Post harvest diseases of
List of diseases
Black Leg - Erwinia carotovora
Dry rot - Fusarium coeruleum
Brown rot - Ralstonia solanacearum
Potato wart - Synchytrium endobioticum
Late blight - Phytophthora infestans
Scab - Streptomyces Scabies
Sclerotium rot - Sclerotium rolfsii
Silver scurf - Spondyocladium atrovirens
Charcoal rot - Macrophomina phaseolina
Black leg - Erwinia spp
Aerial stem rot & tuber soft rot
Black leg begins from a contaminated seed piece
Stem bases - an inky-black to light-brown decay, extend up
the stem from less than an inch to more than two feet
These enlarge into a soft, mushy rot that causes entire stems
to wilt and die
Leaves - roll upward at the margins, become yellow, wilt &
often die
Potato tubers with soft rot have tissues
very soft and watery
have a slightly granular consistency
tissue is cream to tan-colored
black border separating diseased from healthy areas
In the early stages, soft-rot decay - odorless
Later a foul odor and a stringy or slimy decay usually develops
as secondary decay bacteria invade infected tissues
Symptoms of black leg
Survival and spread
Blackleg - Erwinia carotovora subsp. atroseptica
 Carried by contaminated seed tubers
 Usually dormant and do not cause disease unless environmental
conditions are favorable
Aerial stem rot - Erwinia carotovora subsp. Carotovora
 Contained in infested soil or introduced to the crop by irrigation
water, wind-blown rain, and insects
Tuber soft rot - caused by either of these soft-rot bacteria
 Maggot flies (Hylemyia spp. and Phorlin spp.) - spread the black leg
and soft rot
 High soil temperatures and bruising of seed tubers favor seed-piece
 RH - 94 to 100% & temp - 21 to 29oC
 Plant only certified, disease-free seed tubers
Seed treatment
Agallol-3(0.25%) for 5 min
◦ Streptomycin sulphate 0.1 % for 10 min
◦ Streptocycline ( 100 ppm) and copper sulphate ( 40 ppm) for 30 min
Harvest tubers only after the vines are completely dead to
ensure skin maturity
Precautions to minimize cuts and bruises when harvesting and
handling tubers
Storage - 55-60 F with 90-95% relative humidity for the first 1-2
weeks to promote wound healing
Dry rot
F. solani var. coeruleum
Dry dark spots appear on the skin which later becomes
sunken and wrinkled with irregular concentric rings
Spots shrinks and bursts out
Internal tissue becomes brown and shrunken with cavities
filled with numerous white tufts of mycelium
Rotting progress into whole tuber which loses much of water
and become dry hard, shriveled and light in weight
Mycelium – branched, septate
Hyphae break through the skin and form pustules on the surface
Pustule – closely interwoven hyphae which give rise to branched
conidiophores bearing conidia
Mode of spread and survival
Contaminated soil - chief source
Mycelium, conidia and chlamydospores - present in the soil
Conidia floating in the air or found on the floor and walls of stores infect
injured tubers
Temp -15 to 25oC
RH - 50%
 Avoid injuries to tubers
Potatoes should be dried thoroughly and then stored in a cool
To speed the healing process, hold tubers at 50° to 60°F with
good ventilation and a RH of at least 95% for the first 2 to 3
weeks of storage
Brown rot
Ralstonia solanacearum
Bangle blight or bangili
Leaf- turns bronze colour, shrivel and die
Vascular system of stem, root, stolon and tuber turns brown
Ring disease - brown ring in the tuber due to discolouration of
vascular bundles
Whitish bacterial exudate oozes from the vascular system of
cut stems and cut tubers
Casual organism
 Gram –ve, rod shaped bacteria, polar flagellum
Forms no spores and capsules
Mode of spread and survival
 Infected soil and seed tubers - source of infection
Decay plant parts release masses of bacteria in the soil - viable
from season to season
Infection through wounds in roots which spread through
vascular system into the stem
 Soil temp - 25 to 35oC
 Moisture - 50 %
 Optimum pH - 6.2 to 6.6
 Crop rotation - potato-wheat
High degree of resistance - clones of Solanum phureja
Late blight
Phytophthora infestans
Irish famine - 1845-46
 Leaves, stems and tubers
 Water soaked spots appear on leaves, turn purple brown &
finally black colour
 White growth develops on under surface
 Stem breaks at these points and the plant topples
In tubers - purplish brown spots & spread to entire surface
Tuber show rusty brown necrosis spreading from surface to
the center
 Mycelium - endophytic, coenocytic and hyaline
 Sporangiophores – arise from internal mycelium through
stomata on the tubers
 Sporangia - multinucleate, thin walled, hyaline, oval shaped
 Zoospores - biflagellate
Phytophthora infestans
A, zoospores produced within
the lemon-shaped sporangia (B).
Mode of spread and survival
 Infected tubers and infected soil - source of primary infection
Survival of fungus in fruiting stage or as dormant mycelium
in the soil
Persisting of perennial mycelium in affected tubers from the
field, stored and used as seed in next season
 Cool (12 to 15oC) and humid ( above 90 %) weather with
rains alternating with warm (20o C) moist period
 S. demissum and S. phureja - used for breeding for disease
resistant varieties
Varieties - Kufri Naveen, Kufri Jeevan, Kufri Alenkar, Kufri
Bruising of tubers at harvest should be avoided
Regular spraying during growing season gives effective
control- 10 to 15 days interval
◦ Brestan 600g/ha
◦ Zineb 0.2 %
◦ Bordeaux mixture 1.0%
◦ Mancozeb (2 kg/ha)
Streptomyces scabies
Shallow scab – corky tissue which arises from abnormal
proliferation of the cells of the periderm of the tuber
Lesions vary in size and shape and darker than the healthy
Corky lesions 1 to 3mm deep and darker than shallow lesions
Actinomycete attacks young tubers at a early stage of
 Conidia – produced by formation of septa, which contract to
form narrow isthmuses between the cells
Conidia- cylindrical and hyaline
Mode of spread and survival
 Affects cabbage, carrot, eggplant, onion, radish
Contaminated soil and infected tubers - source of infection
Pathogen may survive passage through digestive tract of
animals and hence it may spread with farm yard manure
Control measures
 Use disease free planting materials
 Soil application of PCNB (30kg/ha) at the time of planting
 Green manuring before planting – effectively reduce disease
 Seed treatment - mercuric chloride 0.1 %
 High degree of resistance - S. caldasii var. glabrescens,
S. chacoense & S. commersonii
 Varieties - Menominee, Russet Rural, Sebago
Potato wart
Synchytrium endobioticum
 As small white granular swellings on the eyes
 Remain minute or may become as large as the tuber
 Soft, pulpy, white to begin & become black later
 Do not develop any mycelium
 Produce summer sporangia – thin walled
 Sporangia release zoospores which attack the tubers
Live resting (winter) sporangium
of S. endobioticum.
Mode of survival and spread
 Resting spores - viable in soil for 20-25 yrs
 Withstand passage through the intestines of cattle
 Spread - contaminated manure, soil, infected seed tubers
 Temp - 16.7 to 17.8oc
 Presence of oxygen and nitrates in soil favours the
germination of sporangia
 Resistant cultivars - Kufri Kanchar, Kufri Sherpa, Kufri Jyoti
 Steam sterilization of soil
 Soil treatment – mercuric chloride and formalin 5%
Sclerotium rot
Sclerotium rolfsii
Thick white strands of fungus appear at the collar region of
the stem and roots
White fungal hyphae grow on the tubers which later start
rotting and covered by fruiting bodies
 Silky white mycelium
 Septate and branched hyphae
 Globose, smooth surfaced sclerotia
Mode of survival - mycelium and sclerotia subsists in
Mode of spread - infected soil, in running water & on
farm implements
 Temp - 30-35oc
 Thrives in sandy or loamy soil which are acidic
 Alternate wet & dry soil conditions favour the disease
 Application of ammonium nitrate to the soil
 Seed tuber treatment - [email protected] 15 kg/ha
 Resistant clones - S. acaule, S. multiinerruptum,
S. infundibuliformae
 Resistant varieties - Kufri Bahar, Kufri Jyoti, Kufri Muthu,
Kufri Sherpa
Charcoal rot
Macrophomina phaseolina
Black spot (2-3mm in dia) develops around the lenticels
which appears as whitish specks at the centre
On cutting - internal tissues shows black patches beneath
the spots on the surface of the tuber
 Mycelium - sparse or fluffy
 Hyphae - branched, septate and greyish white or brown
 Sclerotia - minute, black and smooth
 Conidiophores - simple and rod shaped
 Conidia - one celled, hyaline, oval or elliptical
Mode of spread and survival
 Pathogen present in the soil - primary source of inoculum
 Entry of fungus - bruising of skin, insect damage
 Sclerotia – survive in the soil for more than 3 yrs
 Disease is more severe in wet soil
 Temp - 31oc
 Avoid bruising of tubers during harvest, collection and
 Temp of store house should be low
 Early maturing varieties - Kufri Chandramukhi, Kufri Alankar
Silver scurf
Spondyocladium atrovirens
Lesions - brown, slightly depressed and circular with fimbriate
Dotted with minute black specks or sclerotia of the pathogen
Organism invades only the cork cells which are destroyed
and slough off forming a ‘scurf’
 Hyphae – septate, branched, hyaline and become brown with
 Conidia- dark brown, club shaped, thick walled
 Hyphae form minute sclerotia
 Pathogen live from season to season on the affected tubers
and in the soil
 Spread from diseased to healthy tubers in storage
 Use of disease free seed material
 Seed treatment- mercuric chloride - 0.1% for 30 min
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