To introduce an important group of fungus-like organisms and to offer an opportunity to conduct some simple biodiversity exercises.
It is difficult to exaggerate the importance of these "fungus-like organisms" as decay organisms or as parasites. Many species of Saprolegnia and Achlya live saprophytically on decaying plant and animal parts. Some are animal parasites, e.g., the common tropical fish disease "ick" is caused by the oomycete Saprolegnia parasitica.
Others cause important plant diseases. For instance, Phytophthora infestans is the causal agent of late blight of potato, the disease that led to the Irish potato famine.
Other Phytophthora species cause blights and many cause root rots.
Aphanomyces and Pythium species are ubiquitous in soils and are the most common cause of seedling death in excessively wet soils.
These root-rotting oomycetes (Pythium, Phytophthora, Aphanomyces) are the reason we use sterile potting mixes for houseplants and for starting vegetable and flower transplants, rather than taking soil directly from our yards or gardens. Since oomycetes like water-logged soils, we also use pots with holes in the bottom to promote good drainage and healthy plant roots.
Other important plant diseases caused by oomycetes include downy mildews and the white rusts, which are diseases of aboveground plant parts, e.g., leaves, shoots, and fruits. Both downy mildews (e.g., Peronospora, Plasmopara) and white rusts (Albugo) are caused by obligate parasites, that cannot be grown in culture.
Figure 5. Downy mildew of cabbage (left) is caused by Peronospora parasitica (right) which has determinate sporangiophores and sporangia that do not make zoospores. Click image to see a larger view
Oomycetes are not considered true fungi, but are classified as protists. Their cell walls are predominately glucans and cellulose, rather than chitin as in true fungi. Their vegetative nuclear state is diploid (2n), unlike the true fungi. However, most species have a mycelium for a vegetative structure and produce both asexual and sexual spores similar to true fungi. The asexual spores are motile; these zoospores have two flagella and are produced in sporangia.
Sporangia shape varies among genera and species of oomycetes. For example the sporangia of Phytophthora are lemon-shaped in some species and oval in others, those of Pythium are often round, and those of Aphanomyces and Saprolegnia look like very long cylindrical balloons (linear).
Some oomycetes also produce asexual thin to thick walled spores called chlamydospores, which serve as survival structures.
The asexual stage of oomycetes forms first as a means of rapidly exploiting a food source. This is followed by the sexual stage as growing conditions deteriorate. The sexual spores of oomycetes are thick-walled oospores produced from the union of the antheridia and oogonia (male and female sexual structures, respectively). Oospores are important survival structures and sources of genetic recombination. Oospores allow the organism to wait until the next food source comes along, or until environmental conditions improve.