“Major diseases of Potato”

Potato is the most important food crop of the world. The potato is a crop which has always been the ‘poor man’s friend’. Potato is being cultivated in the country for the last more than 300 years. For vegetable purposes it has become one of the most popular crops in this country. Potatoes are an economical food; they provide a source of low cost energy to the human diet. Potatoes are a rich source of starch, vitamins especially C and B1 and minerals.

Bacterialwilt Causalorgansm:

Ralstonia solanacearum- Pseudomonas solanacearum

Disease symptoms:

In addition to the potato, the pathogen also damages plants such as chili, tomato, tobacco and egg plant, as well as several species of weeds. The symptoms of bacterial wilt infection can be seen on all parts of infected plants. Infected plant begins to wilt, starting from the tips of the leaves or where the stems branch out, and then spreading to all parts of the plant.

Leaves become yellow at their bases, then the whole plant wilts and dies. When stems are cut a brown colored ring will be visible. When a tuber is cut in half, black or brown rings will, however, be visible. If left for a while or squeezed, these rings will exude a thick white fluid.

A further symptom is fluid coming out of tuber eyes. This can be signified by soil sticking to tuber eyes when crops are harvested. Serious infection causes tubers to rot.

Survival and spread:

Bacterial wilt pathogen can survive in soil (without a host for several seasons), water, seed tubers, potato plant remnants. The disease can spread from field to field or from plant to plant within field via infected seed, air, water, soil, farming tools, livestock and people.

Favourable conditions:

High temperature, soil moisture, low pH. The disease spread rapidly in the warmer temperatures in storage areas. Infected seed can also be a source of the disease in the field.

Control Measures:

Certified seed free from brown rot disease should be planted.

In case cut potato tubers are being used, they should be kept at 12º C for four days      so that the cut surface hardens. The tubers can also be treated with the solution of Aretan or Agallol.

The crop debris should be collected and burnt.

Late Blight

Phytophthora infestans

This disease damages leaves, stems and tubers. Affected leaves appear blistered as if scalded by hot water and eventually rot and dry out. When drying out, leaves turn brown or black in color. When infections are still active, spots appear on the underside of leaves blanketed in what looks like flour. Affected stems begin to blacken from their tips, and eventually dry out.

Severe infections cause all foliage to rot, dry out and fall to the ground, stems to dry out and plants to die. Affected tubers display dry brown-colored spots on their skins and flesh. This disease acts very quickly. If it is not controlled, infected plants will die within two or three days.

Survival and spread:

The pathogen survives in plant debris in the soil. It spreads through soil and infected seed tubers.

Favourable condition:

High humidity. Low temperature and leaf wetness

Tubers carrying the pathogen are the real carriers and serve as the source of the disease in the subsequent season. Infected seed tubers grow into healthy plants but under favourable conditions for O the disease (10-12 C and RH > 80%) development, the disease infects the stem and lower leaves.

Control Measures:

Use potato tubers for seed from disease free areas to ensure that the pathogen is not carried through seed tuber.

The infected plant material in the field should be properly destroyed.

Grow resistant varieties like Kufri Navtal.

Fungicidal sprays can be effective, if given properly and timely. Sprays should  start a few days before the anticipated time of occurrence of the disease or on the appearance of initial symptoms. Cool humid atmosphere favours the disease and prevalence of such weather can be indication of disease in a short time ahead. Spraying should be done with Dithane M-45 or Dithane Z-78 (2.5 kg per 1000 litres of water per hectare). Spraying should be repeated at 10-12 days interval.

Early Blight

Alternaria solani

Disease symptoms:

This is a common disease of potato occurring on the foliage at any stage of the growth and causes characteristic leaf spots and blight. Normally the disease symptoms become apparent during tuber bulking stage and develop leading to the harvest. The early blight is first observed on the plants as small, black lesions mostly on the older foliage. Spots enlarge, and by the time they are one-fourth inch in diameter or larger, concentric rings in a bull's eye pattern can be seen in the center of the diseased area. Tissue surrounding the spots may turn yellow. If high temperature and humidity occur at this time, much of the foliage is killed.

Lesions on the stems are similar to those on leaves, sometimes girdling the plant if they occur near the soil line.

Survival and spread:

Primary: The pathogen overwinters in infected plant debris in or on the soil where it can survive at least one and perhaps several years. It can also be seed borne.

Secondary: The spores are transported by water, wind, insects, other animals including man, and machinery.

Favourable conditions: Warm, rainy and wet weather

Control Measures:

Fungicidal sprays, preferably with copper fungicides or Zineb given at 15   day intervals effectively control the disease.

Since the same spray schedule controls late blight also, it has become a regular practice among potato growers in many tracts to spray the crop with copper fungicides at least three or four times, starting from about six weeks after planting.

CommonScab Streptomyces:

scabiei Diseasesympts:

Pathogen infects young developing tubers through the lenticels and occasionally through wounds. Symptoms of common potato scab are quite variable and are manifested on the surface of the potato tuber. The disease forms several types of cork-like lesions including surface. Damaged tubers have rough, cracked skin, with scab-like spots. Severe infections leave potato skins covered with rough black welts. Initial infections result in superficial reddish-brown spots on the surface of tubers. As the tubers grow, lesions expand, becoming corky and necrotic.

Survival and spread:

Pathogen can survive in soil, uncomposted manure or seed. It spreads through contaminated soil, seed and water.

Favourable conditions:

Disease is common in fields with low soil pH favoured by high soil moisture. Disease problems may be aggravated by excessive irrigation.

Control Measures:

The pathogen is difficult to control because of long survival both on seed tubers and in soils. However using disease free seed tubers could minimize the disease incidence. Before planting the seed tubers are treated with organomercurial compounds (0.015%for 20 minutes) or Boric Acid (3% for 30 minutes) and dries in shade. The same treatment is repeated before the storage of the tubers. Maintaining high moisture in ridge atleast for a few weeks during the initial tuber formation phase crates adverse conditions for the development of the disease. Following crop rotations with wheat, pea, oats, barley, lupin, soybean, sorghum and bajra checks the disease development.

Follow crop rotation and avoid mono cropping in same field. Before sowing, treat seeds with Emisan 6 @0.25% (2.5gm/ltr of water) for five minutes.

Black Scurf:   

Rhizoctonia solani Diseasesymptos:

Rhizoctonia canker occurs when stolons contact soil borne fungal bodies. Pathogen infects plant tissue and causes stolon blinding thus reducing tuber production and yield. It also infects tubers causing black scurf but this is purely cosmetic, reduces tuber appearance and does not reduce yield.

Survival and spread:

Pathogen is soil and seed borne, remain in soil and plant debris including infected tubers.

Favourable conditions:

High temperature and moisture is the favourable for disease development

Control Measures:

Plant only healthy tubers.

The seed tubers should be dipped in 0.5 per cent suspension of Aretan or Agallol for ten minutes.

Soil can be treated with Brassicol at the rate of 20-30 kg per hectare. Combination of seed and soil treatment gives the best control of the disease.

Viral Diseases (X,SaY) Diseaseymptos:

Potato virus Y (PVY ) is a Potyvirus, causes stipple streak. The necrotic strain generally causes mild foliage symptoms, but necrosis in the leaves of susceptible potato varieties.

Potato virus S (PVS) is a Carlavirus, if plant infected early in the season, show a slight deepening of the veins, rough leaves, more open growth, mild mottling, bronzing, or tiny necrotic spots on the leaves. PVS is transmitted by aphids non-persistently. Potato virus X (PVX) is the type member of the Potyvirus family of plant viruses. Plants often do not exhibit symptoms, but the virus can cause symptoms of chlorosis, mosaic, decreased leaf size, and necrotic lesions in tubers. PVX can interact with PVY and PVS to cause more severe symptoms and yield loss than either virus alone. The source of this virus is infected tubers.

Survival and spread:

PVY is mechanical and aphid transmitted. PVS is transmitted by aphids, including Myzus persicae, the green peach aphid. It is also mechanically transmissible, and transmissible through tubers. PVX is transmitted mechanically, not by an insect vector. Tobacco, pepper, and tomato can also serve as hosts of PVX.

Control Measures:

Seed tubers should be healthy and certified. Do not plant very small sized tubers since they are more likely to be from diseased plants.

Inspect the field regularly and destroy the plants showing the initial symptoms.

Spray the crop with Metasystox or Rogor at the rate of 600-700 milliliter dissolved in 500-600 litres of water per hectare at 10-15 days interval to check the insects that spread this disease.

Black Leg and Soft rot Erwinia carotovora:

Disease symptoms:

Black leg is a rot of the lower stem region. This is encouraged by cool, damp conditions. Soft rot occurs when the bacteria gains access to the tuber through wounds & other entry points. Symptom can range from cultivator damage to fungal lesions. The bacteria dissolve the cell walls and liquefy the tuber invaders. No distinct smell is present in true soft rot.

Surviva landsprea:

The introduction of bacteria is always through a wound in the plant tissue. It can reside in plant residue for short periods. The pathogen may spread through the soil water and infected seed.

Favourable conditions:

Disease is encouraged by cool, humid conditions.

Control Measures:

In the field, avoiding excess irrigation and nitrogen, providing proper and drainage prevents the spread of the disease.

Cultural methods such as adjust planting time to avoid hot weather during plant emergence and harvesting the crop before soil temperature rises above 28 C is recommended.

The crop should be harvested only when the tuber skin is fully cured. Care should be taken to avoid injury to tubers and bruised injured tubers are sorted out.

Treating the tubers with 3% boric acid for 30 min and drying them under shade minimizes infection in the storage. The treated tubers should be stored in either in wellventilated cool stores or cold stores.

Black Heart : Disorder

Oxygen deficiency of internal tuber tissue

Disease symptoms:

Black heart occurs primarily in storage when the tubers do not receive enough oxygen. Blackening of the tuber center follows acute oxygen deficiency associated with either low temperature in confined storage or high field soil temperatures. The tissue dies from the inside out and turns jet black. Smell is absent. Affected tubers rot later.

Control Maesures:

In the field, promote good soil drainage and avoid excessive irrigation.

Do not delay harvest in hot soils.

Positive store ventilation will improve gas exchange, dry crops quicker and help prevent condensation events.

By: Shikha Sharma¹ and Manisha Thakur²

Department of Plant Pathology ¹and Department of Vegetable Science²