Biological characteristics and structure of phytopathogenic fungi – plant disease pathogens

How pathogenic fungi are structured and where they survive during the off-season

Phytopathogenic fungi are the most numerous group of plant disease pathogens in the field and greenhouse. They occupy an intermediate position between the kingdoms of plants and animals, feeding on ready-made organic matter and absorbing it through the entire surface of their hyphae. It is important for an agronomist to understand the biology of these organisms, since almost all agricultural crops are affected by one or another species of fungi.

The structure of the fungus body determines its ability to penetrate a plant and feed at its expense. In lower forms, such as the clubroot pathogen of cabbage, the vegetative body is primitive and consists of plasmodia or individual cells. More complex pathogens form mycelium — a branched network of tubular filaments (hyphae). To feed inside plant tissues, the hyphae form haustoria — specialized suckers that penetrate directly into the host cells.

According to the spectrum of affected crops, pathogens are divided into broad-specialized (infecting many plant species) and narrow-specialized (parasitizing one or several closely related species). The fungal mycelium can be located on the surface of plant organs (exogenous), be completely immersed in the tissues (endogenous), or semi-immersed.

To adapt to unfavorable environmental conditions, hyphae are modified into specialized structures:

• Chlamydospores — vegetative spores with a dense wall, formed during the breakdown of the mycelium.
• Gemmae — cells with thick walls of irregular shape and size.
• Oidia — thin-walled rounded cells that quickly germinate into new mycelium.
• Strands (cords) — fused parallel hyphae for the transport of nutrients.
• Rhizomorphs — dark, branching tangles of hyphae that serve for overwintering and spreading.
• Mycelial mats — suede-like tangles on the surface or inside the substrate.
• Stromata — dense sclerotial-type tangles on which or inside which sporulation organs are formed.

• Sclerotia humidity — 5–10 %
• Stages of sporulation in rust fungi — 5
• Species of phytopathogenic fungi — several thousand

Reproduction methods and development cycles: how to contain an infection

Fungi reproduce by vegetative, asexual, and sexual means. Vegetative reproduction occurs through parts of the mycelium or its modifications — oidia and chlamydospores. During sexual reproduction, the nuclei and cytoplasm of two different cells (gametes) fuse, forming a zygote. The fungal cell wall contains chitin, metabolic products contain urea, and glycogen is accumulated as a storage substance.

Asexual reproduction is carried out by spores (conidia), which are formed at the ends of conidiophores or in special mycelial receptacles. This method of reproduction is especially developed in Ascomycetes (sac fungi). Species that have completely lost the ability to reproduce sexually are grouped by scientists into the class Fungi Imperfecti.

For successful crop protection, it is important to understand exactly where the infection is preserved during the winter period:

• in undecomposed plant residues;
• directly in the soil;
• on seed and planting material;
• on tools and the inner surfaces of greenhouses.

The development of a pathogen represents a regular sequence of stages from a dormant form to new sporulation. The ability of a fungus to form different types of spores during a single life cycle is called pleomorphism. This process is most clearly observed in oomycetes.

1. A zoosporangium with motile zoospores is formed from the overwintering oospore.
2. Zoospores penetrate the tissues of the host plant and form mycelium in the intercellular spaces.
3. Sporophores with asexual spores appear on the surface of the affected leaves or stems.
4. Asexual spores are carried by wind and water, causing new infections of healthy plants.
5. At the end of the growing season, oospores are formed on the mycelium inside the wilting tissues for overwintering.

In rust fungi, the development cycle can be even more complex and include up to five stages of sporulation. Most phytopathogens complete this entire cycle on a single plant. However, some rust fungi require a rotation of two different host plants to complete their cycle.

Knowledge of fungal development cycles is crucial for developing protective measures against plant diseases. Preventive or eradicating measures aimed at reducing primary infection or completely eliminating it are effective against the dormant stages of the fungus. The spread of spores is suppressed or contained by chemical agents; the same can be achieved by creating conditions unsuitable for abundant sporulation and conidia germination, which can only be accomplished in a greenhouse environment.

Peculiarities of plant pathogenesis caused by fungi

Plant disease begins with the penetration of the pathogen into the cells or intercellular spaces of the host plant. To penetrate inside the cells, obligate parasitic fungi use a set of enzymes that break down the cellulose cell wall. Conversely, many facultative pathogens can only penetrate inside the plant through wound surfaces or damage inflicted on the host by other pests, pathogens, or mechanical impacts. The pathogen can be in a dormant form on the surface or inside the plant (e.g., in seed). Based on colonization characteristics, fungi are divided into three groups: non-pathogenic exophytic (ectophytic, epiphytic), living entirely on the surface of plant organs, and endophytic, living inside the tissues of the host plant. Three forms of endophytic phytopathogens are distinguished:

* wound — capable of penetrating inside plant tissues only through mechanical damage;

* primary biotrophs (or obligate) — capable of utilizing the contents only of living plant cells; they do not develop on artificial nutrient media;

* secondary — settling after the plant has been infected by some other organism.

It is known that the penetration of a fungus through a plant cell wall does not always result in the development of a disease, as it must subsequently colonize the plant tissues, which depends on many factors. The main and determining factors include environmental conditions, the susceptibility of the plant to the pathogen, and the aggressiveness of the fungus. For example, the spores of most fungi germinate in the presence of water droplets and at an optimum temperature, which is characteristic for each species. For instance, the optimum for active germination and development of Phytophthora zoospores is 11°, and for conidia — 16-20°. 1-4.3. Systematics of phytopathogenic fungi The systematics is based on the structural features of fungi, their reproduction, development cycle, specialization, and other biological characteristics. According to modern views, and based on historical and practical considerations, the concept of fungi includes some representatives of algae and protozoa (Müller, Löffler, 1995). Fungi are divided into three phyla: Slime molds, Heterokonts, and True fungi. All phyla are divided into classes, but only in the True fungi are there phytopathogenic species in the vast majority of classes. The systematics of fungi changes from time to time 1.4.3.1. Phylum Slime molds, or Myxomycetes (Myxomycota)

Recently, some systematists have removed this phylum from the kingdom of fungi. In this reference book, we have left the systematic position of this phylum, which unites fungi with a primitive organization, unchanged. The vegetative body of slime molds represents a plasma mass with a large number of nuclei (plasmodium), which does not have its own wall or a permanent shape. Subsequently, the plasmodium is transformed into a multitude of spores. They do not form special sporulation organs; the receptacle for spores is the cell wall. The sexual process follows the isogamy type, in which morphologically similar haploid zoospores of different sexes fuse to form a diploid plasmodium. Asexual reproduction is carried out by zoospores. Phytopathogens from the class Plasmodiophoromycetes are obligate intracellular parasites that cause hypertrophy (increase in size) of cells of various plant organs. Pathogens form dormant spores, which, after the decomposition of plant residues, end up in the soil and can persist in it for several years. Germinating zoospores infect the underground organs of plants.

Most other species belonging to the phylum of slime molds are saprotrophs that pose no danger to actively growing plants. Phytopathogens include representatives of two genera:

* genus Plasmodiophora. The most well-known representative of this genus is the agent of clubroot of crucifers through root hairs. Inside the root, myxamoebae are formed; they and their nuclei fuse, after which many mitotic divisions occur without the formation of new cell walls, which leads to the formation of plasmodia. genus Spongospora. The most well-known is the agent of powdery scab of potato Spongospora subterranea (solani), which affects not only potato tubers and stolons but also the roots of tomatoes. Multinucleate plasmodia are localized mainly in the covering tissues, where spongy clumps of tightly fused spores are formed. On the roots, the lesion has the appearance of small white nodules that eventually darken and disintegrate. In greenhouses, the disease is extremely rare.

This phylum unites species ranging from primitive aquatic organisms to highly specialized parasites of terrestrial plants. Of the representatives of the three classes of the phylum, only the class Oomycetes contains phytopathogenic species.

In representatives of this class, the vegetative body is a unicellular multinucleate mycelium consisting of well-developed, branching hyphae. In the autumn period, as a result of a sexual process of the oogamy type, dormant spores (oospores) are formed. Under favorable conditions, in the spring-summer period, oospores germinate, most often into zoosporangia, within which biflagellate zoospores are formed. In moisture droplets or along a surface water film, the zoospores move, penetrate plants, and cause primary infections.

Most members of the class Oomycetes are saprotrophic species, although parasitic species are also known. The division of the class into five orders is based on the morphological characteristics of the mycelium, zoosporangia, and oospores. Phytopathogenic representatives belong to the orders Saprolegniales and Peronosporales.

Order Saprolegniales. This order mainly includes species that live saprotrophically in water bodies or in moist soil. There are several species that parasitize plants. Among them is the pathogen causing seedling disease in certain plants — Aphanomyces cochlioides.

Most Peronosporales are parasites, and only a few species are saprotrophs. The order Peronosporales includes three families: Pythiaceae, Peronosporaceae, and Albuginaceae, the first of which is the most numerous.

On the branched mycelium of Peronosporales, specialized zoosporangiophores (conidiophores) are formed. Oospores are formed within the infected tissues and persist in plant residues and in the soil, where they remain viable for two years or more.

The family Pythiaceae is represented by species that often infect seedlings of weakened plants; some species parasitize algae.

| The mycelium is thin, and zoosporangiophores do not differ from hyphae. The zoosporangia formed on them germinate either as biflagellate zoospores or as mycelium, like conidia. Oospores are spherical. The viability of oospores is maintained for several years.

“ The most harmful are representatives of the genus Pythium. The genus Pythium. Members of this genus have thin, cobwebby mycelium with hyphae 3—6 μm thick. The fungi settle on the roots of plant seedlings and cause diseases known as root rot, damping-off of transplants, or Pythium root rot. The main symptoms of these diseases are thinning and blackening of the roots and the base of the stems. The disease develops under poor tillage, overcrowding of crops, and the absence of crop rotation. \ { | Species of this genus cannot compete in the soil with the saprotrophs living there, but upon contact with plant roots, they are able to penetrate them, causing a pathological process. The pathogenicity of most species is due to the action of toxins that are released by the fungi into the environment and kill plant cells. In addition, roots damaged by Pythium become gateways for other pathogens.

Fungi of the genus Pythium are difficult to isolate from soil samples because they are outcompeted by saprotrophs on artificial media; therefore, the baiting method is used to accurately assess the presence of these fungi. For this, a fruit, such as an apple, is buried in the soil. Pythium has an advantage over other soil fungi in colonizing living cells; thus, pathogens can be quite easily isolated from the sample.

The host range, lifestyle, and morphology of the species are similar, so accurate identification of the isolated fungi is difficult.

The genus Phytophthora includes more than 70 species of fungi with diverse feeding methods: from facultative parasites to facultative saprotrophs. A characteristic feature is the formation of more or less specialized sporangiophores. Zoosporangia are spherical, lemon-shaped, and egg-shaped. They can germinate as mycelium, i.e., function as conidia. At low positive temperatures, they germinate as zoospores.

The most dangerous representative of the genus is the fungus Phytophthora infestans, which infects tomatoes, potatoes, and some other Solanaceae. The mycelium is white, cobwebby. Zoosporangia are lemon-shaped. Zoospores are carried by water currents. Only dormant forms — oospores — persist in the soil. Most other species mainly infect the underground parts of plants. | 3, etc. The most well-known and harmful are such pathogens as P. infestans, which causes late blight of potato, and P. parasitica — one of the pathogens of root rots. Fig. 35. ASEXUAL SPORULATION

The family Peronosporaceae are obligate parasites that settle on living plants of any age at any time. A lilac-gray coating represents asexual sporulation, consisting of densely interwoven sporangiophores, the spores of which carry out the dispersal of the pathogen. Dormant oospores are formed inside plant tissues and can persist in plant residues, in the soil, in overwintering plant organs, or in seeds. The division of the family into genera is based on the morphology of sporangiophores and asexual spores. On agricultural crops, representatives of two genera most often parasitize — Peronospora and Plasmopara, less often Peronoplasmopara [syn.: Pseudoperonospora] and Bremia, species of which cause downy mildew of cucumber, lettuce, cabbage, and other crops.

Members of the family belong to obligate parasites that cause downy mildews (peronosporosis). The characteristics of these diseases are as follows:

The Albuginaceae family (Albuginaceae) are obligate parasites. The most common species is Albugo candida — the causative agent of white rust in cruciferous crops. 1.4.3.3. Phylum True fungi (Eumycota)

All representatives of this phylum possess a vegetative body in the form of mycelium, located on the surface of the nutrient substrate or inside it. Based on the type of sexual process and some other characteristics, fungi are grouped into six classes: Chytridiomycetes, Trichomycetes, Zygomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes. There are no plant disease pathogens among the Trichomycetes.

Class Chytridiomycetes (Chytridiomycetes) are represented by the order Chytridiales. Fungi of this order are obligate parasites. Disease development occurs under conditions of high soil moisture and in acidic soils.

Order Synchytriales of the cruciferous family.

Genus Synchytrium. The vegetative body of these fungi is poorly developed and can appear as a plasmodium or a rudimentary mycelium. The best-known is the potato wart pathogen — Synchytrium endobioticum. In the morphology of its vegetative body and the nature of its effect on the affected tissue, the potato wart pathogen is similar to plasmodiophorid fungi. On the affected potato tubers, as a result of uncontrolled cell division (a phenomenon of hyperplasia), growths are formed. Upon the onset of unfavorable conditions, cysts covered with a thick shell are formed, which can persist in the soil for a long time (up to 10 years or more). Fig. 38. Development of Olpidium brassicae in cabbage root cells...

The mycelium in Zygomycetes is well-developed, branched, multinucleate, and single-celled. The distinction lies in the presence of a sexual process, during which the fusing male and female cells are morphologically identical. This process is called zygogamy. After fusion, a zygospore is formed, capable of surviving in unfavorable conditions. In this class, only representatives of the order Mucorales (Mucorales) are facultative parasites.

Genus Mucor. Representatives of this genus are widely distributed in nature; in terms of nutrition, they are typical saprotrophs.

Colonies can be very low, up to 1 mm in height, velvety white or colored, with sporangia 20-60 µm in diameter, or tall, up to 100 mm in height, white or colored, with sporangia up to 300 µm in diameter. Sporangiophores are simple or branched; their branches do not differ significantly from the main axis. They all end in sporangia of the same size. Sporangia are round, without an apophysis, with a columella and a deliquescent wall (only a collar of the wall remains at the base). Most are represented by heterothallic forms, a few are homothallic. Mucorales are considered conditionally pathogenic species. They cause gray mold of seeds, bulbs, rhizomes, and fruits.

Genus Rhizopus. Representatives of the genus have a well-developed, branched, multinucleate, single-celled mycelium.

They most often form a cleistothecium, more rarely — a perithecium. Ascospores are released passively. This group of orders includes many saprotrophic fungi that develop on plant residues and stored products of plant origin. The most widely known are fungi of the genera Aspergillus and Penicillium. Among the Plectomycetes, there are also semi-parasites that cause diseases of agricultural crops and tree species.