Mechanism of discharge of zoospores from the

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Mechanism of discharge of zoospores from the zoosporangium is varied from species to species and may include: (i) Exooperculate mechanism – discharge of zoospores through an externally located lidded papilla, (ii) Endooperculate mechanism discharge of zoospores through an internally located papilla, (iii) Inoperculate mechanism – discharge of zoospores by rupturing of zoosporangial wall in non-operculum –forming species
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Continuous The number of zoospores formed is proportional to the size of the zoosporangium . The discharged zoospores swims and germinate under favorable condition to form a new thallus. Sexual reproduction may be missing in some species but when present, it involves the fusion of (i) gametes/spores e.g. Synchytrium brownii; (ii) gametangia e.g. Polyphagus euglenae (iii) rhizoids of thalli e.g. Chytriomyces hyalinus (iv) encyted zoospores function as gametangia and copulate e.g. Rhizophlyctis Zygote is the product of fusion and forms a (i) resting spore under unfavorable condition that germinates into a zoosporangim. The zygote can also matures directly into meiosporangium (zoosporagium) The zoosporangial cytoplasm undergoes cleavages to form numerous zoospores that form a new thallus
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Continuation In Synchytrium , commonly known as false rust or wart disease,the asexual life cycle commences with released zoospores locating and infecting a suitable host plant by emptying their protoplasmic content into the host tissue. Protoplasm develops into prosorus which enlarges, undergoes cytological changes including vacuole formation before maturing into a sorus Zoospores function as gametes and copulate during the sexual cycle to form a zygote which then infect the plant host, form a diploid resting spore which develops into a prosorus . The prosorus matures into a sorus within
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Fig. 14: Life cycle of a typical Synchytrium spp. (Order – Chytriales)
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Fig. 15: Life cycle of a typical Allomyces spp. (Order – Blastocladiales)
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Continuation Members of this class, like the plasmodiophorids, are mostly pathogenic and responsible for seedling blight, downy mildew of grape vine, damping off, sudden death of oak, rot of soybean. They are filamentous, coenocytic, multinucleate, exhibit absorptive nutrition and reproduce by spores. Cell wall is composed of β -(1,3; 1,6)glucan and more of cellulose rather than chitin. Some species produce wall-less biflagellate (flagella unequal) flexible-shaped zoospores in specialized sporangia. The sporangia are either terminal or intercalary and demarcated by cross-walls or septa. In some non-zoospore forming species, the sporangia function as spore and germinate to either form germ-tube or produce zoospore.
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Table 5: Examples of Oomycetous fungi, type of disease caused and affected organisms S/n Oomycetous Fungi Infection Organism 1 Phytophthora infestans Potato blight Potato 2 Phytophthora ramorun root rot/dieback shade trees, conifers 3 Phytophthora cinnamomi Oak, Soybean, Eucalyptus 4 Plasmopara viticola downy mildew Grape 5 Sclerophthora rayssiae Maize 6
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  • Fall '19
  • Spore, Slime mold, multinucleate sporangia

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