BSC 314 Lecture 48: Plant Pathogens

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27 Jun 2018
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Plant Pathogens
Many of the fungi are pathogens that infect plants and animals causing diseases of
many kinds. The life cycles of many of these are complex and involve two or more host
plants.
Rusts
The rusts are specialized basidiomycetes that are parasites on plants. They have
complex life cycles, and some produce five different kinds of spores in addition to
basidiospores. Many rusts are heteroecious and complete their life cycles on two
different kinds of host plants whereas autoecious parasites produce all of their different
kinds of spores on a single host species. Well-known examples of heteroecious rusts
are wheat rust, white pine blister rust, and cedar-apple rust.
Smuts
Smuts are parasitic basidiomycetes that produce powdery masses of black spores
enclosed in a membrane. This membrane is often found in the ovaries of species of
grasses, or on their leaves. The smut life cycle is less complex than that of the rusts,
and only one other kind of spore in addition to basidiospores is produced. All smuts
complete their life cycle on only one kind of plant. Smuts live as saprobes in the soil,
however, and readily attack developing seedlings planted in infected soil. Corn smut is
common in the Midwest. Despite the unappetizing appearance of the spore masses and
their dust-like texture, membrane-enclosed corn smut spore masses are considered
delicacies in some cultures and are eaten either boiled or fried.
Yeasts
Yeasts are unicellular fungi that reproduce asexually by budding, a process by which a
new cell is formed from a bulge or “bud” that enlarges and pinches off from the parent
cell. The nuclear material is divided by mitosis, and the new cell receives a nucleus and
cellular organelles before severance from the parent. Yeasts are found in all three of the
fungal phyla, but most are ascomycetes. Many are filamentous most of the time, and
change to the yeast growth form only occasionally.
Yeasts are of great importance to the baking and brewing industries with particular
strains guarded and nurtured closely, because the products of the yeast metabolism
give the distinctive flavors to the brews and cause the bread dough to rise in a
predictable fashion.
A Kingdom Separate from Plants
The fungi (singular, fungus) once were considered to be plants because they grow out
of the soil and have rigid cell walls. Now they are placed independently in their own
kingdom of equal rank with the animals and plants and, in fact, are more closely related
to animals than to plants. Like the animals, they have chitin in their cell walls and store
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