oomycete

Oomycetes, commonly known as water molds, are a group of fungus-like organisms within the Stramenopiles. They resemble fungi in their absorptive nutrition and filamentous growth but are not true fungi; they are more closely related to brown algae and diatoms. Oomycetes typically form coenocytic, or rarely septate, hyphae and have cellulose-containing cell walls. They inhabit fresh water, moist soils, and plant debris, and many are plant pathogens.

Reproduction occurs both asexually and sexually. Asexual reproduction involves sporangia that release motile biflagellate zoospores into water, enabling rapid dispersion. Sexual reproduction produces oospores within oogonia through fertilization, providing durable survival structures. The life cycle allows transitions between saprotrophic and parasitic modes, with some species specialized for aquatic environments.

Ecological and economic importance: Several oomycetes cause severe plant diseases, including Phytophthora infestans (potato late blight), Phytophthora cinnamomi (dieback), and Pythium species (seedling damping-off). Others, such as Saprolegnia, affect fish. Management relies on sanitation, drainage, resistant varieties, crop rotation, and targeted chemicals such as oomycete-specific fungicides; many fungicides for true fungi are ineffective. Oomycetes are a major concern in agriculture, horticulture, and aquaculture, and their study informs plant pathology and biosecurity.