Parasitism and Plant Disease - Merced Countycemerced.ucanr.edu/files/40651.pdf · Parasitism and Plant Disease Disease Triangle Factors affecting the ... Plant Disease Development

Parasitism and Plant DiseaseParasitism and Plant Disease

I D fi i iImportant Definitions:• Pathogen – a disease causing agent

– Pathogenicity – Ability for an organism to interfere with one or more essential functions of another organism – causing disease

Virulence The degree of pathogenicity of a pathogen– Virulence – The degree of pathogenicity of a pathogen

• Parasite – Organisms that live on or in another organism and obtains its food from the laterand obtains its food from the later – Plant Parasite – Organism that becomes intimately associated

with the a plant and multiplies/grows at the expense of thewith the a plant and multiplies/grows at the expense of the plant

• Saprophyte – Organisms that live on dead matter and p p y gsecrete enzymes to break down material for energy


Pathogen SpectrumPathogen SpectrumNon‐ Obligate Parasites

Facultative Facultative Saprophytes Parasites

Obligate Parasites Obligate Saprophytes(Biotrophs) (Necrotrophs)


Parasites• Tend to have narrow host

Saprophytes• Tend to have broad host

range – except viruses• Does not need to kill plant

cells to complete lifecycle

range

• Kills plant tissues to acquire cells to complete lifecycle

• Intimate relationship with plant – continuous

nutrients for growth and complete lifecycle

d ipabsorption of nutrients

• Grows inter‐ and intra‐ll l

• Secrete enzymes and toxins

• Grows inter‐cellularlycellulary

• Infected plants tend to be stunted poor vigor

• “Werewolves”

stunted, poor vigor• “Vampires”


Disease TriangleDisease TriangleDisease occurrence is an interactive event!

All sides must favor disease for the disease process to takeinteractive event! the disease process to take place!

Amount ofDisease

Total of conditions favoring susceptibilityTotal of conditions favoring susceptibilityHost


Disease Triangle

Factors affecting the

Disease Triangle

gPathogen:

1 Virulence1. Virulence

2. Population present

3 Life‐stage of propagule3. Life‐stage of propagule

4. Vectors present


Disease TriangleDisease Triangle

Factors affecting the gHost:

1 Level of resistance1. Level of resistance

2. Growth stage of plant

3 Genetic uniformity of3. Genetic uniformity of neighboring plants

4. Plant Vigor4. Plant Vigor

5. Plant Density

6 Structure of Plant6. Structure of Plant


Disease TriangleDisease Triangle

Factors affecting the gEnvironment:

1 Conditions that favor disease1. Conditions that favor disease

2. Conditions that promote plant growthgrowth

3. Conditions that affect dispersal

Includes levels of humidity, temperature wind speed andtemperature, wind speed, and periods of rain


Disease TriangleDisease TriangleInterpretation of the triangle:‐ Length of each side is

proportional to the sum total f th h t i ti f h

Amount Amount Amount

of the characteristics of each component that favors disease

ofDisease

ofDisease

Amount of

Disease

‐If one side is zero, no disease

‐If one side is very large,

di t ti l i lTotal of conditions favoring

susceptibilityHost

Diseasedisease potential is large

‐ Quantification of the triangle’s area would represent the Hostarea would represent the amount of disease

Plant Disease DevelopmentPlant Disease Development

Diseases are a series of distinct events that occur in succession leading to the perpetuation of the disease and pathogen

This chain of events is called the disease cycle!y

‐Closely related to the

lifecycle of the pathogeny p g

Plant Disease DevelopmentPlant Disease DevelopmentInfectionInfection

Invasion

Colonization

Host Recognition

Colonization

Pathogen h d/Penetration The Disease Cycle

Growth and/or Reproduction

Penetration y

Symptom Development

Attachment

Dissemination of 2° p

Production of

Incubation

Dissemination

Dissemination of 2Inoculum

Production of Dormant Stage1° Inoculum

Dormant Period

Plant Disease Development

P i E


Primary Events:1. Inoculation2 Penetration

3

42. Penetration3. Establishment of

Infection The 2

4

54. Invasion5. Growth and reproduction

of the pathogen

Disease Cycle

5

6of the pathogen (Colonization)

6. Dissemination of the

Cycle

1

6

pathogen7. Survival in the absence of

the host

17

the host


1 I l i


1. Inoculation‐ Initial contact of

pathogen with aInoculum Sources

‐ Survival in perennialpathogen with a susceptible plant site‐ Innoculum: pathogen

brought into infection

‐ Survival in perennial plants, weeds, contaminated soil, soil debris, seeds, brought into infection

court‐ May be spores, sclerotia,

mycelial fragments

, ,transplants, and vectors

Arrival of inoculummycelial fragments, bacteria, or viruses

‐ Primary inoculum –primary infection

Arrival of inoculum1. Passively (i.e. wind)2. Chemotaxis – organisms

ithi th ilprimary infection‐ Secondary inoculum –

secondary infection

within the soil are attracted to plant roots

3. Vector‐transmitted (i.e. insects)insects)

Plant Disease Development2. Penetration 2.4 Recognition between host and

th


2.1 Attachment to host‐ Fungi and bacteria produce gelatinous substances to help

pathogen‐ Triggers plant resistance genes – if present –promotes /prevents infectiongelatinous substances to help

them stick to leaf surface

2 2 Spore germination

infection

2.5 PenetrationDi t F ti f i2.2 Spore germination

‐Mainly Fungi – spore germinates forming germ tube and moves along host to find opening/weak

‐ Direct – Formation of appressorium, and penetration peg

‐ Indirect /along host to find opening/weak

point

2 3 Appressorium Formation

‐wounds ‐ wind/growth cracks,wind blown sand, lesions caused by other pathogens, 2.3 Appressorium Formation

‐ Fungi only – Formation of appressorium and “softening enzymes” and prepares for

vectors

‐natural openings ‐ Stomata, y p ppenetration into plant

p g ,hydrathodes, lenticels

Plant Disease Development2. Penetration (cont.)


Agrios, 1997

Plant Disease DevelopmentPlant Disease Development2. Penetration (cont.)

Photos from: Read, N.D. , Kellock, L.J., Knight, H., Trewavas, A.J. (1992b). Contact g ( )sensing during infection by fungal pathogens.

Plant Disease Development3. Establishment of Infection


‐ Infection: Process by which pathogens establish contact with susceptible cells and procure

t i tnutrients

‐ Successful infections result in t i ibl d t t blsymptoms – visibly detectable

changes in the plant

Obli t f l it f‐ Obligate fungal parasites form haustoria ‐ intracellular “vampires”

P th l t i‐ Pathogen releases enzymes, toxins, and growth regulators

Pl t t ith d fPhotos from: Read, N.D. , Kellock, L.J.,

‐ Plant reacts with defense mechanisms

Knight, H., Trewavas, A.J. (1992b). Contact sensing during infection by fungal pathogens.


4 I i


4. InvasionPathogens spread through plant

Fungi and bacteria: Spread by intracellular and intercellular growth by using enzymes and h l li dhormones, localized

Nematodes: Move intercellularly, localizedlocalized

Viruses, Viroids, and xylem/phloem limited bacteria:xylem/phloem limited bacteria: Move cell to cell intracellularly, systemic


5 G th d d ti


5. Growth and reproduction of the pathogen (Colonization)( )‐ Pathogens continue to spread until the infection is stopped or the plant is deadstopped or the plant is dead

‐ Only fungi and nematodes can actively movecan actively move

‐ All other pathogens rely rapid reproduction and being passively moved

Plant Disease Development5. Growth and reproduction of

th th (C l i ti )


the pathogen (Colonization)‐ Reproduction:

‐ Fungi – spores, inter‐ and g p ,intra‐, surface and interior

‐ Bacteria – cell division, inter‐and intra‐, surface and interior

‐ Viruses – intra‐, inside cells onlyonly

‐ Nematodes – inter‐ and intra‐, surface and interior,

‐ Parasitic Plants – seeds, exterior of plant only

Rate varies on pathogen present, environment, and host


6. Dissemination of pathogen

Agrios, 1997

Plant Disease Development7. Survival of pathogen

Plant Disease DevelopmentBacteria: same way as fungi;without a host

(Overwintering stage)Fungi: mycelium in cankers bud

Bacteria: same way as fungi; infected plants, seeds, tubers, and plant debris; in the bodies of

Fungi: mycelium in cankers, bud scales, seeds, tubers, and plant debris; spores; and sclerotia

insect vectors. Survive better in large slimy colonies than as small groups

‐ Soil inhabitants – survive in soil indefinitely (saprophytes)

groups.

Viruses: survive only in living ( p p y )‐ Soil transients – survive in soil for short period of time (parasites)

plant tissues; roots of perennial plants, seeds of some hosts, and insect vectors.(parasites)

Parasitic Plants: Seeds and

insect vectors.

Nematodes: Survive as eggs in h il lif hvegetative from on host the soil; or lifestages that are dormant in seeds and bulbs


7. Survival of pathogen without a host

Agrios, 1997

Plant Disease DevelopmentDisease Epidemics:Monocyclic: completes 1 disease cycle in a

p1° inoculum

Monocyclic: completes 1 disease cycle in a year‐ 1° inoculum is only inoculum for

entire year

1° infection

entire year‐ Disease increases year to year as

inoculum builds Over‐seasoning Stage

Polycyclic: 2or more disease cycles in a year‐most pathogens‐ disseminated by air and airborne

1° inoculum 1° infection

‐ disseminated by air, and airborne vectors

‐ create explosive epidemics – Late Blight, Powdery Mildew, and Rusts

2° inoculum2° infectiong , y ,

Polyetic – requiring two or more years to complete lifecycle (considered p y (monocyclic)

‐ typical of many vascular wilt pathogens – Dutch Elm Disease

Over‐seasoning Stage

Documents

Parasitism and Plant Disease - Merced Countycemerced.ucanr.edu/files/40651.pdf · Parasitism and Plant Disease Disease Triangle Factors affecting the ... Plant Disease Development