Introduction
Definitions:
Fungi As Plant Pathogens
Anubhav Vinayak
Lisa Tofil
Heather Rice
•  Parasite – organism that gains all or part of its
nutritional requirements from the living tissues of
another organism – the host.
•  Pathogen – organism that causes disease.
A pathogen almost always a parasite but not all
parasites cause diseases.
Pathogens
Pathogens
•  Pathogens
–  Necrotrophic – (kill the host tissues usually by
directly invading them or by producing toxins
or degradative enzymes and then feed on the
tissues that they kill)
–  Biotrophic – (feed on living host tissues by
producing special nutrient-absorbing structures
that tap into host’s tissues)
Seed rots and seedling pathogens
•  Athelia rolfsii
–  One of the most devastating seedling pathogens
in the warmer parts of the world where rainfall
is seasonal
Pathogen
Necrotroph
Host-specialized Non-specialized
Host-specialized
Seedling pathogens (cont)
•  Rhizoctonia solani
–  Common seedling pathogen that attacks the
basal stem tissues of plants
–  Examples
Source: http://www.turffiles.ncsu.edu/diseaseID/diseasedetail.aspx?disease=RHISL
Source: http://www.agroatlas.ru/content/diseases/Olee/Olee_Athelia_rolfsii/Olee_Athelia_rolfsii.jpg
Biotroph
Source:http://www.ctahr.hawaii.edu/nelsons/Misc/
1_potato_sore_shin_rhizoctonia_1.jpg
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Host-Specialized Necrotrophic
Pathogens
Pectic Enzymes
•  Degrade the middle lamella (the cementing
layer)
•  Three major types
–  Pectin methyl esterase (PME)
–  Pectic lyase (PL)
–  Polygalacturonase (PG)
•  Necrotrophic pathogens:
–  Invade normal healthy tissues of plants
–  Overcome the defense mechanisms of their hosts
•  Physical barriers
•  Fungitoxic chemicals
Botrytis cinerea
Source: http://www.ccrc.uga.edu
Tissue Degrading Enzymes
•  Necrotrophic fungi and nongrass plants: F.
oxysporum and V. albo-atrum
–  Pectic enzymes
•  Gramineae (grass family): R. cerealis, F.
culmorum, and T. yallundae
–  Arabinase, xylanase, and glucanase enzymes
–  Small amounts of pectic enzymes
•  Able to degrade typical wall components of their hosts
Plant Defenses Against Necrotrophic
Pathogens
•  Physical barriers:
–  Oxidative burst
•  Produces hydrogen peroxide at or near plant cell surface
•  Hydrogen peroxide strengthens cell wall against attack
–  Cross-links plant cell wall proteins
Source: www.php.wur.nI/UK/Research/
Plant Defenses Against Necrotrophic
Pathogens
•  Hypersensitive response:
–  Upon attack the infected cells die and accumulate
fungitoxic chemicals
•  Physical barriers:
–  Leaf cuticle
•  Cutinase enzyme allows the
fungus to penetrate through
the cuticle
–  Papilla
Source: staff.jccc.net/PDECELL/plants/transpiration.html
•  Thickening of the cell wall
at the point of invasion
Plant Defenses Against Necrotrophic
Pathogens
•  Chemical defenses:
–  Phytoanticipins
•  Converted to an active compound in response to
infection
•  S. lycopersici:
–  Can overcome phytoanticipins
–  Produce enzymes:
  Avenacinase and α-tomatinase
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Plant Defenses Against Necrotrophic
Pathogens
•  Chemical Defenses:
Systemic Acquired and Induced
Resistance
•  Systemic acquired resistance (SAR):
–  Phytoalexins
–  Increased resistance following infection
•  Low molecular weight
•  Protect plants against general attack by fungi
•  Produced in response to infection
–  Activated by an oxidative burst
  Hydrogen peroxide is initial signal
•  Mediated by salicylic acid
•  Induced systemic resistance (ISR):
–  Increased resistance following colonization of
plants roots by rhizosphere bacteria
•  P. fluorescens
•  Mediated by jasmonic acid and ethylene
Vascular Wilt Pathogens
•  Three main fungi:
•  Mode of infection:
1.  V. albo-atrum
2.  V. dahliae
3.  F. oxysporum
•
•
–  Spores enter water-conducting xylem vessels
More than 80 strains (“special forms”)
Strains survive by producing
chlamydospores
•  Symptoms:
–
–
Yellowing and death of leaves
Collapse of leaf stalks
•
Vascular Wilt Pathogens
•  Vectors
•  Wounds
•  Young root tips
–  Carried upwards in the water flow
–  Become trapped on perforated end walls
•  Produce additional spores
–  Remain restricted to the xylem vessels until the
plant dies
Epinasty
Source: www.anbg.gov.au/cpbr/program/sc/path_vari.htm
Plant Response to Vascular Wilt
Fungi
•  Tyloses- (balloon-like
swelling)
•  Pectic gels
•  Release of phenolic
compounds
•  Accumulation of
phytoalexins
Smut Fungi
•  Members of the Basidiomycota
•  Over 100 species:
–  Loose smut of wheat: U. nuda
–  Maize smut: U. maydis
–  Stinking smut of wheat: T. caries
Source: Figure 14.14, Deacon (2006) Fungal Biology, 4th
ed.
Source: www.uky.edu/.../wheat/disease/Ismut/Ismut1.htm
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Smut Fungi
Fungal Endophytes and Their Toxins
•  Mode of infection:
–  Germinating spores penetrate
developing ovary
–  Resulting seeds germinate, colonize,
and enter flowering spike
–  At maturity flowering spike
produces a mass of black spores
Source: www.uky.edu/.../wheat/disease/Ismut/
Ismut1.htm
•  Significance
–  produce secondary metabolites useful for
commercial use
–  induce abiotic tolerance
–  insect antifeedant
Source:
http://cc.oulu.fi/~ampir/kotisivu-3.html
Fungal Endophytes and Their Toxins
Cont.
Fungal Endophytes and Their Toxins
Cont.
•  “Clavicipitaceous endophytes”
•  Dilemmas of endophytes
–  Common species is called Claviceps purpurea
–  These type of endophytes are found in several
pasteur grasses in US, Europe and New
Zealand
•  Lolium (ryegrass)
•  Festuca (fescue)
•  Dactylis glomerata (cocksfoot)
Fungal Endophytes and Their Toxins Cont.
–  Produce mycotoxins
–  Mycotoxins harm grazing animals
–  endophytes increase stress tolerance
–  endophytes prevent insect damage over other
grasses
Fungal Endophytes and Their Toxins
Cont.
•  Types of mycotoxins and effects
–  loliterm B- saturated aminopyrrolizidine alkaloids-“ryegrass staggers” disease of grazing sheep and cattle.
Staggers are loss of coordination and/or tremors occur at
times
–  Egrovaline- causes “fesuce foot” and “fescue toxicosis”
where the animal can experience hyperthermia, weight
loss, reduced pregnancy rates, decreased milk production,
and for horses birth defects or abortions
–  Peramine- tripeptide that is repellent and toxic to insects,
but not mammals
Sources:
http://
www.biology.ed.ac.uk/
research/groups/jdeacon/
FungalBiology/
chap14_3.htm#Fig14.18
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Phytophthora Diseases
Phytophthora Diseases Cont.
•  They are not fungi but are part of Oomycota
•  Phytophthora contains more than 50 species
•  Common known Phytophthora disease was the potatoe
blight
•  Causes
–  Root rot
–  Cankers-develop near base of trees
–  Collar rots and crown rots- develop near base of steams of woody
herbaceous plants
–  Aerial blights
–  Fruit rots
Phytophthora Diseases Cont.
•  Phytophthora infestans
–  Heterothallic fungi
–  Originated in Central Mexico, both mating types
present there (A1 and A2)
–  Moved over to Europe by 1840’s causing Potatoe
Blight
–  Only A1 was dominantly present outside Central
Mexico, but in 1980 began to spread
Sources:
http://www.ent.iastate.edu/images/
plantpath/soybean/rootrot/root_rot.jpg
http://hyg.ipm.illinois.edu/photos/
canker.jpg
•  Phytophthora ramorum: sudden oak death
–  First found in Germany and Netherlands in 1993
–  Described as P. ramorum in 2001
–  Now found in coastal fog belt of northern California
and southern Oregon
–  European mating type is A1
–  North American mating type is A2
–  Separate origin of sources
http://aggie-horticulture.tamu.edu/
publications/tomatoproblemsolver/
ripe/images/photos/sour_rot.jpg
Phytophthora Diseases Cont.
•  Phytophthora infestans Cont.
–  Symptoms
•  Black spreading lesions on foliage
•  Cool humid conditions allow black lesions to produce
sporangiophores that emerge from leaf stomata
•  Potato tubers become infected later (spread of motile zoospores)
•  Tubers rot then destroyed by secondary bacteria invaders
•  May also be spread by wind born sporangia or splashed sporangia
onto leaves
•
Phytophthora Diseases Cont.
Sources Cont.
http://plant-disease.ippc.orst.edu/
disease.cfm?RecordID=1434
Sources:
–  http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/blight.htm ( fig 14.19a )
Phytophthora Diseases Cont.
•  Phytophthora ramorum: sudden oak death cont.
–  Plants affected and symptoms
•  Rhododendron branches, Vibrurnum bushes, ornamental
Rhododendron and Vibrurnum (Europe), and oaks-- “live and
tan oaks” (US)
•  Produces lesions on leaves and terminal shoots of shrubs
(shoot dieback)
•  Cankers at base of trees (destroys cambium)
•  Cracked bark
•  Seepage of dark, viscous sap from bark near base of tree
(gummosis)
•  Sudden death
•  Slow progressive building of infection under bark
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Phytophthora Diseases Cont.
•  Phytophthora ramorum: sudden oak death
cont.
Sources:
http://www.biology.ed.ac.uk/research/groups/jdeacon/
FungalBiology/pramorum.htm (Fig 14.20 a-e)
Biotrophic Plant Pathogens Cont.
Biotrophic Plant Pathogens
•  Characterized by an extended nutritional
relationship with living host cells as
opposed to necrotrophic pathogens which
kills plant tissues
•  Success depends upon
–  ability to avoid eliciting host cell death
–  securing a continuous nutrient supply from
living host tissues
Biotrophic Plant Pathogens Cont.
•  Example Fulvia (Cladosporium) fulvum
–  One strain only feeds from the host if it avoids
inducing hypersensitive response
–  Governed by avirulence genes of the pathogen
and corresponding resistance genes in host
•  gene for gene relationship is common for biotrophic
plant pathogens
–  Specific type of biotroph covered is Haustorial
biotrophs
Source:
http://www.biology.ed.ac.uk/research/groups/
jdeacon/FungalBiology/chap14_3.htm#Fig14.21
Biotrophic Plant Pathogens Cont.
•  Rust Fungi
–  Black stem rust of wheat (Puccinia graminis)
•  Life cycle fig 14.22
Sources:
http://www.biology.ed.ac.uk/research/groups/
jdeacon/FungalBiology/chap14_3.htm#Fig14.22
Biotrophic Plant Pathogens Cont.
•  Powdery Mildew Fungi
–  Part of Ascomycota cause problems throughout all
seasons, especially when dry and hot and if not treated
with fungicides
–  Common types
•  powdery mildew of roses (Sphaerotheca pannosa)
•  powdery mildew of gooseberries (S. mors-uvae)
•  powdery mildew of hawthorn bushes (Podosphaera
clandestina)
–  This species undergoes multiple cycles of
infection in one season like rust fungi
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Question 1
Biotrophic Plant Pathogens Cont.
•  Downy mildews
–  Part of Oomycota
–  haustorial biotroph act like rust fungi and
powdery mildew fungi, but need humid
conditions for infection
–  Examples
•  Blemia lactucae of lettuce
•  Plasmopara viticola on grape vines
•  Pseudoperonospora humuli on hops
•  What structure do pectic enzymes degrade
in plant cells?
A.
B.
C.
D.
Sources:
http://www.hort.cornell.edu/
department/facilities/lihrec/vegpath/
photos/downymildew_lettuce.htm
Question 2
•  What is the name of the signal molecule that
mediates systemic acquired resistance?
A.
B.
C.
D.
Jasmonic acid
Xylanase
Ethylene
Salicylic acid
Question 4
•  What type of fungi has multiple life cycles
including uredospore, teliospores,
basidiospores, and aeciospores?
A.  Powdery Mildew Fungi
B.  Rust Fungi
C. Blemia lactucae
D. Phytophthora ramorum
Primary wall
Middle lamella
Secondary wall
Plasmodesmata
Question 3
•  Which of the following is not a defense
mechanism used by plants to prevent
invasion by necrotrophic pathogens?
A.
B.
C.
D.
Phytoanticipins
Cutinase enzyme
Oxidative burst
Phytoalexins
Question 5
•  Which type of fungi has two types found in
the US (A1 and A2)
A. Phytophthora infestans
B.  Phytophthora ramorum
C. Blemia lactucae
D. Pseudoperonospora humuli
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