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Genome Could Aid Fight against Tree Disease

By Kate Ruder


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Canker on a tan oak caused by sudden oak death.
Two months ago scientists sequenced the genome of the microbe that causes sudden oak death, a disease that has killed tens of thousands of oak trees in California and now threatens to invade forests in the Eastern United States. Knowing the DNA sequence of the pathogen may not help scientists stop the disease in the short term, but some believe it may lead to better ways to test for infected trees. And testing trees has suddenly become a high priority.

Concern about the spread of sudden oak death has reached an all time high. In March 2004, a California nursery was found to have unknowingly shipped plants infected by the microbe to all fifty states. Now, nurseries in over a dozen states, many on the East Coast, have reported infected plants, and scientists are worried that other plants and wild oaks will be infected.

A red oak tree with the disease was found this summer in a preserve on Long Island, New York —the first reported case in the state. Federal and state scientists have collected over 50 samples of DNA from trees in the preserve to test for further infection, and they are awaiting the results. It is not clear how the red oak tree became infected.

The good news is that the genome of the pathogen represents a new tool in the battle against sudden oak death. Using the genome sequence, scientists will develop more specific diagnostic tests for detecting sudden oak death and how it spreads. Right now, there are tests for the disease, but they are time-consuming, costly, and must be done in the laboratory.

“With the genome sequence, we can now develop a test that is super specific,” says Kelly Ivors of the University of Berkeley, who studies sudden oak death.

Lesions from sudden oak death on the tips of bay laurel leaves.
The disease was discovered in 1995, and five years later scientists isolated the fungus-like organism that causes the disease, Phytophthora ramorum. It infects oak tree trunks and causes cankers that weaken them, making them more susceptible to insect pests and fungi.

Plants such as rhododendron and camellia and more than 40 other plants can spread the disease, which harms but does not usually kill them. And a growing number of plant species now seem capable of spreading the disease.

Since the mid-1990’s, the disease has infected trees throughout California and into Southwestern Oregon. Scientists have worried that the large California nursery industry could begin spreading the disease to other states by unknowingly shipping contaminated plants to nurseries on the East Coast.

Sure enough, last March the disease was found in six varieties of camellias at a large wholesale nursery outside of Los Angeles. The U.S Department of Agriculture subsequently ordered a ban on all California nurseries to sell or ship any of the 40 types of plants known to carry disease at a time at a time when business was supposed to be booming.

The California nursery industry, which is the largest in the country, has lost an estimated $25 to $30 million due to sudden oak disease.

The microbe was sequenced along with a related pathogen that kills soybeans in a project funded by U.S. Department of Energy, the U.S. Department of Agriculture, and the National Science Foundation.

The pathogen that causes soybean root rot, Phytophthora sojae, is also nasty and expensive. It cost farmers $1 billion in lost crops in the United States last year. California-based biotechnology company Affymetrix is using the pathogen’s genome sequence to create a DNA microarray so that scientists can analyze genes that are turned on and off during the infection of soybeans.


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