Diseases and pests of potato Vinay Sagar.docx

pests

1. Diseases and Pests of Potato and their Management
Vinay Sagar, Ravinder Kumar, Sanjeev Sharma and Kailash Naga
Division of Plant Protection, ICAR-Central Potato Research Institute, Shimla 171001 (H.P.) INDIA

In India potato is grown under diverse conditions, however its average yield is much lower than other
potato producing countries. One of the main reasons for this is considered to be the diseases and pests
in potatoes. Potato diseases are caused by fungi, bacteria and viruses, which adversely affect the
productivity and quality of the produce. While insects cause direct damage to the crop, they also play
an important role in the transmission of viruses. In areas where untreated seeds without proper crop
rotation are used, tuber borne diseases like black scurf, common scab, dry rot etc. become serious
problems. Among bacterial diseases, bacterial wilt or brown rot is the main problem in some places.
Aphids, leaf hoppers, thrips, white flies, mites, potato tuber moth etc. are the typical pests of potatoes.
The following measures should be taken to identify and manage the main diseases and pests that
damage the potato crop.

A. Important Diseases of Potato
a. Late Blight
This disease is caused by a fungus Phytophthora infestans. The average annual losses due to this
disease have been estimated to 15% of total production in the country. Late blight affects all plant
parts especially leaves, stems and tubers. The initial symptoms appear on lower leaves as pale green
water-soaked spots (2-10 mm) mostly on the margin and tips. In moist weather, spots may appear
anywhere on the leaves, enlarge rapidly and turn necrotic and black, killing the entire leaf instantly.
On the corresponding lower side of the leaves, whitish cottony growth forms around the dead areas.
On stem, light brown lesions develop which elongates and encircles the stem and petioles breaking
them and killing the plant/leaves instantly. Stem infection is more severe under high temperature and
relative humidity conditions. The P. infestans spores get washed down from the infected plant parts to
the soil by rain/dew drops and infect the exposed tubers. The surface of the tubers develops hard
depressions with purplish tinge on the sides. Rusty brown discolouration of the flesh is the typical
symptom. Initially the infected tubers are hard but associated secondary pathogens may set in soft rot
symptoms. The temperature between 10-22°C for two-three consecutive days with relative humidity
above 80 per cent coupled with cloudy weather and intermittent rain are most congenial conditions for
the fast spread of the disease.

Fig. Late blight symptoms on leaves, stem and tubers.

Management
 Use disease free or certified seed of varieties which have moderate to high degree of
resistance to late blight. Select well drained soils for potato cultivation, do proper irrigation
and make high ridges at the time of earthing up to prevent exposure of tubers to the disease.
 As soon as the weather conditions become congenial for late blight, irrigation should be
stopped. Only light irrigation may be given later, if required.
 As a prophylactic measure, spray the crop with contact fungicides like mancozeb 75%WP
(0.25%), propineb 70% WP (0.25%) or chlorothalonil (0.25%) as soon as the weather
conditions become congenial for late blight, or about a week in advance of canopy closure,
whichever is earlier. As soon as the disease is noticed in the field, apply any of the systemic

2. fungicides viz., cymoxanil-based or dimethomorph-based or fenamidone-based (0.3%)
fungicides. After 8-10 days of 2
nd
spray, apply contact fungicides viz. mancozeb (0.2%),
propineb (0.2%) or chlorothalonil (0.2%). However, if weather is highly congenial, repeat
application of systemic fungicides may be resorted to.
 Destroy and remove the haulms from the field when the disease severity reaches >80% to
reduce tuber infection.

b. Early Blight and Leaf Spots
Early blight caused by Alternaria solani appears regularly in central India, plateau of Bihar, Madhya
Pradesh and Maharashtra. The disease has been reported to cause significant losses (up to 20%) in
Kharif crops in Ranchi and adjoining plateau region. The symptoms of early blight mainly appear on
leaves and tubers. Initially the symptoms occur on the lower and older leaves in the form of small (1-
2mm), circular to oval, brown spots. These lesions have the tendency to become large and angular at
later stage. Mature lesions on foliage look dry and papery and often have the concentric rings, looking
like ‘bull’s eye’. Under severe conditions, the entire foliage is blighted and field gives burnt
appearance. The tuber symptoms comprise brown, circular to irregular and depressed lesions with
underneath flesh turning dry, brown and corky. Lesions tend to enlarge during storage and affected
tubers later become shrivelled. Leaf spots are caused by Phoma exigua, P. sorghina, Alternaria
alternata, and Cercospora solani-tuberosi. Depending upon the severity, these leaf spots may cause
significant yield losses. Phoma leaf spots are characterised by pinhead size spots, which may be oval,
circular or irregular, not exceeding 4 mm in diameter. Infected tubers show dark grey large lesions
(upto 1.7cm).

Fig. Alternaria blight and Phoma leaf spot symptoms

Early blight and leaf spot disease occurs due to unbalanced application of fertilizers, especially low
amount of nitrogen in the field. The causative agents of these diseases live in heaps of crop residues,
soil, infected tubers and other solanaceous hosts. A temperature of 17-25°C and relative humidity of
75% are favourable for the spread of the disease. Intermittent dry and wet weather is more conducive
for early blight.
Management
 Always use only disease-free seed tubers for raising the crop.
 Cultivation of solanaceous crops, being collateral hosts, in nearby potato fields must be
avoided.
 Fungicidal sprays are effective in controlling early blight and other leaf spots. Spraying of
crop with chlorothalonil (0.20%), mancozeb (0.20%) or propineb (0.20%) can take care of
these diseases.
 Apply recommended dose of fertilizers especially nitrogen.


c. Black Scurf

3. Black scurf (Rhizoctonia solani) is prevalent both in the plains and hills. The affected tubers look
shabby, unattractive and fetch lessprice in market. The disease affects tubers, sprouts, stems and
stolons. The most common symptom is black-scurf comprising dark brown to black irregular lumps
sticking on the tuber surface. These are sclerotia of the pathogen which adhere close to the tuber
surface and do not wash off easily. The size of sclerotia may vary from pin-head to pea-seed. The
pathogen often causes sprout injury both in storage and in fields after planting. The heavily infected
sprouts get killed i.e. cannot emerge from the soil leading to gappy germination. The emerging
sprouts when infected, later develop cankers causing girdling of stem bases. The cortical region is
affected and the stem may shrink or collapse from such points of infection. Such affected plants show
upward rolling of leaves with pinkish or purplish margin. Often small, green or reddish aerial tubers
are also formed in the axils. The roots and developing stolons also get infected. Such infected roots
later shed away hence infected plants have poor root system. Infected stolons give rise to deformed
tubers. Soil temperature of 18-20°C and high moisture is conducive for the development of the
disease, whereas high soil temperature (28-32
o
C) and high soil moisture favour the development of
sclerotia on tubers.

Fig. Black scurf symptoms on tubers, upward rolling and pigmentation of leaves and aerial tubers
in stem canker affected plant
Management
 The disease can be managed by following proper cultural practices together with seed
disinfestation. Use of healthy seed free from sclerotia of the pathogen helps in disease
management. Growing maize or ‘dhaincha’ (Sesbania aegyptiaca) or brassicas for green
manure as rotation crop also checks the inoculum and disease build up.
 Hot weather deep ploughing, twice at an interval of 20 days, has been found highly effective
in reducing the inoculum levels in the fields. Tuber treatment with boric acid (@3.0% as tuber
dip for 30 minutes or as spray) and drying under shade before cold storage or pencycuron (@
0.25%) spray on tubers before planting has been identified effective for the control of black
scurf. The progeny tubers of such treated seeds are usually free from black scurf. A
continuous use of treated seed for 2-3 crop seasons has been found to completely check the
disease. Wherever possible, crop rotation of 1-2 years may be followed.

d. Dry Rot
Fusarium dry rot is an important post-harvest disease of potato tubers. It causes significant losses in
storage and during transit of both seed and table potatoes. The disease is distributed worldwide and
occurs wherever potatoes are grown. In India, dry rot of potato is common both in hills and plains;
however, it is primarily a problem in country stores and during transit. The losses caused due to dry
rot in plains and hills range between 5-23%. As many as fourteen species of Fusarium have been
reported to cause dry rot of potatoes. The skin of the dry rot infected tubers first becomes brown, then
turns darker and develops wrinkles. These wrinkles are often arranged in irregular concentric circles.
In the later stage of infection, a hole may be observed in the centre of ring with whitish or pinkish
growth of fungal mycelium. On cutting these affected tubers, whitish or brownish tissues are seen
with one or more cavities. Eventually the infected tubers lose much water and become dry, hard and
shrivelled. Infected tubers and soils adhering to the surface of tubers are primary source of inoculums.

4. Store temperature ranging from 15-28
o
C are congenial for development of dry rot. Dry rot of tubers is
aggravated by 5-6 months of storage.

Fig. Dry rot infected tuber
Management
 Use only clean and healthy seed tubers.
 Tuber damage and injury must be avoided during harvest, transport, storage etc.
 Ensure freedom from contaminated soil adhering to tubers.
 Tuber washing followed by drying under shade substantially reduces the infection.
 Tuber treatment with boric acid boric acid (@3.0% as tuber dip for 30 minutes or as spray)
before storage is effective.
 Tubers should be stored in cold stores in plains. In country stores, tubers must be examined
periodically and rotting tubers be sorted out.
 Avoid seed cutting, if possible. Otherwise, seed pieces may be treated with mancozeb (0.2%)
for 10 minutes and dried for 24-48 hrs before planting.

e. Bacterial Wilt and Brown Rot
Bacterial wilt or brown rot is one of the most damaging diseases of potato worldwide. In India, the
disease is prevalent in all potato growing areas except north western plains comprising of Rajasthan,
Punjab and Haryana, north central part of Uttar Pradesh and north western high hills (above 2000
masl, except the Kumaon hills). Losses up to 80 per cent have been recorded under extreme
conditions. The earliest symptom is slight wilting of leaves on top branch during clear sunny days.
The leaves show drooping due to loss of turgidity followed by total unrecoverable wilt. In advanced
stages of wilt, the basal cut end of the stem may show dull white ooze on squeezing. Transversely cut
infected tubers show water-soaked brown circles and in about 2-3 minutes, dirty white slimy droplets
appear in the circle. In advanced stage of brown rot, bacterial mass may ooze out from eyes. The
infected seed tubers, including apparently healthy seed tubers from diseased crop, are important in
spread and carryover of the disease. A temperature of 28-30
o
C is favourable for disease development.
Mean soil temperature below 15
o
C and above 35
o
C do not favour disease development.

Fig. Bacterial wilt symptoms on plants and tubers
Management
• Use only healthy/ certified seed tubers

5. • Do not cut seed tubers. Cutting spreads pathogen even to healthy tubers.
• Apply stable bleaching powder @12kg/ha mixed with fertilizer in furrows while planting. It
reduces wilt incidence by 80%.
• Follow 2-3 years crop rotation with maize, cereals, millet, garlic, lupin, onion, cabbage.
• Pathogen enters in plants through root or stolen injuries. Such injuries cannot be avoided
during intercultural operations. Therefore, restrict tillage to the minimum and it is advisable to
follow full earthing-up at planting.
• The pathogen perpetuates in the root system of many weeds and crops. Clean the field from
weeds and root/foliage remnants and burn or deep burry them.
• Deep plough the fields twice at 20-30 days interval after harvest during summer.

f. Common Scab
Common scab is caused by many Streptomyces species. The symptoms include superficial corky
lesions on tuber surface that affect the quality of the produce. The affected tubers fetch low price in
market due to ugly appearance and also because deeper peeling is required before consumption. Scab
lesions on tubers may be shallow, raised and rough corky pustules or 3 to 4 mm deep pits surrounded
by hard corky tissue. The pathogen is both seed and soil borne. It can survive in soil for several years
in plant debris and infested soil. Soil conditions greatly influence the pathogen. Favourable conditions
include pH between 5.2 to 8.0 or more, temperature in the range of 20 to 30
o
C and low soil moisture.

Fig. Different types of common scab symptoms
Management
 Use only disease-free seed tubers
 Tuber treatment with boric acid (3% for 30 min.) and drying under shade before cold storage
 Frequent irrigation to keep the moisture near to field capacity right from tuber initiation until
the tubers measure 1 cm in dia
 Maintain high moisture in ridges at least for a few weeks during the initial tuberization phase
 Crop rotation with wheat, pea, oats, barley, lupin, soybean, sorghum, bajra, and adopt green
manuring to keep the disease in check.
 Deep plough the fields twice at 20-30 days interval after harvest during summer.

g. Soft Rot
Soft rot of potato causes very high losses in storage depending on the value of the crop and the
severity of the attack. The losses may be as high as 80% in potatoes transported over long distance in
rail wagons during summer months. Initially a small area of tuber tissue around lenticels or stolon
attachment point becomes water soaked and soft. Under low humidity, the initial soft rot lesions may
become dry and sunken. Under high humidity, the lesions may enlarge and spread to larger area.
Tubers in advanced stages of decay are usually invaded by other organisms and the decaying tissue
becomes slimy with foul smell. The tuber skin remains intact and sometimes the rotted tubers are
swollen due to gas formation. At harvest many small rotted tubers with intact skin can be seen. The
infected seed tubers rot before emergence resulting in poor stand of the crop. Pectobacterium
atrosepticum is the primary enterobacteria causing soft rots. Soft rot bacteria may survive in soil, on
tuber surface, lenticels, periderm, cortex, ground tissue and vascular tissue. Contaminated irrigation
water, aerosols of rains, farm implements, soil micro-fauna, nematodes, earth-worms and larvae and

6. adults of some insects help in secondary spread. Excessive moisture creating anaerobic condition,
high temperature, excess nitrogen, tuber injuries and poor ventilation during storage, are the important
factors helping in disease development.

Fig. Soft rot symptoms
Management
 Avoid excess irrigation, provide proper drainage and restrict nitrogen dose to
minimum (150 kg/ha).
 Adjust the planting time to avoid hot weather during plant emergence. Harvest the
crop before soil temperature rises above 28C.
 Harvest the crop only when the tuber skin is fully cured.
 Avoid injury to tubers and sorting out bruised/injured tubers.
 Treat tubers meant for seed purpose with 3% boric acid for 30 minutes and dry under
shade before storage.
 Store the produce either in well-ventilated cool stores or cold stores.

h. Viral Diseases
Viruses infect potato leaves, stalks and tubers. These reduce the size and yield potential of the plants.
Two or more viruses may infect a plant simultaneously to cause mosaic symptoms. Combined
infection of PVX and PVA results in the appearance of spots with creases on the wavy edges of the
leaves, stunted growth, pitted spots on the leaves and streaks on the foliage. The combined effect of
PVX and PVY results in the appearance of wrinkled symptoms leading to stunted plant growth and
leaf drooping or decay of lower leaf veins and intense mosaics symptoms.

Fig. Potato leaves and stalks infected with viruses.
In potato leaf roll virus, the top leaves of the plants are slightly rolled inward and appear yellow,
sometimes a purple-coloured substance appears on the edges. In the secondary infection, the lower
leaves of the plants become tightly curled and appear hard, the plants turn yellow and remain dwarf.
This reduces the yield by 50-90 per cent. Potato apical leaf curl virus is a novel virus transmitted by
whiteflies. Infected potato plant show curling of apical leaves, crinkling and a clear mosaic symptom,

7. later entire plant appears bushy and stunted due to reduced inter-nodal distance. This disease causes
loss of up to 30-40 per cent in crop yield. Viruses cannot pass from one plant to another at their own.
The transmission of potato virus X occurs mainly during agricultural activities by machines or
humans. Potato virus Y, potato virus A. Potato virus S and potato virus M are mainly transmitted by
aphids.
Management
 Use disease free or certified seed procured from a reliable source. Adjust the sowing of potato
during the white fly and aphid free period and keep the fields free from weeds. Do not cut
seed tubers and plant whole tubers only.
 Disinfect all field equipments by dipping in or washing them either with 3% trisodium
phosphate or calcium hypochloride (1%) or sodium hypochloride (1%) solution.
 Do seed treatment with imidacloprid 17.8 SL @ 0.04% (4ml/10 lit) for 10 minutes before
planting.
 Give first spray of imidacloprid 17.8 SL @ 0.03% (3ml/10 lit) at 75% crop emergence and
second spray with thiamethoxam (25WG) @0.05% after 15 days of crop emergence.
 Rogue out the diseased plants carefully along with their tubers and dispose them away from
the field at regular intervals during the growing season
 De-haulm the crop before the aphids cross critical level (20 aphids per 100 compound leaves)
to enforce rigid control of the insect vector and viruses
B. Important Pests of Potato
Aphids: Aphids are sap sucking insects causing both direct and indirect damage to a wide variety of
crop plants globally. They are not serious as pest on potato crop but play a vital role in limiting
disease free seed production by transmitting major potato viruses’ viz. Potato virus Y (PVY), Potato
leaf roll virus (PLRV), Potato virus A (PVA), Potato virus M (PVM) and Potato virus S (PVS).
Among various aphid species, Myzus persicae (green peach aphid; pale yellow green, dark green or
pink in colour) and Aphis gossypii (cotton or melon aphid; pale yellow to brown or grey black or light
to dark green) acts as potential vectors of Potato leaf roll (PLRV) and Y (PVY). A temperature of 18-
25
o
C is very favourable for growth and spread of aphids.

Fig. Aphids sucking sap from potato leaves
Whiteflies: Whiteflies are small milky white insects that cause serious damage by sucking the sap of
the plants, due to which the plants’ growth retards. The female fly lays 150 to 300 eggs during her life
time. Whiteflies are known to transmit Tomato leaf curl New Delhi virus-potato (ToLCNDV-potato),
also known as potato apical leaf curl virus, posing serious threat to healthy seed potato as well as ware
potato production in India.

Fig. Whiteflies feeding on leaves

8. Leaf hopper: Potato leafhopper (Empoasca fabae, Amrasca devastans) is a polyphagous pest
distributed worldwide. The hoppers infest early season potato crop (early varieties) and are most
active from October to March. Both nymphs and adults suck sap from lower side of the leaves causing
extensive damage by direct feeding on the plants. There is yellowing, browning, cupping, curling,
stunting and dwarfing of leaves. The leaf veins turn yellow; the leaf margins turn brown and brittle
followed by drying of entire leaf. The severely infested field gives a burnt look appearing in a circular
ring commonly known as “hopper burn”. It has been worked out that 70-80% hopper burn causes 10-
20 per cent yield loss.

Fig. Leafhopper sucking leaf sap
Thrips: Thrips (Thrips palmi, T. hawaiensis) are tiny, slender insects (1 millimeter or less in size)
with fringed wings. They feed by puncturing the epidermal layer of host tissue and sucking out the
cell contents which results in discoloured flecking, or silvering of the leaf surface. Thrips can readily
move long distances floating with the wind or transported on infested plants. High temperature and
dry climate during September/October are very favorable for them. Thrips are the vectors of tospo
viruses and cause stem necrosis disease in potato.

Fig. Thrips sucking leaf sap
Management of sap sucking insects
 Maintain field sanitation by removing and destroying the weeds, alternate hosts and crop
residues of vectors and viruses.
 Place yellow sticky traps (15 x 30 cm
2
) just above the canopy height @ 60 traps per hectare at
equidistance from each other for mass trapping.
 Trap crops such as cucumber (Cucumis sativus) can be used to provide temporary protection.
Cucumber is a favoured host plant for whiteflies but a non-host for virus, and whiteflies are
known to lose their viruliferous capacity after feeding for few days on this plant.
 Seed treatment with imidacloprid @ 0.04% for 10 minutes and its foliar application @ 0.03%
at emergence with repeated application after 15 days is standard recommendation in seed
potato crop. Foliar sprays of imidacloprid 17.8SL (0.03%) at 85% germination followed by
Thiamethoxam 25WG (5gm/10 lit of water) after 10 days of the first spray.

Mites: Polyphagotarsonemus latus has a world-wide distribution. Both the adults and nymphs
damage the crop by sucking the sap of young leaves. Its attack is confined mostly to new growths
resulting in curling of leaf margins, firmness of infested leaves, necrosis of growing points and growth
inhibition. Internodes get shortened and the lateral buds break more than normal. The blooms abort
and plant growth is stunted when large populations are present. The activity of mites in the plains is
very high between October to mid-December.

9. Fig. Primary symptoms of mite infestation on top leaves of plants
Management
 Spray dicofol 18.5 EC or quinolphos 25 EC @ 2 lit/ha or wettable sulphur @ 2.5 kg/ha. The
first application should be done on appearance of the pest crossing 5-10 mites per plant and
repeated at 7 to 10 days interval depending on the persistence of mite infestation.
 Mite infestation on potato crop can be avoided by delayed planting towards mid-October in
Indo- Gangetic plains.
Cutworms: Cutworms (Agrotis ipsilon, A. segetum) are cosmopolitan and polyphagous in nature. In
India, up to 35-40 per cent tuber damage has been reported on potato crop in some areas. Crop
damage is caused by the caterpillars only. They feed at night on young shoots or underground tubers.
In the early stages of the crop, the caterpillars cut the stem of the young plants near the ground and
feed on the shoots and leaves. After tuberization, they feed by boring and nibbling into the tubers
affecting both tuber yield and its market value. The cutworms are active from October to April in
plains and during summer in hills.

Fig. Cut worm damaging potato plant

Management
 Hot weather ploughing in plains reduces the population of immature stages. A number of
birds, such as crow, mynah, starling etc. feed on insects that get exposed upon ploughing.
 Spray the foliage and the ridges of the field with chlorpyriphos 20 EC @ 2.5 ml / litre.
Spraying should be done in the evening as it is usually during the day they remain hidden in
the soil near the stems.
Leaf-eating caterpillars: Leaf eating caterpillars (Helicorvya armigera, Spodoptera litura) cause the
damage by feeding on tender leaves and shoots at night. Defoliation, cut leaves, cut shoots is the
symptoms of their attack. The infestation can be very serious if number is more. It has been observed
in case of potato that bigger larvae enter the tuber and feed on it if leaves were not available.

Fig. Spodoptera litura (left) and Helicoverpa armigera larvae damaging the potato leaves

10. Management
 Follow deep summer ploughing in potato fields to expose the immature stages of S. litura
and H. armigera to predators and sun.
 Maintenance of field sanitation and removing of unwanted vegetation/weeds should be done
10 to 14 days before planting of the crop.
 Spray the foliage with chlorpyriphos 20 EC @ 2.5 ml / litre.
Potato Tuber Moth (PTM): Potato tuber moth (Phthorimaea operculella (Zell)) larvae damage the
crop by injuring the leaves, boring into petioles and terminal shoots causing wilting of plants. The
eggs are laid on eyes of tubers that are exposed. The larvae enter into the tubers causing galleries and
tunnels leaving frass. The activity of larvae in tubers, placed in heaps, result in production of heat
which promotes significant rotting of the produce. The range of infestation could be 30 to 70% in
farm stored potato.

Fig. PTM larvae infested potato tubers

Management
 Collection of left over tubers, removal of volunteer potato plants, use of un-infested seed
tubers, deep planting, frequent irrigation, covering the exposed tubers in the field with soil
and storage of healthy tubers in moth proof structures are helpful.
 Spray deltamethrin 2.8EC (1350ml) or cypermethrin 25EC (450ml) in 750 liters of water/ha
of water on 30 days old crop. It may be repeated two weeks before harvesting.
 Use of 2-2.5 cm thick layers of dried Lantana or eucalyptus leaves below and on the top of the
potato heaps to check the PTM infestation.
 Use of sex pheromones or water traps @ 4 traps/100m
3
store are effective for mass trapping
of male tuber moths.