Many protists act as parasites that prey on plants or as decomposers
that feed on dead organisms.
LEARNING OBJECTIVES [ edit ]
Describe the actions of parasitic protists on plants
Describe the ways in which protists act as decomposers
KEY POINTS [ edit ]
Plasmopara viticola causes downy mildew in grape plants, resulting in stunted growth and
withered, discolored leaves.
Since downy mildew has a higher incidence in the late summer, planting early in the season can
reduce the threat of downy mildew; fungicides are also somewhat effective at preventing downy
mildew.
Phytophthora infestans causes potato late blight (potato stalks and stems decay into black slime)
and was responsible for the Irish potato famine in the nineteenth century.
Protist saprobes feed on dead organisms, which returnsinorganic nutrients to soil and water.
TERMS [ edit ]
oomycete
fungus-like filamentous unicellular protists; the water molds
saprobe
an organism that lives off of dead or decaying organic material
downy mildew
plant disease caused by oomycetes; causes stunted growth in plants as well as discolored,
withered leaves
Give us feedback on this content: FULL TEXT [edit ]
Plant Parasites
Protistparasites prey on terrestrial plants
and include agents that cause massive
destruction to food crops.
The oomycete Plasmopara
viticola parasitizes grape plants, which
causes a disease calleddowny mildew .
Grape plants infected with P.
viticola appear stunted and have
discolored, withered leaves. The spread of
downy mildew nearly collapsed the French
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wine industry in the nineteenth century. They are easily controlled once discovered, so
careful monitoring of susceptible hosts is key because if left unaddressed, the organism can
quickly spread and completely overwhelm the host species
Downy mildew
Both downy and powdery mildews on this grape leaf are caused by an infection of P. viticola.
Because the downy mildew pathogen does not overwinter in midwestern fields, crop rotations
and tillage practices do not affect disease development. The pathogen tends to become
established in late summer. Therefore, planting early season varieties may further reduce the
threat posed by downy mildew. Fungicides can also be applied to control downy mildew.
Broad spectrum protectant fungicides such as chlorothalonil, mancozeb, and fixed copper are
somewhat effective in protecting against downy mildew infection.
Phytophthora infestans is an oomycete responsible for potato late blight. This disease causes
potato stalks and stems to decay into black slime . Widespread potato blight caused by P.
infestans led to the well-known Irish potato famine in the nineteenth century that claimed
the lives of approximately one million people and resulted in the emigration of at least one
million more from Ireland. Late blight continues to plaguepotato crops in certain parts of the
United States and Russia, wiping out as much as 70 percent of crops when no pesticides are
applied.
Potato Late Blight
These unappetizing remnants result from an infection with P. infestans, the causative agent of potato
late blight.
Agents of Decomposition
The fungus-like protist saprobes are specialized to absorbnutrients from nonliving organic matter, such as dead organisms or their wastes. For instance, many types of
oomycetes grow on dead animals or algae. Saprobic protists have the essential function of
returning inorganic nutrients to the soil and water. This process allows for new plant growth,
which in turn generates sustenance for other organisms along the food chain. Indeed, without
saprobe species, such as protists, fungi, and bacteria, life would cease to exist as all organic
carbon became "tied up" in dead organisms.