4:00 pm to 7:00 pm Mondays except

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Citrus Pathology PLP 5115C - Fall 2008

• 400 pm to 700 pm Mondays except
• Sept 1

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Course Schedule - Fall 2008

• Aug. 25 Concepts of Plant Pathology -
  terminology, the infection process, plant
  pathogen dispersal, relationship to environment,
  ecological considerations, Brlansky
• Sept. 1 Labor Day - Holiday
• Sept 8 Characteristics of Fungal Pathogens -
  classes of fungi, reproduction and spore types,
  life cycles, dispersal, Chung
• Sept. 15 Fungal Diseases of Fruit and Foliage -
  greasy spot, melanose, scab, Alternaria Dewdney

3

• Bacterial and Bacterial-like Pathogens -
  morphology, physiology, multiplication,
  dispersal citrus canker, bacterial spot, CVC,
  greening organisms.Wang
• Sept. 29 Phytophthora Root Diseases of Citrus.
  Graham
• Oct. 6 Control of Fungal and Bacterial Pathogens
  - nature and development of resistance in plants,
  cultural practices and environmental factors in
  relationship to control, characteristics range of
  activity and mechanisms of action of fungicides
  and bactericides. Dewdney

4

• Oct. 13 Postharvest Decays. Zhang
•  MID TERM EXAM
•  Oct. 20 Characteristics of Viral and Viroid
  Pathogens - virus structure, replication, and
  infection process nature and characteristics of
  viroids. Dawson
• Oct. 27. Diseases Caused by Systemic Procaryotic
  Organisms - greening, stubborn, CVC. Davis
• Nov. 3 Citrus Tristeza Virus and Other Citrus
  Viruses. Dawson
• Nov. 10 Viroids - exocortis, cachexia, and
  others. Brlansky

5

• Nov. 17. Field Trip - blight, tristeza, foot
  rot, diseases of fruit and foliage. Brlansky,
  Wang, Graham
• Nov. 24 Tree Declines - blight, Rio Grande
  gummosis, tangerine collapse , etc.
• Control of Viruses, Viroids, and Systemic
  Procaryotic Pathogens - varietal resistance,
  relationship to vectors, cross protection.
  Brlansky
• Dec. 1 Epidemiology of Citrus Diseases. Graham
• Dec. 8. Genetic Manipulation for Disease
  Resistance. Gmitter
• Dec. 15. FINAL EXAMINATION
•  

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Reading Materials

• Nothing assigned
• Some handouts from lecturers
• Compendium of Citrus Diseases
• Google specific disease for more information

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Concepts of Plant Pathology

• What is plant pathology?
• Plants- a living thing that can not move
  voluntarily, has no sense organs and synthesizes
  food from carbon dioxide.
• Pathology the study of the nature of disease or
  any abnormal variation from a sound condition
• Plant Pathology is 1. the study of living
  entities and the environmental conditions that
  cause disease in plants
• 2. the mechanisms by which these factors produce
  disease in plants , 3. the interactions between
  disease-causing agents and the diseased plant,
  and 4. the methods of preventing or controlling
  disease and alleviating the damage it causes

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What do Plant Pathologist Study?

• Diseases caused by fungi, bacteria, mycoplasmas,
  parasitic higher plants, viruses, viroids,
  nematodes and protozoa
• Also plant disorders caused by excess, imbalance,
  or lack of physical or chemical factors (e.g.
  moisture, temperature and nutrients)
• But not plant damage caused by insects, humans or
  animals

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Plant Pathology uses basic knowledge of

• Botany, mycology, bacteriology, virology,
  nematology, plant anatomy, plant physiology,
  genetics, molecular biology, genetic engineering,
  biochemistry, horticulture, tissue culture, soil
  science, forestry, chemistry, physics,
  meteorology, and other branches of science.
• Uses advances other sciences to solve plant
  pathological problems.
• Plant pathology attempts to increase our
  knowledge of the causes and development of plant
  diseases
• Practical goal is to save produce that is
  destroyed by plant diseases to make it available
  to feed and cloth the world.

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Concept of Disease in Plants

• What is a healthy plant?
• When it can carry out its physiological functions
  to the best of its genetic potential
• Functions include what?
• normal cell division, differentiation,
  development absorption of water and minerals
  from the soil and translocation throughout the
  plant photosynthesis and translocation of the
  products to areas of utilization or storage
  metabolism of synthesized compounds
  reproduction and storage of food supplies for
  overwintering or reproduction.

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• Disease whenever plants are disturbed by
  pathogens or by certain environmental conditions
  and one or more functions are interfered with
  beyond a certain deviation from the normal
• The primary cause of disease are either
  pathogenic living organisms (pathogens) or
  factors in the environment.
• Symptoms? Evidence of the reaction of the plant
  to the pathogen or materials that the pathogen
  produces. Macroscopically or microscopically?

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Types of Function Interference

• Root rots root infection
• Vascular wilts and cankers interference with
  the vascular system
• Leaf spots, blights and mosaics infection of
  foliage
• Viral and phytoplasma infections interfers with
  downward translocation of photosynthetic products
• Bacterial and fungal blights interfers with
  flowers
• Fruit rots causes reduce reproduction or
  storage of reserve foods for the new plant

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• Hyperplasia or hypertrophy more cells produced
  or larger cells produced resulting in
  nonfunctioning or abnormal cells or abnomal cell
  growth often diverting food materials from the
  rest of the plant
• In all cases the physiology of the plant is
  affected
• Now define disease
• The malfunctioning of host cells and tissues that
  results from their continuous irritation by a
  pathogenic agent or environmental factor and
  leads to the development of symptoms

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• Is disease a condition?
• Yes, a condition involving abnormal changes in
  the form, physiology, integrity, or behavior of
  the plant that may result in the impairment or
  death of the plant or its parts.
• How do pathogens cause disease in plants?
• 1. weaken the plant by absorbing food from the
  plant, 2. killing or disturbing the metabolism of
  the cells via toxins, enzymes or growth
  regulating substances, 3. blocking the
  transportation of food, minerals, and water, 4.
  consuming the contents of the host cells

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Classification of Plant Diseases

• I. Infectious or biotic plant diseases (type of
  pathogen)
• Caused by 1. fungi, 2. prokaryotes (bacteria or
  phytoplasmas), 3. parasitic higher plants, 4.
  viruses and viroids, 5. nematodes
• II. Disease symptom root rot, canker, mosaic,
  foliage, leaf spot, wilt, fruit disease, post
  harvest, etc.
• III. Field crop disease, fruit tree disease,
  forest disease, turf disease, ornamental disease,
  etc.
• Question to class Which is the most useful?

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History of Plant Diseases

• Old Testament lists blasting and mildews
• Theophratus (370- 286 B.C.) first to study and
  write about diseases of trees, cereals and
  legumes.
• Romans rust god, Rubigo to protect them from
  cereal rusts
• 1792 Micheli role of fungi (dust particles
  taken from a fungus reproduced the same kind of
  fungus
• 1755 Tillet black dust from bunted wheat to
  seed of healthy wheat and produced bunt in the
  plants.
• 1807 Prevost bunt disease caused by a fungus

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• Debary 1853 smut and rust fungi are the causes
  not the results of plant diseases. Remember he
  was a microscopist
• Brefield 1875, 1883, 1912 introduced techniques
  for growing microorganisms in pure cultures.
  Assisted by the accomplishments of Koch, Petri
  and others
• 1878 Downy mildew of grape was introduced into
  Europe form the U.S. Spread rapidly. Millardet
  noticed that vines sprayed with copper sulfate
  and lime to deter pilferers retained their leaves
  during the season. Bordeaux mixture
• 1882 Ward coffee rust in Ceylon warned against
  monoculture of crops

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Bacteria in Plant Disease

• 1876 Pasteur and Koch proved anthrax was incited
  by a bacterium
• E. F. Smith 1895 bacterial diseases of plants
  bacterial wilts of cucurbits, ornamentals,
  crucifers. Established bacteria as
  phytopathogens
• 1977 Chilton Crown gall bacterium transforms
  normal plant cells into tumor cells (anyone know
  the causal agent?)
• 1972 Windsor and Black Rickettsialike organisms
  in the phloem of clover with clover club leaf.
• 1973 and after Fastidious bacteria in the xylem
  of Pierces disease of grapes, phony peach, and
  others (who cultured it?)

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Nematodes in Plant Disease

• 1743 Needham within wheat galls
• 1913-1932 Cobb series of studies on plant
  parasitic nematodes

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Viruses in Plant Diseases

• 1886 Mayer tobacco mosaic juice from plants
  was infectious. contagium fluidium
• 1929 Holmes local lesions could be used to
  measure the number of virus present in plant sap
  prep.
• 1935 Stanley precipated the crystalline protein
  from infected tobacco plants and showed that the
  virus was an autocatalytic protein that could
  multiply in living cells (Nobel Prize)
• Bawden in 1936 virus was protein and RNA
• 1939 virus particles first seen with the electron
  microscope
• 1956 Gierer and Schramm protein could be
  removed from the virus and that the nucleic acid
  carried the genetic info

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• 1962 Agar double diffusion serological test for
  viruses
• 1977 ELISA test for viruses
• 1975 monoclonal antibodies
• Nucleic acid studies 1936 to 1977
• 1971 Diener determined that potato spindle tuber
  disease was caused by a viroid (What is a viroid?
  Is it unique to plants?)
• 1980 Cauliflower mosaic virus, a circular double
  stranded DNA, was first sequenced (8,000 base
  pairs)
• 1982 complete sequence of single stranded TMV
  was done

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Protozoa and Phytoplasmas

• 1909 Lafont observed flagellate protozoa in latex
  bearing cells of laticiferous plants of
  Euphorbiaceae
• 1931 Stahel found flagellates infecting phloem
  of coffee
• 1963 Vermeulen showed evidence of flagellates in
  coffee and in 1976 they were associated with
  coconut and oil palms
• 1967 Doi in Japan observed mycoplasmalike bodies
  in the phloem of plants infected with leafhopper
  diseases
• 1967 Ishiie and showed that symptoms disappeared
  temporarily if treated with tetracycline
  antibiotics
• 1971 Davis et al observed spiroplasmas in corn
  stunt disease

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Diagnosis of Plant Diseases

• Pathogen or Environment Look for sign of the
  pathogen or symptom ( already described) which
  might infer pathogen
• Infectious diseases diseases caused by
  pathogens
• Characterized by their presence on or inside the
  plants
• Parasitic higher plants dodder, mistletoe,
  witchweed or broomrape growing on the surface
• Nematodes presence on or in a plant or in its
  rhizosphere, but must be distinguished from
  nonparasitic ones taxonomic
• Fungi must determine if it is a parasite or
  saprophyte morphology of its structures
  taxonomy and reports of pathogenicity on plant
  species

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• Bacteria diagnosis of the disease and the ID of
  the bacterium in based primarily on symptoms,
  constant presence of the bacterium and absence of
  other pathogens culturing, selective media and
  taxonomic characterization serology, PCR, fatty
  acid analysis avoidance of looking at secondary
  organisms fastidious bacteria difficult (small
  and sometimes in low numbers) and are difficult
  to culture.
• Phytoplasmas and spiroplasmas usually cause
  stunting and yellowing of plants and other
  specific symptoms electron microscopy is used
  Ps not culturable, Ss are often graft
  transmissible and sensitive to antibiotics,
  insect vectored

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• Viruses and viroids distinctive symptoms on
  hosts and often identified this way virus
  transmission tests to specific hosts by sap
  transmission or grafting transmission by certain
  insects serological test available for many,
  PCR, electron microscopy, light microscopy of
  viral inclusions electrophoretic and PCR tests
  for viroids
• Noninfectious diseases lack or over abundance
  of substances, environmental factors

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Kochs Rules

• Pathogen found associated with the disease in all
  diseased plants
• Pathogen must be isolated and grown in pure
  culture on nutrient media and its characteristics
  described or on a susceptible host plant
  (obligate parasites) and it appearance and
  effects recorded
• Pathogen from pure culture must be inoculated to
  healthy plants and must produce the same disease
  symptoms
• The pathogen must be reisolated from the disease
  plant and it characteristics must be the same as
  in step 2.

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• What about viruses and viroids?
• What about circumstantial evidence?

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Break
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Parasitism and Disease Development

• Infectious diseases result from infection of a
  plant by a pathogen
• Parasite an organism that lives on or in some
  other organism and obtains its food from the
  later
• Parasitism relationship between a parasite and
  its host
• Is a plant parasite a pathogen?
• Symbiosis mutual benefit to parasite and host
• Pathogenicity the ability of the parasite to
  interfer with one or more essential functions of
  the plant with parasitism playing an important
  but not always the most impt role.

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• Obligate parasites
• Nonobligate parasites
• Faculative saprophytes
• Faculative parasites
• Parasitism is common of cultivated crops
• In N. America 8000 species of fungi cause
  80,000 diseases, 200 species of bacteria and 75
  phytoplasmas, more than 500 viruses attack crops.
  

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Development of Disease

• What is necessary for a plant disease to occur?
• Plant Pathogen
• Susceptible plant
• Environment for disease to develop or express
  itself
• Called the Disease Triangle
• Variability of each component can change the
  degree of disease severity
• Could there be an additional component of the
  triangle?
• What about insect vectors?

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Stages in Development of DiseaseThe Disease
Cycle

• Infectious diseases have more or less distinct
  events in the succession that lead to the
  development and perpetuation of the disease and
  the pathogen
• Chain of events is the disease cycle
• May correspond to the pathogen life cycle but not
  always
• Disease cycle involves changes in the plant and
  the symptoms of the plant as well as in the
  pathogen and spans time within the growing season
  and from one season to the next
• Eight events in the disease cycle

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The Disease Cycle

• Inoculation
• Penetration
• Establishment of infection
• Colonization
• Growth and reproduction of the pathogen
• Dissemination of the pathogen
• Survival of the pathogen in absence of the host
  (overwintering or oversummering)

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Inoculation

• Pathogen contact with the host- inoculum
• Spores, sclerotia, fragments of mycelium,
  individuals of bacteria, phytoplasmas, viruses,
  viroids, etc.
• Propagules single units of inoculum of a
  pathogen
• Types of inoculum primary,secondary
• Sources of inoculum debris, soil, tubers,
  transplants, seed
• Landing or arrival of inoculum wind, water,
  insects

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Prepenetration Phenomena

• Germination of spores or seeds
• Hatching of nematode eggs
• Attachment of pathogen to the host plant
• Recognition between host and pathogen

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Penetration

• Direct penetration through intact plant surfaces
• Through wounds
• Natural openings

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Infection

• Process by which pathogens establish contact with
  susceptible cells or tissues and procure
  nutrition from them.
• Successful infections result in symptoms in or on
  the host
• Latent infections do not produce symptoms
  immediately
• Incubation period is the time between inoculation
  and appearance of disease symptoms
• Invasion movement throughout the host

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Colonization

• Growth and reproduction of the pathogen

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Dissemination

• Further growth sometimes outside the host to new
  tissues or new plants
• By Air
• By Water
• By insects, mites, nematodes and other vectors
• Humans

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Overwintering or Oversummering

• Survival in plant tissues or plant propagative
  parts
• In infected plant debris
• Soil
• Seeds (parasitic higher plants)

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Disease Cycles

• Monocyclic single cycle pathogens such as
  smuts,
• Polycyclic more than one generation per growth
  season multi cycle pathogens such as downy
  mildews, late blight of potato
• Polycyclic diseases usually cause explosive
  epidemics since they complete many disease cycles
  per year and the amount of inoculum is multipled
  many fold
• Poletic pathogens multi-year pathogens survive
  in perennial hosts and have as much inoculum at
  the beginning of the season as the ended with in
  the last season. Examples are viruses and
  phytoplasmas and fungal vascular wilts

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Terminology

• List of terms to study
• Will provide a list of terms and definitions that
  will be helpful in the coming lectures