Introduction to Fungi, Third Edition

612 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
Fig 22.2 Examples of spore stages in rust fungi. (a) Uredinium of Puccinia obscura on its principal host, the wood-rush
Luzula campestris. Note the spiny urediniospores. (b) Development of a telium within an old uredinium of P. obscura.Some
urediniospores are still seen on top of the smooth-walled teliospores pushing through. (c) Karyogamy in a teliospore of Puccinia
lagenophorae, a demicyclic rust on Seneciovulgaris.The terminal cell still contains two paired nuclei (arrowheads) whereas in the basal
cell the two nuclei have already fused to give one larger (diploid) nucleus. (d) Production of a promycelium (metabasidium) by Puccinia
graminis.Four sterigmata have formed, each about to produce one basidiospore. (e) Spermogonium of P. obscura on its
alternate host, Bellis perennis. Note the flask-shaped body and the ring of stiff periphyses which protrude beyond the upper host
epidermis. (f) Aecia of P. lagenophorae on Senecio vulgaris.
ostiole in a sugary liquid which attracts insects.
These carry spermatia around, and fertilization
occurs if a spermatium makes contact with
a modified periphysis called receptive hypha
of a spermogonium of opposite mating type.
Spermatia are incapable of independent
germination to establish new infections but
perform solely a sexual role in fertilization.
Most rust fungi are heterothallic with a bipolar
mating system, although Narisawa et al. (1994)
have claimed that P. coronata has a tetrapolar
mating system with two mating type loci.