Lecture - 11 Deuteromycotina: Fusarium
Sub-division: Deuteromycotina:- It includes 1 class -
Class - Deuteromycetes:-
> Deuteromycetes occur mostly as saprophytes on a wide range of substrates, but a large number of them are parasites on plants and animals (including humans) and cause a variety of diseases.
Leaf- spots, blights, blotch, wilts, rots, anthracnose, etc. are the important diseases of plants, while diseases like meningitis, candidiasis, skin diseases, nail diseases, dermatomycosis as ringworms, athlete’s foot, etc. occur in animals (including humans).
> The mycelium is made up of well-developed, profusely branched and septate hypha that possess multinucleate cells and simple pore septa.
> The hyphae may be inter- or intracellular, and their cell wall chiefly contains chitin-glucan.
> Deuteromycetes reproduce only asexually. The asexual reproduction may take place by hyphal fragments, budding (common in Blastomycetes), arthrospores (flat-ended asexual spores formed by the breaking up of cells from the hypha), chlamydospores (thick-walled modified cells functioning as resting spores), or most commonly by conidia or conidiospores (nonmotile spores formed externally on the surface of hyphae or on specialized hyphal branches called conidiophores).
> The cell of conidiophore that produces conidia is called conidiogenous cell and the conidia may be produced either at the tip or side of the conidiogenous cell either singly or in chains.
> The conidiophores are either frees from one anther (mononematous) or they may be aggregated to form specialized structures such as synnemata and sporodochia.
In large number of Deuteromycetes, the conidiophores are formed in more specialized and organized fruiting layers present within the specialized fruiting bodies called conidiomata (sing. Conidioma; formerly called conidiocarps). The conidiomata may be acervulus or pycnidium.
> Sexual reproduction lacks, but a parasexual cycle or parasexuality generally operates in their life to fulfil the requirements of sexuality.
> Classification:- It includes 3 sub-classes -
1. Blastomycetidae:-
2 Orders:-
a. Sporobolomycetales:- Eg.- Sporobolomyces
b. Cryptococcales:- Eg.- Cryptococcus
2. Coelomycetidae:-
2 Orders:-
a. Sphaeropsidales:- Eg.- Sphaeropsis, Phoma, Septoria
b. Melanconiales:- Eg.- Melanconium, Colletotrichum
3. Hyphomycetidae:-
2 Orders:-
a. Moniliales:- Eg.- Alternaria, Cercospora, Trichoderma, Helminthosporium, Fusarium, Drechslera
b. Agonomycetales:- Eg.- Rhizoctonia
Fusarium:-
1. Classification:-
Kingdom:- Myceteae
Division:- Amastigomycota
Sub division:- Deuteromycotina
Class:- Deuteromycetes
Sub-class:- Hyphomycetidae
Order:- Moniliales
Family:- Tuberculariaceae
Genus:- Fusarium
2. Habit and Habitat:-
> Fusarium is represented by large number of species which occur both in temperate and tropical regions of the world.
> Majority of the species are saprophytic, some are mild facultative parasite while some species are parasitic and cause serious diseases like wilt or ‘rot’ of economically important plants.
3. Thallus Structure:-
> Mycelium is branched, sepate, hyaline or coloured, inter-or intracellular and uninucleate to multinucleate.
> Hyphae invade the tracheids and vessels of xylem, ramify there, produce toxic substances and block them completely. As a result the plants wilt and die.
4. Reproduction:- It takes place by the formation of asexual spores. These are:
a. Micro conidia
b. Macro conidia
c. Chlamydospores
a. Micro conidia:-
> The micro conidia and macro conidia are produced in same sporodochia. These sporodochia develop on the surface of stem, leaves and other parts of the host plant.
> The fungal mycelium collects near the surface of the host tissue as anpseudoparenchymatous mass. It gets exposed by the rupturing of the epidermis.
> From the fungal hyphae arise many short and cylindrical structures. These are conidiophores.
> The ultimate branches of conidophores which produce conidia are called phialides.
> The phialides are subulate i.e. owl shaped and have some kind of heel (characteristic of Fusarium).
> Micro conidia are small, unicellular or bi-celled, spherial or oval in shape.
> Their size varies from 5×15 µ x2−4 µ.
> They are borne single or in chains on the conidiophores by abstraction method.
> Micro conidia are produced in vast numbers.
> They are easily disseminated by wind and after falling on the suitable substratum, they germinate and infect the host plants.
b. Macro conidia:-
> The macro conidia are long, sickle or crescent shaped, multi-septate (3-5 septa), pointed at the end and broad in the middle.
> They measure 15-50 µ in length and 3-5 µ in breadth.
> Macro conidia are produced in vast numbers.
> They are easily disseminated by wind and after falling on the suitable substratum, they germinate and infect the host plants.
c. Chlamydospores:-
> Under relatively starvation and dry conditions, the mycelial hyphae produce ovoid or spherical thick walled cells. These are called chlamydospores.
> They occur either single or in chains and may be terminal or intercalary in position.
> After maturity they get separated from the parent hyphae and act as resting spores, under favourable conditions they germinate by means of germ tubes to form a fresh mycelium.
> The pseudoparenchymatous mycelium of Fusarium often forms black, compact bodies known as sclerotia. They act as storage organ and also serve as means of perennation and vegetative reproduction.
5. Economic Importance:-
> Fusarium oxysporum causes the most important vascular wilt diseases. It has several specialised forms that infect a variety of host plant.
> Some species of Fusarium produce mycotoxins − Fumonisins and trichothecenes. It is said that these toxins may cause oesophageal cancer. Some of these toxins are said to have been used as biological war fare agents in Vietnam and Afghanistan.
> Few Fusarium species like F. solani, F. Verticilloides, F. proliferatum, F. solani infect human nails (dry rot of nails) and eye. Fusarium contaminated wheat flour when eaten, immune system is weakened (neutropenia).
> F. grwninearum has been used in U.K. to produce a high quality mycoprotein that can be fabricated into a number of meatless food. F. venenatum is produced industrially for use as human food quom in Europe and north America.
