Fungi are of large numbers and diversity, only a classification of some of the most important plant pathogenic genera is presented here.
They are as follows:
1. Fungus-Like Organisms or Pseudofungi (The ‘Lower Fungi’):
Kingdom – Protozoa:
“Unicellular, plasmodial, colonial, very simple multi-cells, phagotrophic, i.e., feeding by engulfing food.”
Out of four protozoan phyla, namely, Dictyosteliomycota, Acrasiomycota, Myxomycota, and Plasmodiophoromycota, the first two lack plasmodia and are informally called cellular slime moulds and the last two form plasmodia and hence are plasmodial slime moulds. Some members of Plasmodiophoromycota are important pathogens.
Phylum – Plasmodiophoromycota:
Contains a single class, the Plasmodiophoromycetes; obligate endoparasites of plants having minute plasmodia.
Some common pathogens and diseases caused by them are the following:
i. Plasmodiophora brassicae – Clubroot of crucifers
ii. Polymyxa graminis – Parasitic on wheat and other cereals, can transmit plant viruses
iii. Spongospora subterranean – Powdery scab of potato tubers
Kingdom – Stramenophila (Chromista):
“Unicellular or multi-cellular, filamentous or colonial, primarily phototrophic, some with tubular flagellar appendages or with chloroplasts inside the rough endoplasmic reticulum, or both contains the brown algae, diatoms, oomycetes, and some other similar organisms.”
Three chromistan phyla, the Oomycota, the Hyphochytriomycota, and the Labyrinthulomycota have been studied by mycologists. Oomycota contains very important plant pathogenic fungi and is taken here.
Phylum – Oomycota:
“Nonseptate (coenocytic) elongated mycelium, glucans as chief constituent of cell wall, cellulose present in minority, majority of species lack chitin, produce biflagellate zoospores with one tinsel and other whiplash flagellum in zoosporangia, sexual spores (oospores) produced by the contact of morphologically different gametangia called antheridia (male) and oogonia (female) that show gametangial meiosis (i.e., meiosis in nuclei of gametangia).”
The phylum contains a single class, the Oomycetes, with nine orders. Plant pathogenic oomycetes mostly belong to orders Saprolegniales and Peronosporales. A very interesting feature has been found occurring in Oomycetes is “gametangial meiosis” rather than “zygotic “.
The sex organs are diploid and their nuclei undergo meiosis before fertilization resulting in haploid nuclei that fuse forming diploid zygote, i.e., the whole life of the fungus is diploid, the haploid condition prevails only in the antheridial and oogonial nuclei after meiosis. This is why the Oomycetes, in their nuclear life cycles, function more like higher organisms than do any of the other fungi.
Some common pathogens and diseases caused by them are the following:
i. Aphanomyces euteiches – Root rot of peas, radishes, and sugar-beets.
ii. Pythium spp. – Damping off of seedlings, seed decay, root rots, and cottony blight of turf grasses.
iii. Phytophthora infestans – Late blight of potato.
iv. Phytophthora cinnamomi – Root rot of trees and shrubs.
v. Phytophthora megasperma – Root rot of vegetable crops and trees.
vi. Phytophthora palmivora – Black pod of cocoa.
vii. Phytophthora parasitica – Black shank of tobacco.
viii. Plasmopara viticola – Downy mildew of grape.
ix. Peronospora parasitica – Downy mildew of Brassica.
x. Peronospora pisi (P. viciae) – Downy mildew of pea and other legumes.
xi. Peronospora tabacina – Blue mould of tobacco.
xii. Bremia lactucae – Downy mildew of lettuce.
xiii. Pseudoperonospora cubensis – Downy mildew of cucurbits.
xiv. Peronosclerospora philippinensis – Downy mildew of maize (corn).
xv. Peronosclerospora sacchari – Downy mildew of maize (in tarai area of U.P. and around Delhi), and sugarcane.
xvi. Peronosclerospora sorghi – Downy mildew of sorghum (jowar).
xvii. Sclerospora graminicola – Downy mildew (green ear) of pearl millet (bajra).
xviii. Sclerophthora rayssiae – Brown stipe downy mildew of maize (corn).
xix. Albugo Candida – White rust (white blisters) of crucifers.
2. The True Fungi (The ‘Higher Fungi’):
Kingdom – Fungi:
“Achlorophyllous (heterotrophic absorptive organisms), typically mycelial (sometimes unicellular as in yeasts), walls chiefly contain chitin.”
There are four phyla, the Chytridiomycota, the Zygomycota, the Ascomycota, and the Basidiomycota. An informal category, the mitosporic fungi (Deuteromycetes), members of which lack sexual phase and have not yet been assigned to taxa in the formal system of classification.
I. Phylum – Chytridiomycota:
The phylum consists of a single class, the Chytridiomycetes. “Each of the zoospores or planogametes possesses a single, posteriorly placed, whiplash (smooth) flagellum.”
Members of this phylum are commonly called “chytrids”. Plant pathogenic chytrids occur only in one order, the Chytridiales, out of five proposed; members of the other orders live as saprophytes. They consist of unicellular and holocarpic vegetative bodies (thalli) that develop within the parasitized cells of host plants. Asexual reproduction is monotonously similar and terminates in zoospores produced within the sporangium.
Released zoospores serve as source of infection (inocula). Sexual reproduction occurs in some thalli, which transform into gametangia. The protoplasm of the entire thallus (gametangium) is fragmented producing planogametes which are morphologically alike to zoospores but function differently. Two planogametes of compatible nature fuse to form zygote (biflagellate), which, like zoospores, acts as infection inoculum.
Some common pathogens and diseases caused by them are the following:
i. Olpidium brassicae – parasitic in roots of cabbage and other plants; can transmit plant viruses
ii. Physoderma maydis – Brown spot of maize (corn)
iii. Synchytrium endobioticum – Wart disease of potato
iv. Urophlyctis alfalfae – Crown wart of alfalfa
II. Phylum – Zygomycota:
“Nonmotile asexual spores (aplanospores) produced in sporangia, zoospores not formed, zygospore (large resting sexual spores) produced within zygosporangium formed by the fusion of two equal or unequal gametangia.”
The phylum consists of two classes, the Trichomycetes and the Zygomycetes. Trichomycetes are the obligate parasites of arthropods. Plant pathogenic members are few and belong to class Zygomycetes. Plant pathogenic zygomycetes are usually weak parasites. They grow mostly as saprophytes on dead or processed plant products, and even when they infect living plant tissues, they first attack injured or dead plant tissues.
These fungi exhibit well-developed coenocytic mycelium, produce non-motile spores (aplanospores) asexually within sporangia, and their sexual reproduction is accomplished by fusion of two morphologically similar or dissimilar gametangia resulting in the formation of a zygosporangium containing a zygospore, a thick-walled resting spore. It has been customary to use the term zygospore to refer to the entire zygosporangium.
But, the recent ultra-structural studies have clearly demonstrated that the two structures are quite different. Zygospores are the structures enclosed within the wall of zygosporangium; each zygosporangium containing a single zygospore.
Some pathogens and diseases caused by them are the followings:
i. Rhizopus stolonifer – Soft rot of fruits and vegetables
ii. Mucor recemosus – Storage rots of a variety of fruits and vegetables
iii. Choanephora cucurbitarum – Soft rot of flowers and fruits of cucurbits
III. Phylum – Ascomycota:
“Sexual spores, called ascospores, are produced inside (endogenously) within sac-like structures referred to as asci (sing, ascus) after karyogamy and meiosis.”
No class is assigned to phylum Ascomycota; Order is the highest taxon. Hawksworth et al. (1945) have recognized 46 orders which are arranged in alphabetical sequence due to lack of reliable data regarding their phylogenetic relationship and evolution. The orders are based on the development of ascomata (the fruiting bodies), and especially the structure and method of discharge of the asci.
The vegetative body (thallus) of plant pathogenic ascomycetes consists of septate, well- developed, slender or stout, and profusely branched mycelium. Most of the ascomycetes have a sexual stage (teleomorph) and an asexual stage (anamorph).
The asexual spores are usually the conidia produced either singly or in chains at the tips of conidiophores arising from the mycelium. The conidia are produced after primary infection, act as the secondary inoculum, and cause all subsequent infections during the growing season.
Sexual reproduction is accomplished on or in the infected parts of the host only at the end of the growing season or when the food supply shortens. It is done usually with the help of sex organs – antheridium, the male sex organ, and ascogonium, the female sex organ. Ascomycetes do not undergo karyogamy directly after plasmogamy; the two are interrupted with a short-lived dikaryophase wherein the two compatible nuclei remain in close association.
However, asci (sing, ascus) containing ascospores are the final products of sexual reproduction. Generally, these are the ascospores that act as the primary inoculum and cause primary infections in the next growing season. The asci in plant pathogenic ascomycetes are produced, singly or in groups, in fruiting bodies called ascomata (=ascocarps), sing, ascoma (= ascocarp); Taphrina deformans and T. maculans causing peach leaf curl and leaf spot of turmeric, respectively, being exceptions wherein the asci are naked, they are not produced inside ascomata (= ascocarps).
Depending upon the morphology and development, the ascomata (sing. ascoma; = ascocarps) of plant pathogenic ascomycetes can be categorized into four basic types. In some (example, powdery mildews), the ascoma is completely closed with a distinct peridium (wall) and is called cleistothecium. In others (example, Claviceps spp. and others), the ascoma opens through a narrow channel (ostiole) and consists of a distinct peridium (wall) and is called perithecium.
The loculoascomycetous ascomycetes (example, Elsinoe causing anthracnose of grape and raspberry, and scab of citrus) the asci are formed directly in cavities within a stroma (matrix) of mycelium, and the cavities do not bear any definite peridium (wall); this type of ascoma is called pseudothecium or ascostroma.
There are plant pathogenic ascomycetes (example, Rhytisma acerinum causing tar-spot of maple leaves; Sclerotinia sclerotiorum causing soft-rot of vegetables) in which the ascoma is fleshy, saucer-shaped structure with distinct peridium (wall) and is called apothecium.
Some important pathogens and diseases caused by them are the following:
i. Protomyces macrosporus – Stem galls of coriander
ii. Taphrina deformans – Leaf curl of peach
iii. Taphrina maculans – Leaf spot of turmeric
iv. Erysiphe polygoni – Powdery mildew of peas
v. Erysiphe graminis – Powdery mildew of cereals
vi. Blumeria graminis – Powdery mildew of cereals and grasses
vii. Erysiphe cichoracearum – Powdery mildew of cucurbits
viii. Sphaerotheca fuliginea – Powdery mildew of cucurbits
ix. Uncinula necator – Powdery mildew of grapevines
x. Podosphaera leucotricha – Powdery mildew of apple
xi. Leptosphaeria sacchari – Ring spot of sugarcane
xii. Sclerotium oryzae – Stem rot of rice
xiii. Sclerotium cepivorum – White rot of onion and garlic
xiv. Claviceps purpurea – Ergot of cereals
xv. Claviceps microcephala – Ergot of pearl millet (bajra)
xvi. Claviceps oryzae-sativae – False smut of rice
xvii. Venturia inaequalis – Scab of apple
xviii. Sclerotinia sclerotiorum – Rots and wilts of vegetable crops
xix. Phyllachora graminis – Leaf spots on grasses
xx. Mycosphaerella musicola and M. fijiensis – Sigatoka disease of banana
IV. Phylum – Basidiomycota:
“Sexual spores, called basidiospores, are produced outside (exogenously) on characteristic club-shaped or tubular structures called basidia (sing. basidium) after karyogamy and meiosis.”
The phylum Basidiomycota consists of three classes – Teliomycetes, Ustomycetes and Basidiomycetes. Class Teliomycetes includes the most renowned group of plant pathogens, the rust fungi (Order – Uredinales) and the Septobasidiales. Class Ustomycetes consists of smut fungi (Order – Ustilaginales). Class Basidiomycetes includes a great number of the other basidiomycetous fungi. Plant pathogenic basidiomycetous fungi consist of septate, well-developed, profusely branched mycelium constituting the vegetative body (thallus).
The mycelium usually passes through three distinct stages of development:
(i) The primary mycelium is the one that develops from basidiospore and is monokaryotic;
(ii) The secondary mycelium is dikaryotic and is characteristically given rise to by the fusion of primary mycelium. Here, a long-lived dikaryotic condition is observed between plasmogamy and karyogamy, and
(iii) The tertiary mycelium is represented by the organized, specialized tissues composing the fruiting bodies, if developed, called basidiomata (= basidiocarps).
The asexual stage, the conidial stage, is much reduced relative to that of ascomycetes. Conidia occur commonly, however, only in smut fungi and are usually absent elsewhere in the class. Production of sex-organs is almost absent (sex-organs, namely, spermatia and receptive hyphae are known only from one order called Uredinales, the rust fungi) and the sexual reproduction accomplishes by means of somatogamy (transfer of cytoplasm along with nucleus from one vegetative cell to the other).
Sexual reproduction in plant pathogenic basidiomycetes culminates in the production of club-shaped or tubular basidium (also called ‘promycelium’) exogenously bearing basidiospores. Only the plant pathogenic basidiomycetes belonging to rot causing group (orders – Polyporales, Tulasnellales, and Agaricales) develop fruiting bodies; the fruiting bodies are absent in rusts (order – Uredinales; except few species) and smuts (order – Ustilaginales). The fruiting bodies are called basidiomata (= basidiocarps) and its singular being basidioma (= basidiocarp).
The rust fungi do not grow saprophytically in nature (in vivo) and are obligate parasites, although some of them have now been grown on special culture media in the laboratory (in vitro). They attack only certain host genera or only certain varieties and have caused famines and ruined the economics of large areas, including entire countries.
The smut fungi also do not grow saprophytically in nature and exist almost entirely as parasites on their hosts, they can be grown in culture on artificial media in laboratory. The smut fungi are considered causing serious grain losses that are equal to, or second only to, the losses caused by the rust fungi.
Some common pathogens and diseases caused by them are the following:
i. Puccinia graminis tritici – Black or stem rust of wheat
ii. Puccinia striiformis – Yellow or stripe rust of wheat
iii. Puccinia recondita – Leaf or brown rust of wheat
iv. Uromyces ciceris-arietini – Rust of chickpea (gram)
v. Uromyces fabae – Rust of pea and lentil
vi. Uromyces phaseoli typica – Rust of beans
vii. Uromyces phaseoli vignae – Rust of cowpea
viii. Hemileia vastatrix – Leaf rust of coffee
ix. Melampsora lini – Rust of linseed (flax)
x. Gymnosporangium juniperi-virginianae – Rust of cedar-apple
xi. Phakopsora pachyrrhizi – Rust of soybeans
xii. Ustilago tritici – Loose smut of wheat
xiii. Ustilago hordei – Covered smut of barley
xiv. Ustilago nuda – Loose smut of barley
xv. Ustilago scitaminea – Smut of sugarcane
xvi. Ustilago avenae – Smut of oats
xvii. Ustilago maydis – Smut of maize
xviii. Sphacelotheca sorghi – Grain smut of sorghum
xix. Sphacelotheca cruenta – Loose smut of sorghum
xx. Sporisorium reilianum – Head smut of sorghum and maize
xxi. Tolyposporium ehrenbergii – Long smut of sorghum
xxii. Tilletia tritici and T. laevis – Common bunt, stinking smut or hill bunt of wheat
xxiii. Neovossia indica (Tilletia indica) – Karnal bunt of wheat
xxiv. Neovossia horrida – Bunt of rice
xxv. Entyloma oryzae – Leaf smut of rice
xxvi. Urocystis tritici – Flag smut of wheat
xxvii. Urocystis cepulae – Smut of onion
xxviii. Armillaria mellea – Root rots of forest and fruit trees
V. Informal Group Mitosporic Fungi (Deuteromycetes):
“Represents an artificial assemblage of all those fungi (mostly of ascomycetes and rarely of basidiomycetes) whose sexual (teleomorphic) stage is thought to have been disappeared during the course of evolution, or is still unknown.”
Fungi which reproduced exclusively by asexual mean, usually by conidia (mitospores), and which are not the asexual stages of any ascomycete or basidiomycete, are called mitosporic fungi. Previously, these fungi were called ‘deuteromycetes’ (imperfect fungi) and were placed under sub-division Deuteromycotina but their being an artificial assemblage was always known; they were never considered equal in status to sexually reproducing fungi. Hawksworth et al. (1995) have de-recognized the sub-division Deuteromycotina as a formal taxon and have discarded the traditional hierarchical divisions into ‘orders’ and ‘families’, which is now universally accepted.
Almost all plant pathogenic mitosporic fungi (deuteromycetes or imperfect fungi) consist of septate, well-developed, profusely branched mycelium constituting the vegetative body (thallus). Like most of the plant pathogenic ascomycetes, the deuteromycetes produce enormous number of asexual spores especially the conidia. Conidia are produced either singly or in chains usually at the tops or sides of characteristic hyphal branches called conidiophores. Conidiophores are also produced either singly or in groups. When produced singly on somatic hyphae, they are called mononematous.
When produced in groups with each conidiophore free from base to apex, they are called fasciculate. Sometimes, the conidiophores are produced in a group in which the individual conidiophores are united at the base and part way up the apex; such conidiophores are called synnematous and the specialized structure containing them as synnema (pi. synnemata).
In other cases, however, the conidiophores are produced radiating from a central cushion-shaped stroma and are packed tightly together; such specialized structure bearing conidiophores is called sporodochium (pl. sporodochia).
In great many plant pathogenic deuteromycetes, the conidia and conidiophores are found produced within more specialized fruiting bodies called conidiomata (= conidiocarps).
The well-developed conidiomata (sing. conidioma) are generally of three types:
(i) Acervuli are flat or saucer-shaped formed usually from the compact mass of hyphae, containing short conidiophores growing side by side on the hyphal bed, and developing mostly sub-cuticularly or sub-epidermally;
(ii) Pycnidia are usually globose or flask-shaped opening mostly through a pore called ostiole, and
(iii) Stromatic conidiomata (= conidiostromata) are usually the cavities or locules developed in mass of hyphae, the conidiophores are present along the inner wall or cavity or locule.
Some common pathogens and disease caused by them are the following:
i. Alternaria solani – Early blight of potato
ii. Alternaria brassicola, A. brassicae and A. raphani – Leaf spot of crucifers
iii. Alternaria triticina – Leaf blight of wheat
iv. Cercosporidium personatum and Cercospora arachidicola – Leaf spot or tikka disease of groundnut
v. Drachslera oryzae – Brown leaf spot of rice
vi. Magnaporthe grisea (Pyricularia grisea) – Blast of rice
vii. Colletotrichum falcatum – Red rot of sugarcane
viii. Colletotrichum capsici – Ripe fruit rot and die back of chillies
ix. Colletotrichum lindemuthianum – Mango anthracnose
x. Colletotrichum gloeosporoides – Bean anthracnose
xi. Fusarium udum – Wilt of pigeonpea (arhar, red gram)
xii. Fusarium oxysporum f. sp. varinfectum – Wilt of cotton
xiii. Fusarium oxysporum f.sp. lini – Wilt of linseed (flax)
xiv. Fusarium oxysporum f.sp cubense. – Fusarium wilt or panama disease of banana
xv. Ascochyta rabiei – Ascochyta blight of chickpea (gram)
xvi. Macrophomina phaseoline – Charcoal rot of soybean
xvii. Rhizoctonia solani – Rhizoctonia stem canker and black scurf of potato.
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