Plant Pathology - Horticulture (vegetables)
Vegetable - Potato
Disease Name - Early Blight
Causal Organism - Alternaria solani
Symptom
Symptoms mainly appears on leaf sometimes on stem and tubers.
Fungus causes leaf spot and blight.
On older leaves small black lesions can be seen on lower surface of leaf.
Lesions are surrounded by chlorotic zone due to alternaric acid.
Concentric rings may appears on the lesions giving them target board appearances.
A number of spots merges and dropping of leaf can be seen.
Elongated spots may be appears on petioles and stems.

Sometimes girdling of plant may seen near the soil.
On tubers, sunken lesions are formed with raised surroundings.
Etiology
Mycelium - Pale to olivaceous brown, septate (acoenocytic).
Conidiophores - Arise through stomata and dark brown in colour.
Conida - Muriforn, long beaked, borne singly.
Disease Cycle-
Infected plant debris remain in field after harvest serve as the main source of inoculum not tuber.
It left in the form of conidia and mycelium.
It also affects host like tomato and chilli and weeds like Solanum nigrum also serve as the source of inoculum.
Infection starts at lower older leaves and producing large number of conidia.
Secondary infection carried out by conidia which thrown by wind, rain and water.
Germ tube form by conidia and enter in the host tissue directly or through stomata.