Biology Reference
In-Depth Information
cola
). Native to eastern Asia, this disease became established in eastern
Europe in the 1800s, apparently by introduction of Asian trees to botan-
ical gardens (Kinloch and Dupper 2002). In Europe, it attacked the east-
ern white pine (
Pinus strobus
), which is native to North America but had
been planted widely. From Europe, it reached North America in about
1900 via importation of white pine seedlings.
The relationship of white pine blister rust virulence to host tree resist-
ance is similar to that of the wheat rusts, except that the host is long-lived
(Hoff and McDonald 1993). The rust infects pines by means of spores
produced on alternate hosts—currants and gooseberries (
Ribes
spp.). It
attacks five-needled pines of the subgenus
Strobus
, to which eastern white
pine and several other species belong. The rust probably evolved in east-
ern Asia, where native members of the subgenus show high resistance to
the rust (Liebhold et al. 1996). From there, the rust spread to Europe in
the early 1800s and to North America in the early 1900s. Several western
pines, especially the whitebark pine (
P. albicaulis
), have been severely
affected by this rust (Kinloch et al. 1998; Zeglen 2002).
Evolutionary interaction occurs between white pine blister rust and its
hosts. Racial variation exists in the white pine blister rust fungus. At least
ten genetic patterns of rust resistance in species of the white pine group
have been detected, some of them due to single genes and others to mul-
tiple genes. The rust has shown an ability, however, to overcome these
resistance systems. In Japan and Canada, strains of the rust differing in
their ability to infect secondary host plants have been observed, and infec-
tion symptoms on pine hosts also point to the existence of at least four
races of the rust (Hoff and McDonald 1993). Rust infections have been
observed on several previously resistant forms of western white pine
(
Pinus monticola
) and sugar pine (
P. lambertiana
). In 1970, for example, a
new rust strain appeared at a site known as Champion Mine in the
Umpqua National Forest, Oregon. This strain killed most western white
pines at this site, which was previously a site from which resistant tree
stock was obtained for propagation. A race of the rust that appeared at
Happy Camp, California, in the 1970s also kills formerly resistant sugar
pines and has recently begun to spread widely (Kinloch et al. 1998).
Other types of fungi show a similar capacity for rapid evolution. Fungi
of the genus
Phytophthora
have followed their hosts to new regions and
adopted new hosts. One of the best documented cases in this genus
involves
Phytophthora sojae
,a form parasitizing soybean (
Glycine max
) and
various species of lupines (
Lupinus
spp.). Thirteen resistance genes are
known in soybeans, and many cultivars with one or more of these genes
