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Exercise: Water Molds (Oomycetes)

Purpose:

To introduce an important group of fungus-like organisms and to offer an opportunity to conduct some simple biodiversity exercises.

Background information:

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.

Figure 1. Phytophthora infestans on the leaves of a potato plant. Note the blighting (brown areas of leaf) and white growth of sporangia and sporangiophores. Click image to see a larger view

Other Phytophthora species cause blights and many cause root rots.

Figure 2. Phytophthora root rot of  conifer can be a serious problem on Christmas tree farms. When a conifer seedling is planted in a poorly drained site, the disease spreads to other trees and kills them. The trees with brown needles have died from Phytophthora root rot. Click image to see a larger view

Aphanomyces and Pythium species are ubiquitous in soils and are the most common cause of seedling death in excessively wet soils.

Figure 3. Damping-off of Verbena caused by Pythium species. The plants in the center cell pack are very small, dead, or dying. Click image to see a larger view

Figure 4. Aphanomyces species cause root rot of many vegetables and flowers, e.g., root rot of peas by Aphanomyces euteiches. Notice the progression (left to right) from healthy, white roots through increasingly more brown roots and shorter plants as the disease becomes more severe. Click image to see a larger view

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

Figure 6. Downy mildew of grapes caused by Plasmopara viticola is a very destructive disease on the European grape Vitis vinifera used to make wine. The wild grapes of America, however, have coevolved with this oomycete, and so, although they can be infected by this pathogen, it causes little damage. Notice the white growth (mycelia and sporangia) on the fruit. Click image to see a larger view

Figure 7. Albugo candida causes "white rust" on brassicas, e.g., on broccoli, cabbage, canola, Chinese cabbage, mustard, radish, rutabaga, turnip, etc. 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.

Movie. Watch zoospores being released from sporangia of Phytophthora sojae. (Courtesy Edward Braun, Dept. of Plant Pathology, Iowa State University)

Note: After viewing the movie, use your browser's back button to return to the lesson.

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).

Figure 8. The sporangia of Aphanomyces euteiches are formed at hyphal tips (terminal) and are linear. Notice the zoospores clustered at the opening of the sporangia. Click image to see a larger view

Figure 9. Pythium undulatum has round sporangia. In Pythium, the sporangium forms a secondary sporangium called a vesicle in which the zoospores form.The zoospores in this vesicle will soon be released. Click image to see a larger view

Some oomycetes also produce asexual thin to thick walled spores called chlamydospores, which serve as survival structures.

Figure 10.Chlamydospores of Pythium species. Click image to see a larger view

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.

Figure 11. Pythium oogonium (female sexual structure) has two antheridia (male sexual structures) attached at the side (paragynous). Click image to see a larger view

Figure 12. Phytophthora cambivora has oogonia with an ornate wall and two-celled antheridia encircling the base of the oogonia (amphigynous). Click image to see a larger view

Figure 13. Some Phytophthora species have smooth oogonia, as in Phytophthora cinnamomi shown here. Click image to see a larger view