BLGY 1643 Lecture 5 Plant Pathology PDF 2024

BLGY 1643 Lecture 5 (Theme 2) Dr Gert Marais [email protected] The importance of Plant Pathology  Why study plant pathology? The causes of plant diseases Types of plant diseases Introduction The impact of disease epidemics...

BLGY 1643 Lecture 5 (Theme 2) Dr Gert Marais [email protected] The importance of Plant Pathology  Why study plant pathology? The causes of plant diseases Types of plant diseases Introduction The impact of disease epidemics Interaction between pathogen, host plant and environment Disease diagnosis Disease management Why study plant pathology? Plants serve as food sources Herbal medicine Plants produce oxygen Plants fixate CO2 Plants serve as energy Produce clothes, furniture, building material, paper, and medicines Why study plant pathology? Worldwide it is estimated that pests account for pre-harvest losses of up to 42% – Insects (15%) – Weeds (13%) – Pathogens (13%) – fungi, bacteria, and viruses Additional 10% is lost post-harvest Why study plant pathology? In South Africa 13% of the land surface can be used for crop production, of which 3% is high- potential arable land. We need, therefore, to optimally utilise the available land to produce food and feed crops. One way to contribute is the protection of crops against pests and diseases. Why study plant pathology? Plant Pathology Plant Pathology is defined as the study of the organisms and environmental conditions that cause disease in plants, the mechanisms by which this occurs, the interactions between these causal agents and the plant (effects on plant growth, yield and quality), and the methods of managing or controlling plant disease. A Plant Pathologist (or phytopathologist) is someone who study plant diseases and can identify diseases, interpreting data, and finding solutions to combat diseases in plants. Why study plant pathology? There is a desperate need for plant pathologists who understand the challenges and need to protect plants against diseases. Food security is going to become more relevant in decades to come and we need to find ways of overcoming problems that limit the optimal production of our plant resources on earth.  The causes of plant diseases Fungi Bacteria Viruses Nematodes Abiotic factors  Three different kinds of pathogens evolved (1) Facultative parasites – can either live saprophytically or parasitically Include pathogens of annual herbaceous crop plants. They must survive between growing seasons as soil mycota e.g. Fusarium (2) Necrotrophs – they are basically saprobical They produce toxins that kill host cells, after which they leak food materials. e.g. Monilinia Three different kinds of pathogens evolved (3) Obligate parasites – they are totally dependent on their hosts for survival. Sometimes the association is so close that the pathogen can only grow on a particular host or even cultivar within a species. e.g. rust fungi such as Puccinia sp.  What are fungi? All fungi are heterotrophs (cannot produce their own food) Composed of filaments that grow in a mass Size and shape determined by the environment Filaments secrete extracellular enzymes to degrade organic compounds Nutrients are absorbed through cell walls Individual filaments – hypha (pl. hyphae) Mass of hyphae – mycelium They can grow saprophytically or as parasites (biotrophs & necrotrophs) They can also produce mycotoxins, metabolites toxic to humans and animals Fruiting structures  What are bacteria? All bacteria are small (1-3 µm), single- celled organisms, much smaller than fungi Bacteria (singular bacterium) are prokaryotic (pro = before, karyo = nucleus) organisms They do not have membrane-bound nuclei or other organelles such as mitochondria Most have cell walls that determine the Figure: Bacterial colonies on a nutrient shape of the bacterium medium. Note that the bacteria have been Round (coccal) diluted through streaking. The small, round colonies are made up of thousands to Spiral or rod-shaped (bacilliform) millions of cells and each probably grew from a single bacterium. Figure 3-2. A bacilliform bacterium, typical of plant pathogens. Note that it has polar flagella.  What are viruses?  All viruses are obligate parasites  They cannot reproduce outside a living host cell, because their reproductive process, called replication, requires amino acids, nucleotides, enzymes, cellular structures, and energy supplied by the host cell  Symptoms are the expression of disease by a plant as a response to a pathogen. Most symptoms are unique to a specific plant x pathogen interaction  Leaf spots  Mottling  Stunting  Streaks  Cankers  Yellowing  Blights  Leaf curl  Rots  Wilt  Mosaic Leaf spot: A self-limiting lesion on a leaf Stunting: Plants that appear smaller than normal Canker: A necrotic, often sunken, lesion on a stem, branch, or twig of a plant Blight: A disease characterised by general and rapid killing of leaves, flowers and stems Rots: The softening, discoloration, and often disintegration of succulent plant tissue as a result of fungal or bacterial infection Mosaic: Symptom of certain viral diseases of plants characterised by inter- mingled patches of normal and light green or yellowish colour. Mottling: An irregular pattern of indistinct light and dark areas Camillia yellow mottle virus Tomato mottle virus Begonia mottle disease Streaks: Yellow stripes following the main veins of the leaves Maize streak virus Bacterial streak disease Canna streak virus Yellowing: Plant parts turn yellow due to ineffective photosynthesis Leaf curl: Leaves curl and shrivel up Wilt: Water and nutrients are cut off due to blocking of xylem Pepper wilting Tomato wilting Tomato wilting The impact of disease epidemics The disease triangle Susceptible host host Disease Conducive environment Pathogen disease environment pathogen  What is an epidemic? The disease pyramid - fourth Susceptible component - TIME host Time environment Time Disease Time Epidemic Conducive DISEASE environment Pathogen EPIDEMIC time pathogen plant Phytophthora infestans Late blight of potatoes Example: Irish potato famine in 1845 Potatoes were introduced to Europe in the 16th century from central America Poor Irish peasants used potatoes as staple food, consuming as much as 6kg per day Only one potato variety, the "Lumper", was used as a monoculture in Ireland because of its ability to produce enough food. In the mid 19th century, Phytophthora infestans was introduced in Ireland, and wiped out the Irish potato crop in the damp, cool summers between 1845-1847. Example: Irish potato famine in 1845 The disease was attributed to excess of water in the plants, or even the effects of the newly introduced steam locomotives This was the birth of plant pathology when Rev. M.J. Berkley concluded that the disease was caused by a fungus A million people died due to starvation The Irish population decreased from 8 million to 4 million, many emigrated to North America Memorial statues: Dublin, Ireland