Discuss briefly the life cycle of Fungi Alternaria.

Ans. Classification and Systematic Position:

Mycota -Fungi

Eumycotina -Deuteromycetes

Deuteromycetes -Moniliales

Moniliales -Dematiaceae

Dematiaceae -Alternaria

Alternaria -A. solani

(Modern) -(Early blight of potato)

Occurrence: The fungus alternaria occurs commonly on potato plants and causes the disease early blight of potato. The fungi appears as yellow spots on the leaves which are scattered and usually rounded. Sometimes the whole surface of the leaf is covered with such spots or leasons. The spots are merged to each other and with the result large black of dark brown spots are developed. On studying these spots by the help of hand lens the concentric rings may be present. Sometimes petioles tubers etc. may also be got affected. The peal of infected tubers become dark brown, and the irregular or rounded spots develop on it. The pulp of the potato just below the peel becomes rusty or brown in appearance.

Structure: The vegetative body of the fungus is light brown, slender, branched intracellular or intercellular mycelium which is found in the host tissue and therefore, can only be seen by cutting sections of the infected host plant. After sometime, the mycelium becomes dark brown.

Reproduction: Being imperfect in nature, the fungus reproduces only by asexual means with the help of conidia. These are large, multicellular, septate, brown and develop usually in chains. As a rule the conidia are beaked and muriform. They develop on short, dark-coloured, conidiophores, which cannot be differentiated from the septate vegetative mycelium. The conidiophores emerge through the stomata of the leaf from the dead centre of the infected spots which appear as concentric rings measuring 50-90μ × 9μ. These infected spots are called ‘Target Board’.

Each conidium possesses 5-10 transverse and a few longitudinal partitions. When mature, they are detached from their conidiophore and are liberated in air or soil. Each conidium measures 130-300μ in length and 12-20μ in breadth. In case of saprophytic species, the conidia germinate directly in soil and on a land plant, in case of parasites. In the latter case, the conidium produces 5-10 germ-tubes which infect healthy host through the stomata. Cases of direct penetration of the infection tube in the host-cells are of rare occurrence.

Symptoms A. solani, causes the ‘Early Blight of potato’ is a very common disease in India and outside. After 3 or 4 weeks of sowing, the disease first appears as pale brown spots, scattered on the lower leaflets. Gradually such spots increase on the upper leaves as well. With the growth of the plant, these spots become large and show circular or concentric necrotic ridges which produce a ‘target board’ effect. Leaves dry up soon and the plant droops down, thus causing colossal damage to potato crop. In severe cases, the spots may develop on petioles and stems. An enzyme alternaric acid is secreted by the pathogen which causes leaf infection of tomato + potato. Control:

(1) Rotation of crops.

(2) Growing of resistant varieties.

(3) Spraying with fungicides like Bordeaux mixture every week after the plants become 6′-8′ tall.

(4) Eradication of diseased plants.

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