Biology Reference
In-Depth Information
the southeastern United States in 1946. It appears to be native to central
Mexico and is now affecting various native North American pines in the
southeastern United States and in California. Resistant Monterey pines
have been identified in some stands that have been affected by the canker
disease.
As we noted in chapter 11, the sudden oak death fungus (
Phytophthora
ramorum
) appeared in California in 1995. It spread into Oregon in 2001
and has also been found in nurseries and gardens in several European
countries. The geographic origin of this fungus, however, still remains
unknown. Differential resistance appears to exist to the sudden oak death
fungus in coast live oak (
Quercus agrifolia
) along the Pacific coast of North
America (Dodd et al. 2002). The distribution of trees with sudden oak
death symptoms is patchy where the disease occurs, and the disease itself
appears to be absent in many parts of the range of coast live oak. Oak
shoots from southern California also appear to show some resistance to
experimental infection, compared to those from central and northern
California. Genetic variation also appears to exist in susceptibility of Cal-
ifornia bay laurel (
Umbellularia californica
) to the sudden oak death fungus
(Hüberli et al. 2002). This species appears to be a critical vector for the
spread of the fungus. Many other trees and shrubs carry mild foliar infec-
tions of the fungus, however, so its impacts and evolutionary future are
difficult to predict.
Also as noted in chapter 11, several members of the white pine group
exhibit genetic variation in resistance to white pine blister rust (
Cronar-
tium ribicola
) (Kinloch and Dupper 2002).These include sugar pine (
Pinus
lambertiana
), western white pine (
P. monticola
), southwestern white pine (
P.
strobiformis
), and limber pine (
P. flexilis
). The genetic system involves a
gene-for-gene relationship between the host tree and the rust. Host trees
are homozygous resistant (RR), heterozygous (Rr), or homozygous sen-
sitive (rr).Trees carrying the resistance allele (R) respond to rust infection
with a hypersensitive reaction of cells surrounding the infection locus.
These cells collapse and die, forming a barrier that prevents spread of the
infection (Kinloch 1992). Efforts to breed fully resistant varieties of west-
ern pines, based on this resistance gene and other genetic factors, are
underway in several laboratories (see, e.g., Bingham 1983).
The selective impacts of blister rust on pines may also influence over-
all genetic variability of the affected tree species. For western white pine,
for example, trees from stands that had been heavily affected by blister rust
showed lower genetic polymorphism and heterozygosity than trees in
lightly affected stands (Kim et al. 2003). On the other hand, western white
