Biology of fungal pathogens and classification of greenhouse crop disease agents

Biology and life cycle of fungal pathogens

The vegetative form of most fungi is represented by thread-like hyphae, which form mycelium when aggregated. The internal structure of hyphae can be cellular or non-cellular, being called septate and non-septate respectively. Fungal mycelium has various textures—from fluffy to compact; hyphae are sometimes combined into mycelial cords or rhizomorphs.

The diameter of hyphae and their growth rate depend on the fungal species and environmental conditions. For the growth of most greenhouse phytopathogens, the optimal temperature fluctuates within the range of 15—25 °С.

The color of a fungal colony, whether it is a saprophyte or a parasite, is often determined by the color of the spores it forms. Spores perform the functions of reproductive and dispersal organs of fungi; they are formed as a result of asexual or sexual processes.

Spores formed by vegetative mycelium as a result of simple cell division are genetically identical to the parent form, but since their quantity is very large, a very small fraction of mutants, genetically different from the parent hypha, sometimes appears among them. Mutants are very important for adapting the pathogen to a new environment, for example, to a certain fungicide or a resistant host plant. New traits of the pathogen can also be a consequence of sexual recombinations in the genetic material of both parent forms.

The structures forming sexual and asexual spores, as well as the classification of fungi infecting greenhouse crops, are presented below.

Classification of fungal pathogens of greenhouse crops

Continuation OR Group,. subgroup Class Genus d sh i Hyphomycetes (leaf spots, vascular wilts, molds, mushroom pathogens) Alternaria Botrys Chrysospinit Cercospora Cladosporium Chrysospory Corunnespora Clitarioscarpon Clitariophytum Pasillum Deuteromycetes Kua Fusarum Heterosporium Musovone Mycephora Myrotecium Raphospora Riuorphora Ratshana Usorhators Septocium Sethyfit T/ueau:0p!$ Trichoderma Sodit Verticillium Phvorhal (Mycelia Steria) spores themselves often possess species specificity, characterizing the species, and in combination with the vegetative characteristics of the mycelium, serve as the basis for the classification of the fungus. In general, asexual spores serve as the most important agents of epiphytotics, while sexual spores often facilitate the long-term survival of many phytopathogens. Fungi are divided into four main groups, and although a detailed description of their classification in this book is inappropriate, some of the main characteristics of each group are still worth mentioning. In addition to sexual and asexual spores, some fungi form other structures that facilitate their dispersal or ability to survive in unfavorable conditions. This function is taken over by individual hyphal cells; their walls thicken and a spore analogue is formed. Such cells are called chlamydospores, as opposed to aleuriospores—cells forming at the tips of hyphae. Mycelial aggregations also acquire resistance to unfavorable environmental conditions; they form into a very compact structure surrounded by a dark shell. Such bodies are called sclerotia.

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This group also includes fungi that exist as simple unicellular organisms and do not form mycelium. Recently, they are increasingly classified into a separate group, for example, the genera Plasmodiophora and Myxomycota. Sexual or asexual reproduction of these fungi occurs only in the presence of a stage

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Ascomycotina. This large group of fungi includes yeasts which, unlike other representatives, do not form mycelium. Almost all Ascomycotina have septate mycelium, the hyphae of which branch at an obtuse angle and sometimes at a right angle. This group includes many harmful pathogens, for example, the causal agents of powdery mildew (Sphaerotheca fuliginea), sclerotial and stem rots of many plants (Sclerotinia sclerotiorum, Gautieria species). The most typical for species of this group is the sexual spore formed in a sac-like structure — an ascus. Each ascus usually contains eight ascospores, which are formed in structures called ascocarps. These can be spherical cleistothecia with an inconspicuous opening (Pleospora spp. and powdery mildew fungi), flask-shaped perithecia (Mycosphaerella spp.), and disc-shaped open apothecia (Sclerotinia spp.). Ascospores are often ejected from the ascus, which is an important factor in disease development. Sexual and asexual spores germinate without a motile stage and form germ tubes, through which the fungus infects the host plant under favorable conditions. Asexual spores, which vary greatly in shape and size, are called conidia, and the stalks on which they are formed are called conidiophores. Sometimes they are also formed in special structures similar to those of Deuteromycotina.

Basidiomycotina. It is generally believed that this group unites the evolutionarily most advanced fungi, which constitute a very diverse collection of genera. The mycelium is septate, often with rectangular branching hyphae. In some Basidiomycotina representatives, the mycelium can be aggregated

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Deuteromycotina. This large and diverse group includes many phytopathogens, usually without a regular phase of sexual reproduction. Most representatives belong to ascomycetes. The mycelium is typically septate, often with branching at an obtuse angle. Asexual conidia are formed in various structures. One of the most typical structures for phytopathogenic fungi of this group is a usually brown or black pycni-

Phytopathogenic bacteria are typically very small (1–4 μm in length) unicellular spherical or rod-shaped organisms. Bacterial cells have a cell wall, but they lack an organized nucleus, although the cells contain RNA and DNA. Their growth rate can be very high, and reproduction occurs through simple division of the mother cell into two daughter cells. Many species of bacteria move in a liquid medium with the help of flagella attached to the cell polarly, i.e., at one end, or peritrichously, i.e., over the entire surface of the cell. Representatives of one group of phytopathogenic bacteria, Streptomyces, which form mycelium and chains of spores, resemble fungi. Another group is similar to rickettsiae, which are obligate parasites. Spores are formed only in some species, but for the majority of phytopathogenic bacteria, this process is atypical, and they can hardly survive for a long time under unfavorable conditions. At the same time, certain phytopathogenic species persist in dry plant debris and plant exudates for more than a year. Bacteria are classified according to their shape, reaction to Gram staining (Gram-positive or Gram-negative), and according to biochemical test data showing the ability of the bacteria to utilize such substrates as sugars (through fermentation, oxidation, or hydrolysis), complex carbohydrates (pectin, cellulose), and proteins.

The majority of phytopathogenic bacteria are Gram-negative and are classified as aerobes (requiring oxygen) or facultative aerobes (growing at low O2 concentrations). Such pathogens include species of Agrobacterium, Erwinia, Pseudomonas, and Xanthomonas. Gram-positive phytopathogens belong to the genus Corynebacterium.

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Bacteria are easily spread by irrigation, soil, or drainage water, by raindrops and splashes of irrigation water, and even by water aerosols during active overhead irrigation or spraying of plants. Other transmission routes include infected seed, soil, and vegetatively propagated diseased plants. Various insects (especially flies), humans (on clothing or tools), and even fungal spores released into the air can also act as vectors of bacteria from plant to plant.

For optimal growth, bacteria often require higher temperatures than fungi, and perhaps for this reason, bacterial plant diseases are especially dangerous in regions with warm climates, in subtropics and tropics. However, there are important exceptions here as well. Diseases such as bacterial canker of tomatoes (Corynebacterium michiganense), bacterial wilt of carnation (Pseudomonas caryophylli), leaf tumors and crown gall (Corynebacterium fascians and Agrobacterium tumefaciens) cause significant harvest losses in temperate climates. Despite high temperatures typical for greenhouse production, greenhouse crops are affected by a relatively small number of bacterial diseases.

Compared to fungi and bacteria, viruses have the smallest particles, visible only under an electron microscope. They lack a cellular structure; a virus consists of a nucleic acid surrounded by a protein coat. The nucleic acid determines the behavior of the virus (host plant range, virulence for pathogenic forms, and symptoms induced in the plant).

Multiplying within plant cells, viruses are able to use the host's metabolites to form a large number of viral particles instead of the plant's nucleoproteins. The question of whether viruses are living organisms has long been a subject of discussion. Naturally, viruses are well adapted to the environment in which they multiply; however, the majority of them are unable to maintain viability for long outside a living plant cell.

Viruses vary in shape and size. They can be rod-shaped, like the tomato mosaic virus, whose particles reach 300 nm in length (1 nanometer is one millionth of a millimeter) and 18 nm in width; flexuous, for example, the lettuce mosaic virus, with a size of 750x13 nm; spherical or isometric (sometimes up to a 20-sided icosahedron), like the arabis mosaic virus with a diameter of 30 nm. Finally, some viruses are bullet-shaped, for example, the lettuce necrotic yellows virus with particles 227x66 nm in size. Nucleic acids without a coat, called viroids, also cause diseases, for example, chrysanthemum stunt.

Viruses are transmitted from plant to plant in a wide variety of ways, although the mechanism of spread is usually specific to each pathogen species. For example, the tomato mosaic virus is characterized by a mechanical type of transmission, where even with a light touch to a diseased plant, a sufficient number of viral particles is transferred from destroyed leaf hairs to a neighboring healthy plant to cause infection. At the same time, the tomato mosaic virus, the most infectious of all phytoviruses, has no natural means of transmission, not even by sucking insects feeding on affected plants.