Case Study
case study for early disease diagnosis

The  source of the Sudden Oak Death epidemic, which has killed more than a
million trees throughout coastal California has been genetically traced to two sites
about 60 miles apart: a plant nursery in Scotts Valley, Santa Cruz County, and
ornamental landscapes around homes in Marin County. A UC Berkeley scientist
speculates that both sites may have become infected from a single plant shipment
originated in Asia bearing the plant pathogen Phytophthora ramorum.

Phytophthora strains caused the potato blight famine in Ireland in the 1840’s and a
1920 epidemic that devastated soft-fruit growers in Scotland. An epidemic in
Australia caused by yet another strain has transformed forests into grassland,
triggering a major ecological disaster.

Because disease symptoms develop slowly, the P. ramorum infection in California
may have occurred more than ten years before it was first detected at a Scotts Valley
nursery in 2001. By then the disease was already widespread.

Analyzing genetic markers of samples taken throughout California, scientists
identified 35 unique strains of the pathogen and established the infestation sources
by identifying the locations with the most ancestral populations. They also estimated
the pathogen’s natural propagation by calculating the distance between plants with
identical strains: the vast majority of spread occurred within 200 to 300 yards, though
the wind can blow spores as far away as three miles.   

These findings illustrate the usefulness of plant disease DNA fingerprinting and the
importance of disease surveillance. Early plant disease diagnosis allows the
implementation of control strategies that limit devastating pathogen damage.
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