Here is an elaborated discussion on Plant Pathogenic Fungi, highlighting:- 1. Introduction to Plant Pathogenic Fungi 2. Degree of Plant Pathogenic Fungi Parasitism 3. Biotrophs and Necrotrophs 4. Classification of Plant Pathogenic Fungi.
Introduction to Plant Pathogenic Fungi:
Fungi represent organisms that are “eukaryotic, achlorophyllous, spore-bearing, reproducing usually asexually and sexually, and their mostly filamentous branched vegetative structures are typically surrounded by cell wall made up of chitin or cellulose or both together with many other complex organic molecules.”
They are clearly delineated from the plants and other autotrophs because their vegetative body (or thallus), even when it forms tissues, is never differentiated into root and shoot and, most important of all, has no specialized vessels for internal transport of nutrients.
The vast majority of fungi are strictly saprophytic (obligate saprophytes) living on dead organic matter, which they help decompose. On the other hand, many fungi live as parasites and most of them cause diseases in humans, animals, and plants. More than 8000 fungal species, however, can cause diseases in plants. Almost all plants are attacked by some kinds of fungi, and each of the plant pathogenic fungi can attack one or many kinds of plants.
Though one can find mention of plant diseases caused by fungi in the pages of first recorded history, the scientific proof regarding the disease causing nature of fungi was first given by Prevost (1807) confirming Tillet’s (1755) earlier report that the bunt disease appeared severely in wheat plants grown from seeds that had been dusted with the black powder (spores) obtained from diseased kernels. Moreover, Prevost provided convincing evidence that the fungus is the cause, not the effect, of the disease.
A great majority of plant pathogenic fungi have a vegetative body consisting of more or less elongated, continuous, branched, microscopic filaments called mycelium (pl. mycelia). An individual branch or filament of the mycelium is called hypha (p. hyphae). The mycelium (or hyphae) is generally uniform in thickness and the length of mycelium may vary from a few micrometers to several meters.
The mycelium of primitive forms (called ‘lower fungi’) of plant pathogenic fungi is usually coenocytic (aseptate), i.e., the mycelium contains nuclei scattered more or less uniformly in the protoplasm, which is continuous throughout the length. The advanced forms of fungi (called ‘higher fungi’), however, consist of mycelium well differentiated into cells by means of septa (cross-walls) and each fungal cell contains one or more nuclei.
Degree of Plant Pathogenic Fungi Parasitism:
The plant pathogenic fungi, being achlorophyllous, are heterotrophs, i.e., they do not synthesize food by their own but depend on other sources for their food procurement. These heterotrophs develop either complete or partial parasitic relationship with their hosts during their life. A parasite is an organism that becomes intimately associated with other organism (host) and lives at the expense of the latter. The relationship between a parasite and its host is called parasitism. However, the plant pathogenic fungi adopt, varying degree of parasitism.
Some plant pathogenic fungi are such that they cannot live elsewhere except their living hosts and are called obligate parasites, i.e., host-parasite interactions are obligatory for these parasites. Other plant pathogenic fungi can live on either living hosts or dead organic matter in nature depending upon the circumstances and are, therefore, called non-obligate parasites.
Some non-obligate parasitic fungi live most of the time or most of their life-cycles as parasites but, under certain conditions, may live as saprophytes on dead organic matter and are referred to as facultative saprophytes, other non-obligate parasitic fungi live most of the time or most of their life-cycles on dead organic matter but, under certain conditions, may adopt parasitic mode and are, therefore, called facultative parasites.
Usually the degree of parasitism of a plant pathogenic fungus does not correlate with the severity of disease it causes on its host. Actually, disease severity is always determined by many other factors in addition to the parasitic ability of the pathogenic fungus. Many diseases caused by weak parasitic fungal pathogens (facultative parasites) are much more destructive to the host plants than those caused even by obligately parasitic fungal pathogens.
This is, most probably, because the facultative parasites are basically saprophytic and they liberate enzymes even after being parasitic on host plants, these enzymes disintegrate (kill) the host tissue in advance thus helping the pathogen to draw nutrients from dead cells. This however, makes the disease incidence quite severe.
Contrary to it, all the obligate parasitic fungal pathogens and most of the facultative saprophytic ones do not kill host tissue immediately after infection. They establish such a relation with their host, either by penetrating living cells and sending in the haustoria or by establishing close contract with them, that they can absorb nutrients continuously without killing the host cells.
Otherwise if host cells are killed, the obligate parasitic pathogen fails to obtain its nutrients and usually dies but, a facultative saprophyte turn to be a saprophyte in these circumstances and survives.
There are also examples of fungal pathogens that grow as saprophyte near the roots of plants and secrete certain toxic substances; the latter damage the roots and suppress their development thus causing short supply of necessary nutrients and water to plants, which makes them diseased.
Biotrophs and Necrotrophs:
Although there are an enormous variety of pathogens, an important distinction can be made between those which coexist with host tissues for an extended period of time without causing severe damage, and others which rapidly kill all or part of their host.
The former category, referred to as biotrophs, do not kill their host immediately as they are dependent upon living cells of the host for their development. Extreme biotrophy resembles mutualism in that it is difficult to discern any marked pathogenic effects.
In contrast, the pathogens that rapidly kill all or part of their host are called necrotrophs. The latter are often opportunistic and grow intracellularly producing cytolytic factors and utilize the dead host tissues as a resource. The ability to attack a living host distinguishes these organisms from the saprophytes, which subsist exclusively on dead organic matter.
The contrasting features of biotrophs and necrotrophs are summarized below:
Biotrophs:
1. Morphological and Biochemical Features:
(i) Host cells not rapidly killed.
(ii) Specific parasitic structures (e.g., haustoria) typically formed.
(iii) Host penetrated directly or via natural openings.
(iv) Few or no toxins or cytolytic enzymes produced.
2. Ecological Features:
(i) Host range narrow.
(ii) Cannot grow away from the host
(iii) Infect healthy hosts at all stages of development.
Necrotrophs:
1. Morphological and Biochemical Features:
(i) Host cells rapidly killed.
(ii) No any specific parasitic structure formed.
(iii) Host penetrated via wounds or natural openings.
(iv) Toxins and cytolytic enzymes produced.
2. Ecological Features:
(i) Host range wide.
(ii) Can grow saprophytically away from the host.
(iii) Infect juvenile, debilitated, or senescing tissues.
Necrotrophs may grow on both living and dead host tissues in nature. Pythium and Rhizoctonia, for instance, occur growing actively in soil, or on subterranean or aerial plant surfaces. If a suitable host is absent, these pathogens may successfully grow on dead organic matter. Biotrophs, in contrast, show either very limited or even no such ability to grow on dead organic matter.
These differences in patterns of natural occurrence are reflected in their growth on laboratory culture media. Most necrotrophs are nutritionally undemanding, i.e., they grow well on a wide range of simple media. Biotrophs, on the other hand, in extreme cases, cannot be grown on any known culture media in laboratory.
Biotrophs and necrotrophs represent the absolute categories and are accompanied by intermediate categories of continuous degradation. These categories are called Hemibiotrophs.
For instance, Phytophthora infestans fungus, which causes late blight of potato, exhibits a high degree of host specificity and other biotrophic characters, such as haustoria, but it also causes relatively rapid necrosis of invaded tissues. Some pathogens behave both as a biotroph and as necrotroph during their life cycle.
Venturia inaequalis, fungus that causes apple scab disease, grows beneath the cuticle of host leaves for many days without causing obvious necrosis, but as the lesions age the host tissues are eventually killed and the typical scabs manifest. Some species of Colletotrichum, the fungus that causes anthracnose, enter host cells by direct penetration. But, such host cells remain alive for several days and, subsequently, the necrotic lesions develop.
Classification of Plant Pathogenic Fungi:
Earlier, the true fungi (eumycetes) were classified into four classes – Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes. But, later on, it was realized by the mycologists that the fungi included in class Phycomycetes show great heterogeneity and, therefore, should be given separate treatment.
This resulted in the creation of nine classes instead of four, which have been Chytridiomycetes, Hyphochytridiomycetes, Plasmodiophoromycetes, Oomycetes, Zygomycetes, Trichomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes. Later on, fungi have been classified time to time in different ways during the last some decades.
The classification of fungi is the most recent one proposed by Hawksworth, Kirk, Pegler, Sutton, and Ainsworth in 1995. This classification first appeared in the 8th edition of Ainsworth and Bisby’s Dictionary of Fungi (1995). Hawksworth’s classification is now universally accepted and there is no going back.
According to it the organisms studied by mycologists are spread in three different kingdoms in the domain Eukarya. Some fungi, often referred to as the lower fungi, are now considered to belong to the kingdom Protozoa or to the kingdom Stramenophila Chromista. The true fungi often referred to as the higher fungi belong to the kingdom Fungi (earlier called Eumycota or Mycetae).
The taxon ‘Division’ is replaced by ‘Phylum’, a term earlier used only by zoologists and which has a suffix ‘mycota’. The fungi that cause diseases on plants are a diverse group.
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