Classifying different pathogens that cause disease in plants and animals

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describe a variety of infectious diseases caused by pathogens, including microorganisms, macroorganisms and non-cellular pathogens, and collect primary and secondary-sourced data and information relating to disease transmission, including: (ACSBL097, ACSBL098, ACSBL116, ACSBL117)

  • classifying different pathogens that cause disease in plants and animals (ACSBL117)

Pathogens that cause diseases in plants:

  • Fungi:
    • Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. Significant fungal plant pathogens include:
      • Ascomycetes
        • Fusarium spp. (Fusarium wilt disease)
        • Thielaviopsis spp. (canker rot, black root rot, Thielaviopsis root rot)
        • Verticillium spp.
        • Magnaporthe grisea (rice blast)
        • Sclerotinia sclerotiorum (cottony rot)
      • Basidiomycetes
        • Ustilago spp. (smuts)
        • Rhizoctonia spp.
        • Phakospora pachyrhizi (soybean rust)
        • Puccinia spp. (severe rusts of cereals and grasses)
        • Armillaria spp. (honey fungus species, virulent pathogens of trees)
  • Fungus like organisms:
    • Oomycetes
    • The oomycetes are fungus-like organisms and have similar pathogenic strategies.
    • They include some of the most destructive plant pathogens including the genus Phytophthora, which includes the causal agents of potato late blight and sudden oak death.
    • Significant oomycete plant pathogens include:
      • Pythium spp.
      • Phytophthora spp., including the potato blight of the Great Irish Famine (1845–1849)
      • Phytomyxea
        • Some slime molds in Phytomyxea cause important diseases, including club root in cabbage and its relatives and powdery scab in potatoes. These are caused by species of Plasmodiophora and Spongospora, respectively.
  • Bacteria
    • Most bacteria that are associated with plants are actually saprotrophic and do no harm to the plant itself.
    • Bacterial diseases are much more prevalent in subtropical and tropical regions of the world.
    • Five main types of bacterial pathogenicity factors are known: uses of cell wall–degrading enzymes, toxins, effector proteins, phytohormones and exopolysaccharides.
    • Significant bacterial plant pathogens:
      • Burkholderia
      • Proteobacteria
        • Xanthomonas spp.
        • Pseudomonas spp.
      • Pseudomonas syringae pv. tomato causes tomato plants to produce less fruit, and it "continues to adapt to the tomato by minimizing its recognition by the tomato immune system."
  • Phytoplasmas and spiroplasmas
    • Phytoplasma and Spiroplasma are genera of bacteria that lack cell walls and are related to the mycoplasmas, which are human pathogens.
    • They are normally transmitted by sap-sucking insects, being transferred into the plant's phloem where it reproduces.
  • Viruses, viroids and virus-like organisms
    • Under normal circumstances, plant viruses cause only a loss of crop yield. Therefore, it is not economically viable to try to control them.
    • Plant viruses are generally transmitted from plant to plant by a vector, but mechanical and seed transmission also occur. Vector transmission is often by an insect (for example, aphids), but some fungi, nematodes, and protozoa have been shown to be viral vectors. In many cases, the insect and virus are specific for virus transmission such as the beet leafhopper that transmits the curly top virus causing disease in several crop plants.
  • Nematodes
    • Nematodes are small, multicellular wormlike animals.
    • They are a problem in tropical and subtropical regions of the world, where they may infect crops.
    • Potato cyst nematodes (Globodera pallida and G. rostochiensis) are widely distributed in Europe and North and South America.
    • Root knot nematodes have quite a large host range, they parasitize plant root systems and thus directly affect the uptake of water and nutrients needed for normal plant growth and reproduction, whereas cyst nematodes tend to be able to infect only a few species.
  • Protozoa and algae
    • A few examples of protozoa causing plant diseases (e.g., Phytomonas, a kinetoplastid). They are transmitted as durable zoospores that may be able to survive in a resting state in the soil for many years. Further, they can transmit plant viruses.
    • Some colourless parasitic algae (e.g., Cephaleuros) also cause plant diseases.
  • Parasitic plants
    • Parasitic plants such as mistletoe and dodder are included in the study of phytopathology. Dodder, for example, is used as a conduit either for the transmission of viruses or virus-like agents from a host plant to a plant that is not typically a host or for an agent that is not graft-transmissible.

Pathogens that cause diseases in animals:

  • Bacterial
    • The vast majority of bacteria, which typically range between 1 and 5 micrometres in length, are harmless or beneficial to humans. However, a relatively small list of pathogenic bacteria can cause infectious diseases.
    • One of the bacterial diseases which is lethal in humans is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa.
    • Pathogenic bacteria contribute to other globally significant diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter, and Salmonella.
    • Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis, and leprosy.
    • Chlamydia is a phylum of intracellular parasites. These pathogens can cause pneumonia or urinary tract infection and may be involved in coronary heart disease.
    • Other groups of intracellular bacterial pathogens include Salmonella, Neisseria, Brucella, Mycobacterium, Nocardia, Listeria, Francisella, Legionella, and Yersinia pestis. These can exist intracellularly, but can exist outside of host cells.
    • Bacterial meningitis is a bacterial inflammation of the meninges, that is, the protective membranes covering the brain and spinal cord.
    • Urinary tract infection is predominantly caused by the bacterium Escherichia coli. Bacteria can ascend into the bladder or kidney and causing cystitis and nephritis.
    • Helicobacter pylori is a type of bacteria responsible for diseases in gut.
    • Vibrio cholerae is a bacterium which is the causative agent of cholera.
    • Bacillus anthracis is the causative agent of Anthrax, a disease that is commonly seen in farm animals.
  • Viral
    • Some of the diseases that are caused by viral pathogens include smallpox, influenza, mumps, measles, chickenpox, ebola, rubella, hepatitis, foot and mouth disease in cattle and the most notable one, AIDS.
    • Pathogenic viruses are diseases mainly of the families of: Adenoviridae, Picornaviridae, Herpesviridae, Hepadnaviridae, Flaviviridae, Retroviridae, Orthomyxoviridae, Paramyxoviridae, Papovaviridae, Polyomavirus, Rhabdoviridae, Togaviridae.
  • Fungal
    • Fungi comprise a eukaryotic kingdom of microbes that are usually saprophytes (consume dead organisms) but can cause diseases in humans and animals.
    • Some of the disease-causing fungal genus includes Candida, Aspergillus, Cryptococcus, Histoplasma, Pneumocystis.
  • Prionic
    • According to the prion theory, prions are infectious pathogens that do not contain nucleic acids. These abnormally folded proteins are found characteristically in some diseases such as scrapie, bovine spongiform encephalopathy (mad cow disease) and Creutzfeldt–Jakob disease.
  • Other parasites
    • Some eukaryotic organisms, including a number of protozoa and helminths, are human parasites (i.e., they cause various infectious diseases).
  • Algal
    • Examples of algae acting as a mammalian pathogen are known as well, notably the disease protothecosis. Protothecosis is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as prototheca that lacks chlorophyll.

Extract from HSC Biology Stage 6 Syllabus. © 2017 Board of Studies NSW.