Introduction to Fungi, Third Edition

620 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
stomatal surface acting as the thigmotropic
signal (Rubiales & Niks, 1996). There is also
adult-plant resistance, usually an interaction
between various genes which retard the progress
of infection to such a degree that the losses are
reduced to economically acceptable levels.
22.3 Puccinia graminis, the cause of
black stem rust
A connection between the barberry bush (Berberis
vulgaris) and rust disease on wheat and other
cereals had been suspected for centuries by
farmers who noticed the frequent occurrence of
crop damage around or downwind from barberry
bushes. Further, an altogether different-looking
fungus known as Aecidium berberidis was known
to infect barberry leaves. The formal proof that
the barberry and wheat pathogens were different
stages of the same species was made by Anton de
Bary who, in 1865, performed cross-inoculation
experiments to show that basidiospores derived
from germinating teliospores from wheat were
able to cause spermogonial and aecial infections
on the barberry. This story has been recounted
many times, but nobody has told it better than
Large (1940).
Black stem rust affects especially wheat but
also most other cereals and a range of wild
grasses. Crop losses can be severe as a reduction
in quantity as well as quality of the grain yield.
The host epidermis can become ruptured over
much of its surface in severe infections, thereby
debilitating the plants (see Fig. 22.8d). Serious
epidemics have occurred in the past, e.g. in the
USA in 1904 and especially during the war year
1916. In fact, losses were so severe (up to 50% in
the Great Plains; Eversmeyer & Kramer, 2000)
that P. graminis was seriously considered and
developed as a biological weapon by the US
Government during the 1960s (Line & Griffith,
2001). On susceptible wheat cultivars and without
chemical protection, P. graminis can cause
total crop failure.
Although it is an ecologically obligate
biotroph, Puccinia graminis is able to infect an
astonishingly wide range of grass and cereal
hosts. Gäumann (1959) listed 365 host species in
Fig 22.8 Puccinia graminis. (a) Spermogonial pustules on the upper surface of a leaf of Berberis vulgaris.Notethedropsofnectar.
(b) Aecia on the underside of a Berberis leaf.The outer frilly layer is the white peridium, within which is a mass of orange-coloured
aeciospores. (c) Wheat leaf showing uredinia which appear as reddish-brown powdery masses. (d) Wheat straw showing telia as
black raised pustules.