites on plants. Colonies usually have a powdery or mealy surface. The vegetative mycelium is spreading, permeates the substrate, and often develops as a more or less dense coating on its surface. Conidiophores are erect, more or less branched, rarely simple, sometimes slightly swollen at the upper ends, rounded or with knotty projections, with small tooth-like sterigmata. Conidia are located on the latter, usually clustered in heads. They are usually colorless, smoky or brownish, sometimes with an olive tint. Single-celled conidia are gathered at the ends of the conidiophores into heads of spherical, ovoid, elliptical, or slightly elongated shape; colorless or more or less dark-colored, sometimes with stalks. A number of species form sclerotia. The most frequently encountered is B. cinerea, the pathogen of gray mold of cucumber, tomato, cabbage, lettuce, and other crops.

- Ur A Fig. 46. CULTIVAR V. stegeae ON AGAR MEDIUM. | } A | 0 u A | Iv r 0110 |] |1 Genus Cercospora. Conidiophores in bundles, conidia long, spindle-shaped, colorless, with several septa. Pathogens of Cercospora leaf spot of beet, spinach, sorrel, etc. Occasionally, in greenhouses, the pathogen Cercospora capsici is found, causing Cercospora leaf spot of pepper. Genus Cladosporium. For these fungi, conidial sporulation in the form of a small tree is characteristic, the branches of which consist of spores. The trunk of the tree is formed by brownish or pale-olive conidiophores, usually erect, septate, unbranched, more rarely with 1-2 branchlets. Cylindrical conidia extend directly from the conidiophore, which in turn give rise to short cylindrical spores, and from them, unicellular egg-shaped conidia bud off. Conidia of this fungus are constantly present in the air. Dangerous plant pathogens are C. fulvum (pathogen of brown leaf mold tomato) and C. cucumerinum (pathogen of cucumber scab). Genus Helminthosporium. For fungi of this genus, the shape of the conidia is characteristic. They are dark-colored, straight or slightly curved, cylindrical, spindle-shaped with several transverse septa. Conidia are formed at the apex and along the sides of the conidiophore. In most species, a sexual stage has not been found. In greenhouses, they occur extremely rarely, causing root rot of seedlings. Genus Fusarium. Mycelium white, white-pink, red, light-cream, straw-yellow, grayish, lilac-purple, or brownish. Conidia colorless, multicellular, often of two types. Macroconidia spindle-shaped or sickle-shaped, narrowed at both ends m “A ci G E %, A 7 M: \ r, ‚ \ 7 A ax Ur > I = m i Ch d A E UK i” k. eo A <. u N y | \\ y 1+. \ ya N |] an | | | | TSIN SHCHI A AN AS Fig. 48. SPORULATION Fig. 50. CULTIVAR Fusarium sp. ON AGAR Helminthosporium. k: Bases with a more or less pronounced foot or papilla, more rarely without a foot. ro Upper cell short, conical, beak-like, sometimes more or less rounded or elongated, gradually or suddenly narrowing, sometimes thread-like, usually with 3-5, rarely with a larger or smaller number of septa. Conidia are formed in the aerial mycelium, often in a continuous layer on a stroma in sporodochia or in pionnotes — mucous masses on a network of hyphae or directly on the substrate, light-colored in mass. Small one- and two-celled conidia (microconidia) are formed in the mycelium singly, in false heads, or in chains on conidiophores. In shape, they more often resemble an elongated ellipse, more rarely spherical, egg-shaped, pear-shaped, club-shaped, or spindle-shaped. Chlamydospores in hyphae are single, in chains or nodules, intercalary or terminal, sometimes in macroconidia, colorless or of various yellow-brown shades. Sometimes white, yellow, brown, purple, or blue sclerotia are present. Fig. 51. MACRO- AND MICROCONIDIA Fusarium oxysporum.

Known sexual stages belong to the genera Nectria, Calonectria, Gibberella, Hypomyces.

Genus Phialophora. A distinctive feature of this group of imperfect fungi is the method of conidia formation. The cell wall of the conidium is formed anew; the wall of the conidiogenous cell does not participate in its formation. Such spores are called phialospores, and they are formed on phialides, which are cells thickened at the base and slightly drawn out in the upper part.

In greenhouses, P. cinerescens causes carnation phialophorosis.

Genus Stemphylium. Sexual stage — genus Pleospora. Fungi of this genus are close in biology and morphology to the genus Alternaria. Unlike the latter, chains of conidia do not form on the conidiophore, and there is no elongated apical cell. 3-5 conidia are located on the conidiophore.

Z Pycnidia of black color form on leaves, fruits, stems and (0 2990 other parts, predominantly of living plants, immersed or r» \® e^< 0. semi-immersed, sometimes almost superficial, scattered or clustered, (U em =. 4. varying in shape from lens-shaped and spherical-flattened to spherical, RU, d, round and spherical-conical, with a rounded ostiole. The wall of pycnidia e 23 de usually depends on the color of the substrate; delicate, thin-walled, almost transparent Suasya d or thick-walled, the lower part of pycnidia sometimes without a wall. Pycnospores with d one, rarely two septa, colorless. The most well-known

RE < G. disease is ascochyta blight of cucurbits, the pathogen of which is As. d cucumis. The sexual stage, which appears on dying plants, | o t belongs to the species Didymella bryoniae. Genus Phoma [syn.: Dothroma]. p O 1 Pycnidia are more often with an opening at the apex, sometimes with a rostrum or papilla,: ee E spherical or flattened. On the inner side of the wall, at the base and PC. vy ya PYCNIDIA AND PYCNO- on the sides, simple or branched SPORA Ascochyta conidiophores are located, producing conidia. Conidiophores are simple, arranged radially, but in some species, they are absent, and conidia develop directly from the inner cells of the pycnidial wall. Pycnidia are in most cases immersed, scattered, or clustered. Conidia are of various shapes, less than 15 μm long, colorless or slightly yellowish, unicellular. axes In greenhouses, black rot of tomatoes occurs, the pathogen of which

Genus Septoria - septoria. Conidial sporulation of spherical or pear-shaped form. It develops in pycnidia, which are spherical and embedded in the substrate; only a small part with an opening in the form of black dots emerges on the surface. Conidia, or pycnospores, are formed inside the pycnidia on their walls. They cause plant diseases with symptoms of dry rots. Fig. 57. PYCNIDIA AND SPORES. The spores are colorless, filamentous, and multicellular. In protected ground, septoria affects tomato and celery.

Genus Phomopsis - phomopsis. Ascogenous stage - Diaporthe. Fruiting bodies (perithecia) of dark brown or black color are formed on plant debris, on dead leaves, and feed saprotrophically. Asci are formed at the bottom of the perithecium and form a hymenial layer, however, paraphyses are absent. Upon ripening, the stalks of the asci dissolve, and in the mature perithecia, they are loosely located in mucus. The conidial stage (pycnidia), having the shape of spheres, parasitizes leaves, stems, and fruits. In protected ground, a disease is encountered that causes fruit and stem rot, the pathogen of which is Phomopsis vexans.

Order Mycelial or Sterile Fungi (Myceliales)

-They do not form sporulation. The life cycle includes sclerotia and vegetative mycelium. Genus Rhizoctonia - rhizoctonia. The fungus does not form sporulation. The life cycle includes sclerotia (a preservation form) and vegetative mycelium. The mycelium, in the form of felt-like cords with thin branches of violet, red, or brown color, envelops the roots of plants and penetrates the soil. Rhizoctonia solani - potato rhizoctonia is among the most well-known pathogens. The fungus leads a saprotrophic lifestyle in the soil; it also parasitizes the underground organs of plants, forming a more or less noticeable plexus and black crusty sclerotia on them, which are quite firmly attached to the substrate. Hyphae are brownish, in places almost colorless, 6-10 μm thick. The pathogen settles on the underground organs of tomato and other plants. It inhabits everywhere in regions with a humid and cool climate.

Genus Sclerotium - sclerotium. It has approximately the same life cycle as the genus Rhizoctonia. The most frequently encountered species are: Sclerotium cepivorum - the pathogen of basal rot of bulbs in vegetable and ornamental crops; S. rolfsii - the pathogen of southern sclerotial rot of bean, tomato, and other crops. Fig. 58. Rhizoctonia solani CULTURE ON AGAR MEDIUM.

1.4.4. General measures of plant protection against fungal infections

Adherence to crop rotation, regulation of climatic and soil conditions, and introduction into production of resistant plant cultivars and hybrids are general recommendations applicable to any type of infectious disease. Special chemicals intended for the control of fungal infections are called fungicides. These substances have different natures and different mechanisms of action. The most well-known and used are sulfur and copper salts, which have long been used to combat many fungal pathogens. Modern fungicides are generally characterized by specificity towards certain groups of pathogens and are capable of preventing the development of zoospores and conidia or inhibiting the development of mycelium. A distinctive feature of many preparations is the systemic nature of distribution throughout the plant, which facilitates their application and increases the effectiveness and duration of the protective action.

Besides chemical fungicides, there are also biological means of plant protection. Microorganism producers release toxins and antibiotics that have a suppressive effect on the enzymatic systems of pathogens. Certain microorganisms are capable of parasitizing directly on disease pathogens.

A special group consists of preparations that increase plant immunity and growth regulators, which in a number of cases allow for effectively restraining the development of diseases or reducing their harmfulness due to the stimulation of growth processes.

In recent years, increasing interest has been shown in work devoted to the development of induced immunity by incorporating foreign genes into the plant genome. In plant cells, in response to a pathogen attack, proteins are induced that determine non-specific or specific resistance. These proteins, formed in the presence of a pathogen, are usually called PR-proteins. Among them, chitinases, found in many species of higher plants, are the most studied. Chitinases hydrolyze the main component of the fungal cell wall (chitin), which determines the fungicidal activity of plants.

Modern methods of genetic plant protection

A clear example of the effectiveness of such techniques is the production of transgenic plants of tobacco carrying the chitinase gene of bean, which ensured them increased survival in soil infested with rhizoctonia.

Plants can be modified using introduced genes responsible for the synthesis of phytoalexins. The following approaches are applied:

• Introduction of foreign stilbene synthase genes, which are involved in the synthesis of the phytoalexin resveratrol, into the potato genome; this provides increased resistance to late blight and Fusarium wilt.
• Introduction of genes into the tobacco genome to ensure resistance to