2. Systematic Position
Kingdom - Mycota
Division – Eumycota
Sub-division Deuteromycotina
Form class Moniliales
Form-order - Hyphomycetes
Form-family - Dematiaceae
3. The form-species of Alternaria are saprophytes or parasites,
and are distributed throughout the world.
The saprophytic species grow on the dead organic material
and help in its Integration, whereas the parasitic species
several diseases in plants.
Some important petogenic species of Alternaria are as
follows:
A. solani causes early blight of potato and other solanaceous
plants
A cucumarina infects the members of the family Cucurbitaceae
like watermelon, cucumber and gourd,
A tenuis causes black point disease in wheat,
A brassicae infects the members of the family Brassicaceae like
mustard and cauliflower
4.
5. Symptoms
The disease symptoms appear when the plants are 3 weeks
old.
The early symptoms are in the form of small. yellowish brown
spots on the leaves, which enlarge to form concentric rings.
These are known as target board symptoms.
The spots formed on the leaf reduce the photosynthetic area
and eventually the leaf dries and falls off,
In severe infection, entire lamina, petiole and stem is badly
damaged.
Alternaria solani even infects the tubers,
and the potato pulp turns brown and
becomes corky in texture.
6. Vegetative mycelium
The profusely branched and septate mycelium
is light brown in colour.
The cells are multinucleate.
In the early stage of infection, the hyphae of
the parasitic species are intercellular but later
on they become intracellular.
7. Reproduction
Sexual stages are not known in Alternaria and the asexual
reproduction takes place by exogenously formed conidia.
The conidia are produced singly or in chains at the tips of the
branched or unbranched conidiophores.
The conidiophores are undifferentiated and emerge through the
stomata of the infected leaf.
The dark coloured conidia are multicellular and obclavate, obovoid
muriform in shape.
They are both transversely and longitudinally septate.
There are about 5-10 septa in each conidium, and the number of
septa is probably dependent on environmental conditions.
The conidium measures 30-130 um in length and 12-30 um in
breadth.
It is surrounded by a two layered wall, where the outer layer is
pigmented.
8.
9. Germination of conidia.
The conidia are readily disseminated by wind.
On being detached from the conidiophore, they
germinate on a suitable substratum.
Humid conditions favour their germination.
Each conidium produces 5-10 germ tubes, usually one
from each segment.
The germ tubes enter through the stomata of the host
leaf and soon develop into new mycelia within the
intercellular spaces.
10. Alternaric acid has been isolated from the culture filtrate
of this fungus, and it is regarded that the pathogenecity
of the fungus is due to the presence of this acid.
Perfect Stage of Alternaria
According to Webster (1980), the perfect stage of
Altenaria is a loculoascomycetous fungus,
Pleospora infectoria
Alternaria infectoria
