In this article we will discuss about:- 1. Genus of Alternaria 2. Mycelium of Alternaria 3. Reproduction.
Genus of Alternaria:
The form-genus Alternaria occurs universally. It has several form-species. Many of these are encountered with as contaminants in laboratory media and on petri-dish cultures.
In nature the alternarias grow mostly as saprophytes on plant debris and dying plant parts and also in the soil. Several form-species are parasitic on plants.
The important plant pathogens are:
1. Alternaria solani. It causes early blight (not to be confused with late blight caused by Phytophthora infestans) of potato and other members of family Solanaceae.
2. Altenaria brassicae and A. brassicicola cause leaf spot disease in crucifers such as mustard, cabbage, cauliflower, and knol kohl.
3. Alternaria tenuis. It is the causative agent of leaf blight of wheat seedlings. Agarwal (1959) reported that A. zinneae parasitizes Ageratum conyzoides.
Mycelium of Alternaria:
It is not large and extensive but is short, septate, branched, light brown but becoming darker with age. The colonies of Alternaria are woolly but more compact with the underside very dark-coloured.
In the parasitic species, the hyphae are intercellular at first, but later penetrate cells of the invaded tissues and thus become intracellular. The cells are usually multinucleate.
Reproduction in Alternaria:
Alternarias have no sexual or perfect stage. They multiply asexually by the method of sporulation. The characteristic asexual spores which are produced exogenously are the conidia (Fig. 16.9).
The conidia are produced at the tips of ordinary hyphae which are comparatively short and dark coloured. Special hyphae termed conidiophores are not recognisable. The conidia are large, dark coloured, several celled and beaked.
The number of cells varies from 8-14 or even more. The component cells occur packed in muriform, conical masses. The septa dividing the spore into cells are both transverse and vertical and their number is not fixed.
According to Tandon and Srivastava, it is affected by environmental conditions. The conidia of Alternaria thus belong to the category of dictyospores. Usually they are borne end to end in chains of two or three.
Occasionally they may occur singly at the tip of a hypha. Each conidium originates as a bud from the terminal cell of a hypha (Fig. 16.10A). The conidia are wind borne and thus invade laboratories where they contaminate cultures. They are also abundant in the house dust.
The conidia germinate readily in the presence of moisture and suitable temperature by putting out a number of germ tubes (B).