Biology Reference
In-Depth Information
Zealand from somewhere outside the Australasian region (Brown and
Hovmøller 2002). At the time, more than 90% of the wheat on the North
Island was susceptible to this rust strain.
Wheat stem rust (
Puccinia graminis
) has a similar life cycle, with the
usual alternate host being common barberry (
Berberis vulgaris
) in Europe
and North America. Some native North American barberries (
Berberis
and
Mahonia
spp.) are occasional alternate hosts. Although the primary host is
wheat, varieties of several other small grains, such as barley, oats, and rye,
are also susceptible, as is jointed goat grass (
Aegilops cylindrica
), a wild rel-
ative of wheat and a common grain field weed. Like wheat leaf rust, stem
rust overwinters in southern areas of North America and spreads north-
ward during the growing season for small grains.
Both leaf and stem rusts of wheat have the ability to evolve new vir-
ulence races very quickly. The primary strategy for protection of wheat
from these rusts has been the breeding of resistant strains of wheat. For
wheat leaf rust, about 46 resistance genes, derived from cultivated wheats
or their relatives, are now known (Kolmer 2001). These resistance genes
tend to correspond to individual virulence genes in the rust. The intro-
duction of a wheat variety with a new resistance gene is typically followed
by redevelopment of virulence by the rust within a period of a few years.
The origin of increase in virulence appears to be gene mutation during
the life cycle phase involving asexual production of urediospores rather
than during the sexual phase (Kolmer 2001).
Wheat stem rust, one of the oldest known diseases of cultivated grain,
has a similar evolutionary capacity. More than 200 races of this rust are
known. New varieties of wheat that are resistant to stem rust are contin-
ually under development. As for leaf rust, new races of stem rust can arise
quickly, in this case by both mutation and sexual reproduction (Palm
2001).
Stripe rust of wheat and barley (
P. striiformis
) is not known to have an
alternate host but does infect many wild native and introduced grasses in
North America (Line 2002).This rust, apparently of Asian origin, entered
western North America by natural dispersal and infected native grasses
prior to European settlement. In the 1800s and early 1900s, it adapted to
wheat, barley, and other cultivated grains. Races adapted to cultivated
grains are now worldwide in distribution. This rust, although lacking a
sexual phase, is highly variable, and many races capable of infecting pre-
viously resistant grain varieties have appeared.
Alien rust diseases also infect woody plants. In North America, the
most serious rust disease of trees is white pine blister rust (
Cronartium ribi-
