Potato diseases..

  Late blight of potato: Phytopthora infestans

Symptom:

It affects leaves, stems and 

tubers. Water soaked spots appear on 

leaves, increase in size, turn purple 

brown& finally black colour White 

growth develops on under surface of 

leaves. This spreads to petioles, rachis& 

stems. It frequently develops at nodes. 

Stem breaks at these points and the plant topples over. In tubers, purplish brown spots 

and spread to the entire surface on cutting, the affected tuber show rusty brown necrosis

spreading from surface to the center. 

Pathogen

The mycelium is endophytic, coenocytic and hyaline which are inter cellular with 

double club shaped haustoria type. Sporangiophores are hyaline, branched intermediate 

and thick walled. Sporangia are thin walled, hyaline, oval or pear shaped with a definite 

papilla at the apex. The sporangium may act as a conidium and germinate directly to form 

a germ tube. Zoospores are biflagellate possess fine hairs while the other does not.

Mode of spread and survival

The infected tubers and the infected soil may serve as a source of primary 

infection. The diseased tubers are mainly responsible for persistence of the disease from 

crop to crop. The air borne infection is caused by the sporangia.

Favourable conditions

RH->90% , Temp.-10-25°C and Night temperature:10°C. Cloudiness on the next day 

Rainfall at least 0.1mm, the following day.

Management

A regular spraying and dusting during the growing season give effective control. 

First spraying should be given before the commencement of the disease and subsequent 

should follow at reqular interval of 10 -15 days. Protective spraying with mancozeb or 

zineb 0.2 % should be done to prevent infection of tubers. Destruction of the foliage few 

days before harvest is beneficial and this is accomplished by spraying with suitable 

herbicide. Tuber contamination is minimized if injuries are avoided at harvest time and 

storing of visibly infected tubers before storage. The resistant varities recommended for 

cultivation are Kufri Naveen, Kufri Jeevan, Kufri 

Alenkar, Kufri Khasi Garo and Kufri Moti.

Early blight: Alternaria solani

Symptoms 

It is present in both hills & plains. Brown-

black necrotic spot-angular, oval shape 

characterized by concentric rings .Several spot

coalesce & spread all over the leaf. Shot holes on fruits. 

Pathogen

Hyphae are light brown or olivaceous which become dark coloured with age. The 

hyphae are branched, septate and inter and intra cellular. The coniophores emerge 

through the stomata or between the epidermal cells. The conidia are club shaped with a 

long beak which is often half the long of the whole conidium. The lower part of the 

conidium is brown while the neck is colorless. The body of the conidium is divided by 5 

– 10 transverse septa and there may or may not be a few longitudinal septa.

Favourable condition

Dry warm weather with intermittent rain .Poor vigor. Temperature: 25-30°C. 

Poorly manured crop.

Mode of spread and survival

The conidia and the mycelium in the soil or in the debris of the affected plants can 

remain viable for more than 17 months. These conidia or the new conidia found on the 

overwintered mycelium bring about the primary infection of the succeeding potato crop. 

Secondary infection is more important in the spread of the disease. The conidia formed 

on the spots developed due to primary infection are disseminated by wind to long 

distances. The conidia from the affected plant may also be disseminated to the adjoining 

plants by rain and insects.

Management

Disease free seed tubers should be used for planting. Removal and destruction of 

infected plant debris should be done because the spores lying in the soil are the primary 

source of infection. Very early spraying with Zineb or captan 0.2% and repeating it for 

every 15 – 20 days gives effective control. The variety Kufri Sindhuri possesses a fair 

degree of resistance