Biology Reference
In-Depth Information
Rusts
Rust fungi belong to the Uredinales, a highly
specialized order of the Basidiomycetes. In com-
mon with mushrooms they have spores of the
sexual stage borne in fours on a club-shaped
hypha known as a basidium, but apart from this
they differ very decidedly from woody and fleshy
Basidiomycetes. The term rust is applied both to
the pathogen and to the disease it inflicts. There
are more than 4000 species of rusts, all obligate
parasites on ferns or seed plants. Many are heter-
oecious, completing their life cycle on two dif-
ferent kinds of plants; but some are autoecious
(monoecious), having all spore forms on a single
host species. There are only two families,
Melampsoraceae and Pucciniaceae.
Many rusts show physiological specialization,
the existence within a species of numerous strains
or races that look alike but attack different vari-
eties of crop plants, thus greatly complicating the
problem of breeding for rust resistance. Rusts
with a complete life cycle have five different
spore forms, numbered 0 to IV.
0.
Pycniospores
(spermatia) formed in
pycnia
(spermagonia). The pycnia resemble pycnidia
of Ascomycetes, are usually on upperside of
leaves. They discharge one-celled
pycniospores with drops of nectar, and these,
usually distributed by insects attracted to the
sweet secretion, function in fertilization.
I.
Aeciospores
(aecidiospores), one-celled,
orange or yellow, formed, often in chains,
in a cuplike sorus or
aecium
, which has
a peridium (wall) opening at or beyond the
surface of the host.
II.
Urediospores
(uredospores, summer spores,
red rust spores), one-celled, walls spiny or
warty, reddish brown, on stalks or in chains
in a
uredium
(uredinium or uredosorus), over
which the epidermis of the host is broken to
free the spores. Resting II spores, formed by
some rusts, have thicker and darker walls.
III.
Teliospores
(teleutospores, winter spores,
black rust spores), one or more cells, in
telia
(teleuto sori), either on stalks, as in the
family Pucciniaceae, or sessile, in crusts or
cushions as in the Melampsoraceae.
IV.
Basidiospores
(sporidia) on a basidium or
promycelium formed by the germinating
teliospore. Basidium is usually divided trans-
versely into four cells, with one sporidium
formed from each cell at the tip of a sterigma.
In heteroecious rusts spore stages 0 and I are
formed on one host and II and III on another, and
are so indicated in the information given with
each species. Stage IV always follows III on
germination. Although most autoecious rusts
have all spore forms, on one host, there are
a few short-cycle (microcylic) rusts with some
spore stages dropped out. For a detailed life his-
tory of a heteroecious rust,
Puccinia graminis
.
Gardeners frequently mistake a reddish discol-
oration of a leaf, perhaps due to spray injury or
weather or a leaf-spot fungus, for rust. True rust is
identified by the presence of rust-colored spores in
powdery pustules or perhaps gelatinous horns.
With rusts, the discoloration of tissue is yellowish,
not red, and it is due to increased evaporation from
the broken epidermis. Plants are often stunted.
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