BASIC PLANT PATHOLOGY Topics What is plant pathology Definition of disease Abiotic, biotic and decline diseases Signs, symptoms and patterns Mechanisms of disease action The disease triangle, square and tetrahedron Classification of organisms
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BASIC PLANT PATHOLOGY • Topics • What is plant pathology • Definition of disease • Abiotic, biotic and decline diseases • Signs, symptoms and patterns • Mechanisms of disease action • The disease triangle, square and tetrahedron • Classification of organisms • 8. Organisms causing biological diseases • 9. Pathogen/insect relationships
1. What is plant pathology – the study of the organisms and environmental factors that cause diseases in plants, mechanisms by which they cause disease, and methods of managing diseases and reducing damage. 2. Definition of disease - sustained physiological and structural damage to plant tissues caused by biological and non-biological agents ending sometimes in plant death.
YOU TREE Skin diseases Foliagediseases Cankers Ozone Heart rot Vascular system diseases – Dutch Elm disease Athlete’s foot fungus Root diseases
3. Biotic, abiotic and decline diseases Biological – fungi, bacteria, etc. non-biological (abiotic) decline (combination – distinct causes may be unknown).
Non-biological agents - excesses of temperature and moisture, nutrient deficiencies or excesses, air pollution, etc.
4. Signs, Symptoms and Patterns a. Signs - indications of presence of disease causing organism – e.g., fruiting body or mycelium of fungi b. Symptoms - change in host - exudations, resinosis, necrosis (death of tissue or tree), hypotrophy (dwarfing), hypertrophy (overgrowths - galls, witches brooms). c. Patterns – random – biotic agents - systematic - abiotic
How do we establish that fungi caused the observed disease • Rules of proof “Koch's postulates” • Establish constant association of organism and disease symptoms. • Isolate organism and grow in pure culture. • Inoculate healthy plant and produce disease symptoms • Re-isolate organism.
Mechanisms of disease action • Production of enzymes (pectinase, cellulase) • Toxins • Hormones (giberellin) • Physically plugging conducting tissues • Nutrient competition
Classification of fungi(Fusarium solani – important plant pathogen) Domain- Eukarya Eurarya Kingdom- Fungi Fungi Phylum- ----mycota Ascomycota Class- ----mycetes Euascomycetes Subclass- ----tidae Pyrenomycitdae Order- ----ales Hypocreales Family- ----aceae Hypocreaceae Genus Fusarium Species solani Forma speciales f. sp. gladioli Race 1
Classification of insects(Asian long-horned beetle) Domain- Eurarya Kingdom- Animalia Phylum- Arthropoda Class- Insecta Subclass Pterygota Division Endopterygota Order- Coleoptera Suborder Polyphaga Superfamily Chrysomeloidea Family Cerambycidae Subfamily Lamiinae Tribe Monochamini Genus Anaplophora Species glabripennis 1
8. Organisms causing biological disease • Fungi and fungus-like organisms • Viruses, viroids, and prions • Bacteria • Phytoplasmas • Nematodes • Parasitic plants • Protozoans
Fungi cause the greatest problems on woody plants because they have enzymes to break down cellulose and lignin. Bacteria and viruses more of a problem on soft tissued plants.
Perhaps a million species of fungi, but only 100,00 are known. 10,000 are plant pathogens
Phylum Role Fungus-like organism Oomycota Damping off, feeder (fine) root diseases True fungi Zygomycota Saprophyte, endomycorrhizal fungi, few pathogens (storage molds) Ascomycota Saprophyte, ectomycorrhizal fungi, (Deuteromycota) foliage diseases, cankers, vascular wilts, fine root diseases, powdery mildews, stain fungi Basidiomycota Saprophytes, ectomycorrhizal fungi, stem and branch decays, structural root diseases, rusts
FEATURES OF FUNGI AND FUNGAL-LIKE ORGANISMS Phylum Features Hyphae Cell walls Spores Sexual Asexual Oomycota no septa glucans/cellulose Oospores zoospores (fungal-like) TRUE FUNGI Zygomycota no septa glucans/chitin Zygospores sporangiospores Ascomycota septa glucans/mannans Ascospores conidia Deuteromycota septa glucans/mannans None conidia Basidiomycota septa glucans/mannans Basidiospores conidia, (with (rarely) clamps)
Features of fungi The vegetative body of fungi is the mycelium (made up of thread-like hyphae – with and without septa) which grow through or on substrates. Fungi disperse widely by spores which can be either asexual or sexual. The most commonly produced spores are asexual conidia or conidiopsores. Other asexual spores are sporangiospores. Sexual spores include zygospores, ascospores and basidiospores; they are produced in fruiting bodies. Spores can be airborne, spread by rain splash or water or by insects, other animals and humans. Oomycota were once included in the Fungi but are now considered to be fungal-like organisms. Oomycota have asexual spores (zoospores) and sexual spores (Oospores). Many fungi have resting stages - chlamydospores or sclerotia
Nectria perithecia Peziza apothecia Ascomycota
Nectria Canker – Ascomycota
Basidiomycete hyphae showing clamp connections. Also associated yeast cells and bacteria Basidiomycete hyphae decaying wood cells – clamp connections and Ca oxalate crystals
White Pine Blister Rust – Basidiomycota
Other Pathogens • Viruses, viroids, and prions • Bacteria • Phytoplasmas • Nematodes • Parasitic plants • Protozoans
Viruses, viroids, and prions • Viruses – protein coat and nucleic acid • Viroid – low MW RNA • Prion – infectious protein molecule
Viruses and viroids • Needs a vector • Insects • Nematodes • Grafting or vegetative propagation • Can move through plant in phloem or xylem, or stay localized in foliage • Can become part of plant genome
Symptoms • Foliage streaking, spotting, mottling • Brooms or rosettes • Growth reduction • No symptoms
Bacteria • 1-celled prokaryotes with a cell wall • Rods (bacilli), spirals, or spherical (cocci) • Easily exchange genetic material on plasmids
Symptoms • Bacteria cause disease by enzymes that digest cell walls, toxins, or tumors • Typical symptoms are: • Water soaking • Wetwood • Shoot blight • Bleeding cankers • Galls
Bacterial Wetwood • Very common on elms and poplars • Caused by anaerobic bacteria in the xylem following wounding and wound closure • May prevent decay fungi from colonizing
Crown Gall • Caused by Agrobacterium tumifaciens, a soil bacterium • Usually occurs at the soil line, but sometimes on aerial parts • Requires wounding for infection • Galls interfere with nutrient and water flow • Used in genetic engineering – can insert DNA into plant genome
Important genera of bacteria Causing plant diseases Pseudomonas Xanthomonas Agrobacterium Bacillus Clostridium Streptomyces Bacterial leaf spot