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.
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” i.Establish constant association of organism and disease symptoms. ii.Isolate organism and grow in pure culture. iii.Inoculate healthy plant and produce disease symptoms iv.Re-isolate organism.
5.Mechanisms of disease action Production of enzymes (pectinase, cellulase) Toxins Hormones (giberellin) Physically plugging conducting tissues Nutrient competition
6. The disease triangle - plant, pathogen, and environment
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
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
Nematodes Worm-like animals in Phylum Nemahelminthes Various parasitic habits Usually in the soil or on roots, more of an issue in agriculture " If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable..." -N.A. Cobb, 1914 -N.A. Cobb, 1914
Pine Wilt Nematode Bursaphelenchus xylophilus, vectored by Monochamus sp. (Cerambycidae) Introduced to Asia from North America
Root nematode Pine wilt nematode symptoms Pine wilt nematode
Mode of Action Plant parasitic nematodes have a stylet to pierce plant cell walls Cause injury by feeding, toxins Vectors of other diseases Contributing factors in declines Indirect damage
Parasitic Plants Evolution towards parasitism has occurred at least 8 times in the flowering plants. Many are in the order Santalales: Loranthaceae – leafy mistletoes Santalaceae – root and stem hemiparasites Viscaceae –Phoradendron – leafy mistletoes –Arceuthobium – dwarf mistletoes
True Mistletoes Phoradendron sp. - –Common on oaks – South of 40-45 lat. –Cause water stress, usually not lethal –Spread by birds
Symptoms Parasitic plants are usually visible Brooming Galls Reduced growth
Protozoans such as phytoflagellates can parasitize milkweed, tomato, onion and chive plants.
9. Insect/Pathogen relationships a. Virus spread by plant hoppers, aphids b. Fungi spread by: bark beetles – Dutch elm disease fungus wood wasps (Sirex) – Hematostereum c. Nematodes spread by bark beetles – Pine wilt disease d. Bark beetles and root diseases are closely associated
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