Introduction to Fungi, Third Edition

OTHER CEREAL RUSTS
627
race characterization has been written by Roelfs
(1984).
The serious race 15B epidemics of the early
1950s (see Fig. 22.12) resulted in the replacement
of the then prevalent cultivars by others such as
‘Selkirk’, which carried several resistance genes
and remained resistant to P. graminis f. sp. tritici
in North America for several decades (Roelfs,
1985). Several wheat varieties more recently
released have also retained their resistance for
many years in the field (Roelfs, 1984; Dyck &
Kerber, 1985), so that no major stem rust
epidemic has occurred in the USA during the
past 50 years. This success is probably due to two
main reasons, namely the combination of several
resistance genes in one cultivar which the rust
fungus is apparently unable to overcome, and
the stabilization of rust races by the elimination
of sexual reproduction as a consequence of the
barberry eradication campaign.
22.4 Other cereal rusts
Although potentially the most damaging cereal
rust, P. graminis has lost much of its menace
for reasons discussed in the preceding section.
Several other rust species now cause more
serious crop losses than P. graminis. These are
briefly discussed below. Cereal rusts are primarily
controlled by breeding of resistant cereal
cultivars, and this is an ongoing battle against
new races which, once arisen, are capable of
spreading rapidly and on a global scale (Chen,
2005; Kolmer, 2005). Chemical control by the
application of fungicides is practised to protect
crops grown for seed production, and to control
severe outbreaks of rust if resistance fails. Fungicides
used include the systemic strobilurin-type
compounds and the ergosterol biosynthesisinhibiting
triazoles (see Fig. 13.15), as well as
various protectant compounds.
22.4.1 Puccinia triticina (brown leaf
rust of wheat)
Puccinia triticina was formerly named P. recondita
f. sp. tritici, but Zambino and Szabo (1993) and
Anikster et al. (1997) have shown that it is not
closely related to P. recondita, which infects rye
but is only of minor significance. Puccinia triticina
has displaced P. graminis as the economically
most important rust of wheat especially in North
America and Eastern Europe (Samborski, 1985).
This species is macrocyclic, with Thalictrum speciosissimum
(¼ T. flavum; Ranunculaceae) serving as
the alternate host, although it seems to survive
mainly in the uredinial state. Its epidemiology is
therefore similar to that of P. graminis f. sp. tritici
(Eversmeyer & Kramer, 2000). The temperature
optima for urediniospore germination and infection
(about 18°C) and sporulation (25°C) are
lower than those of P. graminis (Singh et al.,
2002). Teliospores are not readily formed, and
the disease symptoms are easily recognized by
the chocolate-brown uredinial lesions on wheat
leaves. There is no infection of the stem, in contrast
to P. graminis. Triticale, a hybrid between
wheat (Triticum) and rye (Secale), is also affected.
About 50 Lr (‘leaf rust’) resistance genes are
known, and wheat leaf rust is currently controlled
mainly by using cultivars containing
a pyramid of several of these Lr genes.
22.4.2 Puccinia striiformis (stripe rust or
yellow rust on wheat and barley)
The alternate host of this rust species has not
yet been found, and it may no longer have one.
Therefore, its life cycle is probably confined to
the grass or cereal hosts on which uredinia and
telia are produced. This rust can be distinguished
from P. triticina by its yellow uredinia which are
arranged in stripe-like rows following the leaf
veins. There are several formae speciales, but their
host ranges overlap to a greater extent than
in the other cereal rusts and they are therefore
less clearly delimited. The most important
forms are those on wheat (f. sp. tritici) and
barley (f. sp. hordei). The temperature optima of
P. striiformis are the lowest of any of the common
cereal rusts, with urediniospore germination
and penetration around 10°C, and urediniospore
production at 12 15°C (Singh et al., 2002). This
species overwinters by using winter crops and
volunteers as a ‘green bridge’, and can cause
early epidemics in spring. Zadoks and Bouwman
(1985) have estimated that a single uredinium