Spotted fever is found predominantly in animals and is transmitted by the tick. Humans are accidental hosts. Q fever is found mostly in animals. Humans acquire disease primarily by inhalation of contaminated aerosols. APPLICATION IN AGRICULTURE Plant diseases have been associated with Rickettsia-like organisms(RLO). 1. Citrus Greening bacterium 2. Clover leaf RLO 3. Grapevine infectious necrosis RLO Assignment 1 (10 marks) The recent discovery between Rickettsia and viruses may become a basis for fighting HIV infection. Human immune response to the scrub typhus pathogen, Orientia tsutsugamushi rickettsia, appears to provide a beneficial effect against HIV infection progress, negatively influencing the virus replication process. Discuss the possible contribution of 27
4. Grapevine Pierce's RLO 5. Grapevine yellos RLO 6. Witch's broom disease 7. Peach phony RLO 28
CHAPTER 4: FUNGI F ungi are eukaryotic organisms that include microorganisms such as yeasts , molds , and mushrooms. Fungal cells have cell walls that contain chitin , compared to plant cell walls, which contain cellulose. Fungi reproduce via spores, which are often produced on fruiting bodies Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. Fungal cells contain membrane-bound nuclei with chromosomes that contain DNA They also have other membrane bound organeles like mitochondria, ribosomes, and vesicles for sugar/alcohols, disaccharides and polysaccharides storage. Fungi lack chloroplasts and are heterotrophic organisms, requiring preformed organic compounds as energy sources. They reproduce by both sexual and asexual means, and like lower plants produce spores. The cells of most fungi grow as tubular, elongated, and thread-like structures called hyphae. Hyphae may contain multiple nuclei and extend at their tips. Each tip contains a set of aggregated enzyme filled vessicles. 29
Unique features: 1. Some species grow as single-celled yeasts that reproduce by budding or binary fission. 2. Dimorphic fungi can switch between a yeast phase and a hyphal phase in response to environmental conditions. 3. The fungal cell wall is composed of glucans and chitin. Fungi are the only organisms that can combine these two structural molecules in their cell wall. 4. Most fungi lack an efficient vasicular system (xylem and phloem) for long-distance transport of water and nutrients. 5. To overcome these limitations, some fungi, such as Armillaria , form rhizomorphs that resemble and perform functions similar to the roots of plants. Microscopic structure Most fungi grow as hyphae, which are cylindrical, thread-like structures 2–10 µm in diameter and up to several centimetres in length. Hyphae grow at their tips (apices); New hyphae are formed through new tips along existing hyphae The apical growth and branching leads to the development of interconnected network of hyphae the mycelium.
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