Introduction to Fungi, Third Edition

UREDINALES: THE RUST FUNGI
613
Homothallic species are also known, and these
generally do not produce spermogonia. A generalized
account of rust life cycles has been given
by Buller (1950).
Following fertilization, the nucleus from
the donor spermatium divides repeatedly and
migrates down the receptive hypha into the
receiving primary mycelium which thereby
undergoes dikaryotization. When compatible
nuclei reach the proto-aecium, this becomes
converted into an aecium which breaches the
lower epidermis. Heterokaryotic aeciospores
develop and are released by a sudden roundingoff
of the flattened wall separating adjacent
spores, thus flicking the spores into the air.
Aeciospores are relatively thin-walled and bear
warty rather than spiny surface ornamentations.
They are often brightly coloured due to the
abundance of carotenoids accumulating in lipid
droplets (Plate 12a). Aeciospores of P. graminis
are unable to re-infect Berberis, but are infective
on the principal grass host. The resulting secondary
mycelium produces urediniospores, thus
completing the life cycle.
Examples of the different spore types and
the pustules (sori) producing them are shown in
Fig. 22.2. Spores and sori have been given various
names, and we follow the naming used in the
Dictionary of Fungi (Kirk et al., 2001). It is customary
to assign Roman numerals 0, I, II, III or IV
to the distinct spore types, and these numbers
provide a convenient shorthand for describing
the range of spores found in a given rust.
The most important terms are summarized in
Table 22.1.
The various spore stages of rust fungi differ
greatly in their length of survival. Although
details are dependent on species and conditions
of storage (temperature, state of hydration,
light), it may be generalized that the maximum
survival period in the field is in the order
of days (spermatia and basidiospores), weeks
(aeciospores), a few months (urediniospores)
and several months to more than a year
(teliospores).
22.2.2 Derived life cycles and Tranzschel’s
Law
Rust fungi which must alternate between two
different host plants in order to complete their
life cycle are called heteroecious. In contrast,
autoecious species are confined to one host
plant. Species whose life cycle contains all five
possible spore stages are called macrocyclic. In
this terminology, P. graminis is a macrocyclic
heteroecious rust whereas P. menthae, which
produces all five spore stages on one host
(mint), is macrocyclic but autoecious. Many
rusts have derived life cycles in which one or
more spore stages have been omitted. One
common variation is the absence of uredinia in
heterocyclic rusts. For instance, Gymnosporangium
fuscum (see p. 629) produces spermogonia and
aecia on the leaves of pear trees and has Juniperus
spp. as its principal host for production of telia.
Such rusts which lack uredinia are called
demicyclic or -opsis forms.
An autoecious version of the demicyclic theme
is presented by P. lagenophorae which produces
spermogonia, aecia and telia on Senecio spp.
(see Figs. 22.2c f). In functional terms, the aecia
could be considered uredinia because the aeciospores
infect the same host species on which they
were produced, and because in old aecia the
Table 22.1. Generally accepted terminology of the sori and spore states of rust fungi.
0 I II III IV
Sorus Spermogonium Aecium Uredinium Telium Basidium
Pycnium Aecidiosorus Uredosorus Teleutosorus Metabasidium
Aecidium Uredium Promycelium
Spore Spermatium Aeciospore Urediniospore Teliospore Basidiospore
Pycniospore Aecidiospore Uredospore Teleutospore Sporidium
Urediospore