In this coastal
region of
California disease called Sudden Oak Death is caused by a fungus-like
organism
known as an oomycete. The oomycete (Phytophthora ramorum)
infected
tanoak (Lithocarpus densiflorus) and true oaks trees of the
genus Quercus, as well as many other native and cultivated
plants.
From this region the infection spread to other areas along the western
US coast, including Oregon and Canada. The original source of the
infection is unknown (but see below). The disease also can infect other
tree species, woody shrubs, and especially Rhododendron and Viburnum
species. Some of the leaves and shoots of the plants may die, but
these species usually survive. The infection is spread by wind blown
rain,
which carries the disease to uninfected plants. Infections of oak
and tan oak trees produce symptoms, including bleeding cankers on the
trunk.
Deformations disrupt the vascular tissue and, therefore, the
nourishment,
usually resulting in death of the tree. It is widely believed that the
disease has been introduced and spread by the distribution of nursery
plants.
Inspections are held in nurseries to attempt to control the spread of
the
disease, and importation of trees is monitored. The disease was
noted
in Germany and the Netherlands in 1993 and recently in many other parts
of central Europe. Because the mating types of the disease found
in Europe and in North America are different, there is concern that if
the two mating types were to come together, sexual recombinants could
produce
more virulent genotypes. In 2004 infected plants were discovered
to have been distributed from a southern California nursery to
nurseries
in 39 states in the USA. --C. K. Erbil
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