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?

BacteriaActinomycetes phytoplasmas fastidious bacteria

Fungi Nematodes

HostPathogen

Environment

Disease Triangle

Disease!

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?

BacteriaActinomycetes phytoplasmas fastidious bacteria

Fungi Nematodes

Plant PathologyG. 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 / BlacklegOrganism: 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

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

Soft rot bacteria

Disease: Bacterial Ringrot (BRR)Organism: Clavibacter michiganensis

subsp. sepedonicusSymptoms: 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

Almost exclusively seed borne Zero tolerance in seed Confined to vascular tissues Can spread during seed cutting Survives on equipment and in storages

Bacterial Ring Rot

Disease: Common scabOrganism: Streptomyces scabiesSymptoms: Corklike scabby areas or pitted

depressions on the tuber surfaceSource and Spread:

Endemic to many soils or introduced on seed, infects upon contact with tuber skin

Key Features

Superficial only Only develops while tuber is growing Progress stops in storage “Cosmetic” disease

Common Scab

Fungal Diseases

Plant PathologyG. 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 BlightOrganism: Phytophthora infestansSymptoms:

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

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

Late Blight

Disease: Early blightOrganism: Alternaria solaniSymptoms:

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

Early blight

Late blight

Key Features

Attacks senescing tissues May show up on lower leaves first Favored by alternating wet and dry Mostly foliage but tubers can be affected

Early Blight

Disease: Verticillium wiltOrganism: Verticillium dahliae or albo-atrumSymptoms:

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

Soil borne disease One major reason for fumigation Some varieties worse than others Mostly wilt but some SED is possible

Verticillium wilt

Disease: Rhizoctonia CankerOrganism: Rhizoctonia solaniSymptoms:

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

Seed and soil borne Cankers girdle new shoots More susceptible before emergence Yield unchanged, quality affected Cosmetic = “dirt that won’t wash off”

Rhizoctonia

Disease: Fusarium tuber rot “dry rot”Organism: Fusarium coeruleum and sambucinumSymptoms:

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

Dry rotFusarium coeruleum

Key Features

Seed and soil borne? Must have a wound to infect tubers Cut seed tubers = huge wounds Seed piece decay Storage rot

Fusarium dry rot

Disease: Pink rotOrganism: Phytophthora erythrosepticaSymptoms:

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

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

Pink rot

Disease: Leak, Pythium Watery wound rotOrganism: Pythium ultimumSymptoms:

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

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

Pythium leak

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 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

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 PathologyG. Agrios