Potato Science Lecture 7
Potato diseases – foliar, soilborne,
viruses, and tuber rots.
What is plant disease?
 Anything that causes disfunction
 Many (most?) are caused by “infectious
biological agents” that are parasitic
 Others are physiological or “abiotic”
(Lecture 15)
 We will discuss several important
infectious diseases of potato
Causal agents of disease
 Viruses
Viroids
 Prions?
 Bacteria
Actinomycetes
 phytoplasmas
 fastidious bacteria
 Fungi
 Nematodes
Disease Triangle
Disease!
Environment
Disease Triangle
 Plant Pathology is about interactions
 Host can be altered by choice of variety
 Use of certified seed reduces pathogen
 Alter irrigation or drainage changes
environment
 Usually these factors can’t be
controlled very well
 Disease is the result
Epidemic
 Susceptible host
 Large population of virulent pathogen
 Very favorable environment
 Over a large area
 Usually wind-borne
 Can be very expensive
 Even deadly
Important Terms
 Primary inoculum
 Secondary inoculum
 Symptom
 Sign
 Incubation period
 Latent period
Symptom
Sign
Infection
 Many pathogens require a wound
 Some use “natural openings”
 Stomates
 Lenticels
 Some penetrate directly
 Mechanical
 Enzymatic
 Combination
Infection
 Some require a “vector”
 Provides mobility
 Provides wound
 Insects (aphid, thrip, leafhopper)
 Fungi (powdery scab vectors PMTV)
 Nematode (Stubby root vectors TRV)
Causal agents of disease
 Viruses
Viroids
 Prions?
 Bacteria
Actinomycetes
 phytoplasmas
 fastidious bacteria
 Fungi
 Nematodes
Plant
Pathology
G. Agrios
Bacterial Diseases
Bacteria: Characteristics
 Very small
 Require microscopic techniques to see
 Biochemical techniques also needed
 Rapid generation times (20 minutes!?)
 Prokaryotic
 Rigid cell walls
 Phytoplasmas = no cell wall = ameoboid
Bacteria: Characteristics
 Damage often due to enzymatic activity
 Some may clog vascular system
Specific Bacterial
Diseases
Disease:
Soft rot / Blackleg
Organism: Pectobacterium carotovorum
(=Erwinia carotovora)
Symptoms: Foliar blackleg appears as a black
stem rot starting at the seed that kills the
stem. Soft rotted tissues appear creamy and
very soft, can become discolored and
odiferous
Source and Spread:
Inoculum originates from multiple sources
and spreads during handling, field spread is
in water
Key Features
Soft rot bacteria
 Are everywhere
 Very opportunistic secondary invader
 Thrives with or without O2
 Facultative anerobe
 Storage rot – major player
 Seed piece decay – major player
 Aerial stem rot
Disease: Bacterial Ringrot (BRR)
Organism: Clavibacter michiganensis
subsp. sepedonicus
Symptoms: Plants can show wilting and
leaf rolling, tubers show a slimy yellow
exudate in the vascular ring
Source and Spread:
Seed tubers provide inoculum which
spreads during seed cutting and handling
Bacterial Ring Rot
“BRR”
Key Features
Bacterial Ring Rot
 Almost exclusively seed borne
 Zero tolerance in seed
 Confined to vascular tissues
 Can spread during seed cutting
 Survives on equipment and in storages
Disease:
Common scab
Organism: Streptomyces scabies
Symptoms: Corklike scabby areas or pitted
depressions on the tuber surface
Source and Spread:
Endemic to many soils or introduced on
seed, infects upon contact with tuber skin
Key Features
Common Scab
 Superficial only
 Only develops while tuber is growing
 Progress stops in storage
 “Cosmetic” disease
Fungal Diseases
Plant
Pathology
G. Agrios
Fungi: Characteristics
 Most plant diseases caused by fungi
 Larger, some can be seen with naked eye
 Also require microscopic techniques
 Biochemical techniques also needed
 Complex life cycles in some
Fungi: Characteristics
 Eukaryotic
 Multicellular
 Main body is thread-like “hypha”
 Many “hyphae” = “mycelium”
 Many form spores and other structures
 Spores sexual or asexual
 Overwintering and dispersal structures
Fungi: Characteristics
 Wind, soil, water, seed, equipment
 Single or multicycle
 Many ways of entering plant
 Foliar, tuber or both affected
Fungi: Characteristics
 Wind, soil, water, seed, equipment
 Single or multicycle
 Many ways of entering plant
 Foliar, tuber or both affected
Specific Fungal
Diseases
Disease:
Late Blight
Organism: Phytophthora infestans
Symptoms:
Leaf and stems lesions, foliage destruction,
tuber rot
Source and Spread:
Seed, cull piles and volunteer potatoes provide
inoculum, sporangia move with wind and water
Key Features
Late Blight
 Most important disease of potatoes
 Responsible for Irish famine
 Very rapid disease development
 Spores windborne
 Must have wet conditions
 Effects foliage and tubers
 Recent changes in capabilities
Disease: Early blight
Organism: Alternaria solani
Symptoms:
Brown to black leaf lesions (bullseye) appear first
on the older leaves, leaf death and defoliation,
sunken surface tuber lesions
Source and Spread:
Inoculum in soil overwinters on debris, moves
onto the plants is from splashing water, additional
spore movement in air and water , tubers infected
during harvest
Early blight
Early blight
Late blight
Early blight
Key Features
Early Blight
 Attacks senescing tissues
 May show up on lower leaves first
 Favored by alternating wet and dry
 Mostly foliage but tubers can be affected
Disease: Verticillium wilt
Organism: Verticillium dahliae or albo-atrum
Symptoms:
Wilt of stems and leaves, early death of
foliage, necrotic stem vascular streaking
Source and Spread:
Inoculum occurs naturally in the field and
overwinters on refuse, disease moves with
seed and soil, only plants in infested fields
are infected
Verticillium wilt
Verticillium wilt
Key Features
Verticillium wilt
 Soil borne disease
 One major reason for fumigation
 Some varieties worse than others
 Mostly wilt but some SED is possible
Disease: Rhizoctonia Canker
Organism: Rhizoctonia solani
Symptoms:
Reddish brown lesions on underground stems
and stolons that occasionally result in girdling
or “damping off”, black “scurf” on the surface
of mature tubers
Source and Spread:
Overwinters in soil or on seed tubers as
sclerotia which invade developing sprouts or
stolons in the spring.
Rhizoctonia
Rhizoctonia
Key Features
Rhizoctonia
 Seed and soil borne
 Cankers girdle new shoots
 More susceptible before emergence
 Yield unchanged, quality affected
 Cosmetic = “dirt that won’t wash off”
Disease: Fusarium tuber rot “dry rot”
Organism: Fusarium coeruleum and sambucinum
Symptoms:
After a period in storage brown lesions form
under the tuber periderm, infection area
enlarges and becomes sunken, tuber eventually
mummifies
Source and Spread:
Inoculum source is primarily seed tubers and is
spread during seed handling, hyphae invade
wounds
Fusarium
sambucinum
Fusarium coeruleum
Dry rot
Key Features
Fusarium dry rot
 Seed and soil borne?
 Must have a wound to infect tubers
 Cut seed tubers = huge wounds
 Seed piece decay
 Storage rot
Disease: Pink rot
Organism: Phytophthora erythroseptica
Symptoms:
Field infection can cause wilting but is
primarily a tuber rot, spreads quickly
and uniformly through the tuber, flesh
rubbery but intact and turns pink upon
exposure to air
Source and Spread:
Endemic to some soils, spores infect
tubers through stolons, lenticels, or
buds
Pink rot
Key Features
Pink rot
 Tissues remain relatively firm
 Rubbery texture
 Rot usually progress in straight line
 Tissues turn pink upon exposure to air
 Can be seen in field and storage
Leak, Pythium
Watery wound rot
Disease:
Organism: Pythium ultimum
Symptoms:
Solely a tuber rot, starts as a discolored area
around a wound, rot moves quickly through
the flesh with a dark line demarcating line
between infected and healthy tissue
Source and Spread:
Endemic to most soils, enters tubers at
wound sites during harvest and handling
Key Features
Pythium leak
 Must have a wound
 Associated with high pulp temperatures
 Very rapid decay
 Often rots center of tuber, leaving shell
 Tissues very soft
 Readily “leaks” clear fluid
Diseases not discussed
 Powdery scab
 White mold
 Black dot
 Phoma
 Southern bacterial wilt
 Compendium of Potato Diseases
Disease Management
Principles
Management
 Usually aimed at populations
 Single individuals not important
 EXCEPT Trees, other perennials
Management
 Diseases difficult to cure
 Most management aims at
protecting
Control Strategies
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Regulatory
Cultural
Biological
Physical
Chemical
Regulatory
Aimed at excluding a
pathogen from host
or geographic area
Cultural
 Avoiding contact between plant
and pathogen
 Create unfavorable environment
or avoid favorable conditions
 Eradication or reduction of
inoculum
Biological
 Host resistance
 Microorganisms
antagonistic to pathogen
Physical and Chemical
 Protection from inoculum
 Curing an infection
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Control Methods
Exclusion
Eradication
Resistance
Direct protection
Integrated control
Guidelines for making
disease management
decisions
Before Planting
 Use only certified seed
 Fall fumigation? Spring not as good
At Planting
 Avoid unfavorable conditions
 Use a seed piece treatment
 Fusarium dry rot
 Rhizoctonia stem canker/black scurf
 Late blight?
 Single drop or healed seed
 In furrow fungicide applications
Growing season
 Fertility and water management
 Scouting, forecasting
 Fungicide applications may be needed for:
 Early blight
 Late blight
 White mold
 Insecticide for PLRV (virus - insect vector)
Vine Kill and Harvest
 Adequate time for skin set before harvest
 Wound-obligate pathogens
 Post harvest fungicides for some diseases
 Phosphorus acid (late blight, pink rot)
 Biologicals (bio-save) (dry rot
 New post harvest under development
Storage
 Wound healing period
 Pesticides applied in storage?
 Tools for the storage manager
 Air flow
 Temperature
 Humidity
Virus Diseases
Plant
Pathology
G. Agrios