Citrus Pathology PLP 5115C Fall 2017

Transcription

1 Citrus Pathology PLP 5115C Fall 2017 Meeting times: Mondays at 1pm to 4pm Instructor: Dr. Nabil Killiny Office: Building 7103, Room 5, Citrus Research and Education Center Office hours: Drop-in office hours Mondays 9:00-10:30, or by appointment, or anytime.

2 Course Schedule - Fall 2017 Due Date* # Topic Instructor August, 21 1 Course overview Killiny Overview of citrus diseases History of major citrus diseases. August, 28 2 Epidemiology of citrus diseases Dewdney September, 4 Holiday September, 11 3 Fungal Diseases of fruit and foliage Dewdney September, 18 4 Management of fungal and bacterial diseases Dewdney September, 25 5 Characteristics of viral and viroid pathogens Levey October, 2 6 Citrus viral and viroid diseases Brlansky Case study1: Citrus leprosis (Citrus leprosis virus) Case study2: Viroid diseases October, 9 7 Citrus viral diseases Dawson Case study: Citrus tristeza (Citrus tristeza closterovirus) October, 16 8 Citrus Root Pathology Case study 1: Citrus nematode (Tylenchulus semipenetrans) Duncan

3 Due Date* # Topic Instructor October, 23 9 Citrus Root Pathology Evan Case study 2: Phytophthora Diseases. (Foot rot, root rot, brown rot, gummosis, and Phytophthera- Diaprepes (PD) complex) Case study 3: Huanglongbing (Candidatus Lebribacter asiaticus) October, MID-TERM EXAM November, 8 11 Characteristics of bacterial pathogens Killiny Eubacteria Mollicutes November, Citrus Bacterial Diseases Case study: Citrus stubborn (Spiriplasma citri) Case study: Citrus variegated chloroses (Xylella fastidiosa) Killiny November, Citrus Bacterial Diseases Case study1; Citrus greening (Candidatus Liberibacter asiaticus) Case study 2: Canker (Xanthomonas citri) Wang

4 Due Date* # Topic Instructor November Citrus responses to various pathogens Killiny Phytohormones Metabolic responses Volatile organic compounds Micro-RNA Pathogenesis-related proteins December 4 15 Genetic manipulation for disease resistance Killiny Transgenic plants Virus-induced gene silencing (CTV) Expression vectors (CTV) CRISPR/Cas system Citrus breeding December 10 Final Exam

5 Reading Materials Nothing assigned for now! Some handouts from lecturers Compendium of Citrus Diseases Plant Pathology - Agrios Essential Plant Pathology- Schumann and D Arcy Plant Virology Hull Plant Bacteriology Kado

6 Concepts of Plant Pathology What is plant pathology? Pathology: the study of the nature of disease or any abnormal condition in an organism. Plant Pathology is 1. the study of living organisms 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

7 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

8 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 of 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. Done by control and management of diseases.

9 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.

10 Plant disease (one definition): 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. Definition of symptoms: Evidence of the reaction of the plant to the pathogen or materials that the pathogen produces (or environmental factors). Macroscopically or microscopically.

11 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, phytoplasma and systemic bacterial infections: interferes with translocation of photosynthetic products Bacterial and fungal blights: interferes with flowers Fruit rots: causes reduce reproduction or storage of reserve foods for the new plant

12 Hyperplasia or hypertrophy: more cells produced or larger cells produced resulting in nonfunctioning or abnormal cells or abnormal cell growth; often diverting food materials from the rest of the plant

13 In all cases of disease the physiology of the plant is affected Now let s 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

14 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

15 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?

16 History of Plant Diseases

17 Begins with fungal diseases OldTestament lists blasting and mildews Greek philosopher Theophratus ( B.C.) first to study and write about diseases of trees, cereals and legumes. Romans: created a special rust god, Rubigo, to protect them from cereal rusts. Each spring celebrated Robigalia 1729 Micheli: His Nova plantarum genera (1729) was a major step in the knowledge of fungi. 1st to point out that fungi have reproductive bodies (spores) (dust particles taken from a fungus reproduced the same kind of fungus 1755 Tillet: black dust from bunted wheat to seed from healthy wheat and produced bunt in the plants.

18 1807 Prevost: bunt disease caused by a fungus 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

19 Historical Perspectives with Different Causal Agents

20 Bacteria in Plant Disease 1876 Pasteur and Koch proved anthrax was incited by a bacterium (Anthrax is an infectious disease of both humans and other animals caused by bacteria called Bacillus anthracis. Infection in humans most often involves the skin, the gastrointestinal tract, or the lungs). 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 and after: Fastidious bacteria in the xylem of Pierce s disease of grapes, phony peach, and others (who cultured it?)

21 Nematodes in Plant Disease 1743 Needham within wheat galls : Cobb series of studies on plant parasitic nematodes

22 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 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

23 1962 Agar double diffusion serological test for viruses 1977 ELISA test for viruses 1975 monoclonal antibodies Nucleic acid studies: 1936 to 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 strandedtmv was done

24 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

25 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

26 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 and PCR is used; P s not culturable, S s are; often graft transmissible and sensitive to antibiotics, insect vectored

27 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 (look for specific symptoms)

28 Koch s Rules (Postulates) 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.

29 What about viruses and viroids? What about a positive PCR and accompanying sequence determination? Is this enough? What about circumstantial evidence?

30 Break

31 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? Terms to consider: symbiosis and pathogenicity

32 Symbiosis and Pathogenicity Symbiosis: mutual benefit to parasite and host Pathogenicity: the ability of the parasite to interfere with one or more essential functions of the plant with parasitism playing an important but not always the most important role.

33 Considerations Obligate parasites Nonobligate parasites Facultative saprophytes Facultative parasites In N. America: 8000 species of fungi cause 80,000 diseases, 200 species of bacteria and 75 phytoplasmas, more than 500 viruses attack crops.

34 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?

35 Stages in Development of Disease: The 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 This 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

36 The Disease Cycle Inoculation Penetration Establishment of infection Colonization and growth Reproduction of the pathogen (could be further growth) Dissemination of the pathogen Survival of the pathogen in absence of the host (overwintering or oversummering)

37 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

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

39 Penetration Direct penetration through intact plant surfaces Through wounds Natural openings

40 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

41 Colonization Growth and reproduction of the pathogen

42 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

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

44 Types of 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

45 Terminology List of terms to study that will be helpful in the coming lectures

46 plant pathology healthy plant Plant disease symptoms Macroscopic symptoms microscopic symptoms vascular wilt systemic infection hyperplasia hypertrophy

47 Koch s Rules Infectious disease symbiosis pathogenicity Obligate parasites Nonobligate parasites Facultative saprophytes Facultative parasites disease triangle disease cycle Monocyclic Polycyclic Poletic pathogens