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In this article we will discuss about:- 1. Genus of Fusarium 2. Mycelium of Fusarium 3. Reproduction.
Genus of Fusarium:
The form-genus Fusarium includes a large number of species and many forms within species. Many of these are saprobic or saprohytic. Some are only mild facultative parasites.
Others are parasitic. All, however, have a saprophytic stage. Some of the parasitic species are soil inhabitants and can live indefinitely as soil saprohytes but some are soil invaders. The latter soon die if they are not able to infect a suitable host plant.
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The parasitic form-species cause rot of stores fruits vegetables and other commodities and are responsible for dry rot of potato tubers of colocasia corms (Tandon al, 1956) or soft rot of rhizomes of ginger during storage.
Some are mild root parasites and some primarily cortical invaders which cause stem cankers, foot rots and pre-emergence damping off diseases.
The most destructive species, however, are:
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1. Fusarium udum which causes wilt of Arhar (Cajanus cajan).
2. F. lycopersi which causes wilt of tomatoes (Lycopersicum esculentum).
3. F. lini. It is a causative agent of wilt of Flax.
4. F. orthaceras. It causes wilt of gram (Cicer aurietinum).
5. F. vasinfectum. It produces wilt of cotton.
6. F. cubense. It is parasite on Banana and causes the wilt disease.
All the above-mentioned species are vascular parasites, and are thus often referred to as vascular fusaria. They are now placed under a single species Fusarium oxysporum as forms (Snyder and Toussoum, 1965).
Mycelium of Fusarium:
It is extensive. The hyphae are septate and branched. They are both intercellular and intracellular. When young they may be colourless or with a tinge of pink, purple or yellow and become dark-coloured at maturity.
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The dark mycelium produces thick bands which plug the vascular tissues (Fig. 16.6 A-B) and produce toxic secretions. The toxins are carried up in the xylem vessels. As a result the plant wilts and dies.
Reproduction in Fusarium:
Asexual Reproduction. (Fig. 16.7):
It takes place by the formation of three kinds of asexual spores, microconida, macroconidia and chlamydospores. Sclerotia are also formed.
a. Microconidia (Fig. 16.7):
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They are very small conidia abstricted from the tips of simple or branched conidiophores. The conidiophores are distinguishable from the vegetative hyphae.
The microconidia vary in form from the rounded to oval. They are often held in small masses. At times they are elongated or crescent-shaped.
b. Macroconidia (Fig. 16.7):
They are large, multi-cellular usually two to four- celled conidia. In form they are elongated, sickle-shaped or crescent- shaped. They are produced at the tips of simple or sparingly branched conidiophores which are assembled to form a sporodochium type of fructification (Fig. 16.8).
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The microconidia and macroconidia are produced in vast numbers. They are distributed by the wind. On falling on a suitable substratum they germinate and initiate new infections.
c. Chlamydospores (Fig. 16.7):
They are rounded, oval, thick-walled cells formed in the hyphae. They may be formed singly or in chains of two or more. They become separated from the parent hyphae after maturing and function as resting spores.
Under suitable conditions, the chlamydospores germinate by means of germ tubes to form a fresh mycelium.
d. Sclerotia:
The mycelium often forms compact, resting bodies of thick-walled hyphae These are called the sclerotia. They function as storage organs and also serve as means of perennation and vegetative propagation.
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The conidia and mycelia are short-lived. The fungus persists as chlamydospores and sclerotia which germinate under suitable conditions. Infection usually takes place through the fibrous root system near the root tip region or through wounds.
The parasite then invades the root cortex and finally establishes itself in the xylem vessels. It does but little damage to the cortex. The vascular fusaria are favoued by relatively high soil temperature.
