Download October 13, 1997

October 13, 1997
PLANT PATHOLOGY - EPWS 310
SSN:
Examination II, Plant Pathology EPWS 310, Fall 1997
GENETICS OF DISEASE RESISTANCE
&
FUNGAL DISEASES OF PLANTS - PROTOZOA AND OOMYCETES
Try to answer each question. Write your name on every page. The value of each question is
indicated. There are Ten questions on both sides of the paper. The examination is worth 100 points.
Question 1: (5 points)
Name the disease caused by the fungi listed and identify the host:
a) Pythium
Damping off/Root rot - any host
b) Plasmodiophora brassicae
Club root of crucifers
c) Plasmopara viticola
Downy mildew of grape
d) Phytophthora capsici
Phytophthora root rot of pepper
e) Phytophthora infestans
Late Blight of potato
Question 2: (5 points) Define the terms Horizontal Resistance and Vertical Resistance. What
type(s) of diseases is vertical resistance used to control?
HR - Polygenic resistance, due to many characteristics of plants
VR - Monogenic resistance, due to a single gene that confers either a receptor or single gene
resistance product
VR is used to control biotroph dieases
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Question 3: (5 points) How do the fungal pathogens Phytophthora infestans and Plasmopara
viticola differ in the mode of pathogenesis? How are they similar? Compare and contrast
penetration, colonization and the nature of the disease each pathogen causes.
Phytophthora - Bio-necrotroph, foliar pathogen intracelular colonization
Plasmopara - Biotroph foliar and fruit pathogen intercellular colonization
Similar - Deciduous sporangia, fruiting bodies appear through stomata
Differences: Phytophthora is a necrotroph, intracellular pathogen v's biotroph
intercellular pathogen
Question 4: (10 points)
a) What is a receptor molecule? Molecule that sits on host cell membrane to detect avr gene
products(elicitors) of pathogen.
b) List two plant characteristics that may contribute to the horizontal resistance of plants? Cuticle,
leaf shape orientation hairiness etc etc
b) What is the source of resistance genes used in cultivated crops? Wild relatives near the center
of origin
d) What is an avr gene? An elicitor of the resistance response. Often cell wall component of the
pathogen
_______________________________________________________________________________
_______________________________________________________________________________
____
e) What is the two primary genetic processes leading to variability in plant pathogens? Mutation
and recombination
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Question 5: (30 points) Provide short definitions for the following terms:
a) Plasmogamy - - Fusion of two cells _____________________________
b) Zoospore - - ASEXUAL, motile, dispersal spore of protozoa and oomycetes
c)
Zygote
-
d)
Direct
germination
e)
Diploid
Chlamydospore
f)
Sporangium
g)
Oospore
i)
Amoeba
l)
-
m)
Asexual
structure
fusion
of
by
gametes
germ
overwintering
that
overwintering
contains
spore
tube
spore
zoospores
of
oomycetes
Early death of seedlings before during or after emergence
Phagotrophic,
motile
cell
of
the
myxomycetes
coenocytic protoplasm of the myxomycetes ("the blob")
Coenocytic
Dikaryotic
sexual
Germination
SEXUAL,
-
Haustorium
-
Sac-like
j) Plasmodium -
k)
of
-
-
h) Damping-off -
product
-
nucleii
-
Intracellular
not
feeding
-
separated
organ
two
of
nucleii
by
the
cell
downy
in
walls
mildews
one
cell
n) Immunity Non -host, absolutely no infection or penetration by pathogen leading
to no disease at all
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o)
Conjugation
-
Sexual-like
exchange
4
of
gentic
material
in
bacteria
Question 6: (10 points) Oomycetes may be necrotrophic, or biotrophic in their mode of
pathogenesis, What do each of these terms mean? List the BEST method of disease control for type
of disease.
Necrotroph - live son dead organinc matter, kill s cells before colonization
Biotroph - live only on living cells must grow among living cells and be "stealthy"
Necrotrophic diseases - sanitation
Biotrophic - resistance
Question 7: (10 points) Complete the disease reaction types for the following table showing
complementary interaction of two host genes for resistance and the corresponding two pathogen
genes for virulence. (Note:"" indicates an incompatible, resistance reaction and a "" indicates a
compatible , susceptible reaction).
Resistance (R) genes or susceptibility (r)genes in the host
plant
Virulence (a) or
avirulence (A)
genes in the
pathogen
R1R2
R1r2
r1R2
r1r2
A1A2
-
-
-
+
A1a2
-
-
+
+
a1A2
-
+
-
+
a1a2




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Question 8: (10 Points) Draw and label a typical Oomycete life cycle showing where mitosis and
meiosis occur and the ploidy levels at all stages. State which structures are for dispersal and
which for survival (10 points).
Question 9: (5 points) What two characteristics of the Plasmodiophoromycetes make them serious
plant pathogens?
Virus
transmission
and
biotrophs,
water
dispersed
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Question 10: (10 points) Discuss the evolutionary trends that are apparent in the Oomycetes. Where
did they originate? How have they shown adaptation to the terrestrial environment?
From unspecialized necrotrophs to specialized biotrophs. Originated as algae
Wind-blown sporangia
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