Botany - B.Ye. Yakubenko 2017

Part Two. System of the Organic World
Chapter V. System of the Organic World
5.4. KINGDOM CHROMISTA

The kingdom Chromista includes eukaryotic organisms that are both autotrophic (such as diatoms, haptophytes, and cryptophytes) and heterotrophic, fungus-like organisms, particularly oomycetes. Phylogenetically, chromists are closer to heterokont Algae—sharing a cellulosic Cell wall—than to true Fungi, which is why they have been classified into a separate kingdom.

Phylum Oomycota. Organisms of this phylum possess a well-developed, coenocytic mycelium. The Cell wall is composed of Cellulose, entirely lacking Chitin. Asexual reproduction occurs via zoospores equipped with two flagella—one tinsel and one whiplash. Sexual reproduction is oogamous, resulting in The formation of an oospore. The phylum Oomycota comprises approximately 800 species grouped into 3 classes and several orders, with Saprolegniales and Peronosporales being the most prominent. Key Representatives of the order Saprolegniales include Saprolegnia and Achlya, which infect fish eggs, larvae, and fry, as well as weakened or injured fish. Saprophytic forms play an essential role in the Mineralization of organic matter in aquatic environments. These represent the aquatic oomycetes. Members of the family Peronosporaceae, on the other hand, illustrate the adaptation of fungus-like organisms to terrestrial life, representing the most ancient and evolutionarily advanced group within the phylum.

Among the Peronosporales, Phytophthora (family Pythiaceae) causes severe agricultural damage by attacking plant species of the Solanaceae family. It targets the leaves and tubers of potatoes, as well as the foliage and fruit of tomatoes. Infection is initiated by biflagellate zoospores in mid-to-late summer. The spores penetrate the leaves and develop into a branched, coenocytic mycelium that destroys chlorophyll, leading to the formation of brown necrotic spots on the leaves. Branched hyphae emerge through the Stomata on the lower epidermis, terminating in zoosporangia containing zoospores. These sporangia are dispersed by wind and rain splashes, rapidly infecting neighboring plants. Under favorable weather conditions and high humidity, the cycle from infection to sporulation takes just three days. Washed into the soil, the zoospores infect tubers, causing a destructive 'wet rot'. Other notable members of the Peronosporales include Sclerospora, which attacks cereals; Plasmopara, which causes downy mildew in grapes; and Peronospora, which induces downy mildew in tobacco, cucumbers, onions, beets, poppies, and other crops.

Control measures against phytopathogenic oomycetes focus on breeding resistant (immune) plant varieties, adhering to optimal agricultural practices, and applying chemical treatments.