Introduction to Fungi, Third Edition

PUCCINIA GRAMINIS, THE CAUSE OF BLACK STEM RUST
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22.3.2 Puccinia graminis on cereals
A symptom of infection on wheat leaves and
stems is the appearance of brick-red pustules
(uredinia) between the veins. Uredinia contain
stalked, one-celled dikaryotic urediniospores
which burst through the epidermis (Figs. 22.8c
and 22.10a,b). These have a spiny wall which has
four thinner areas (germ pores) near the middle
of the spore. The urediniospores are detached by
wind and blown to fresh wheat leaves upon
which they germinate by extruding a germ tube
from one of the germ pores. Germination
requires free water (e.g. night-time dew) and
proceeds optimally at about 20°C. Infection
Fig 22.10 Puccinia graminis f. sp. secalis. (a) T.S. stem through a uredinium.The stalked unicellular urediniospores are protruding
through the ruptured host epidermis. A teliospore (t) has also been formed. (b) Higher-power detail of urediniospores. Note the
germ pores (g) and the haustoria (h) in the host cells. (c) Germination of urediniospores on host leaf. Note the directional growth
of the germ tubes perpendicular to the long axes of the epidermal cells, towards the stomata. (d) T.S. leaf sheath through a telium.
The stalked teliospores are projecting through the ruptured epidermis.Drawing to same scale as (a). (e) Germination of teliospores
to form metabasidia bearing sterigmata and basidiospores.One basidiospore is giving rise to a secondary spore.