Issue 9

Revealing The Genetics of a Pathogen

If you've ever considered how you'd design a pathogen, consider Phytophthora ramorum, the cause of sudden oak death. According to SRS geneticist Tom Kubisiak, from the SRS Southern Institute of Forest Genetics, DNA markers show that it reproduces clonally (by making identical copies of itself), not sexually. "With a disease organism, that's good news-if there were two mating types, you'd have sexual reproduction and the kind of genetic shuffling that could permit the evolution of increased virulence."

Kubisiak's genetics work includes testing a new detection tool, a rapid diagnostic that can uniquely identify most Phytophthora species including P. ramorum. From among 10 diagnostic techniques being tested by various labs on a collection of blind samples featuring most of the currently named Phytophthora species, the technique was the only one that produced no false negatives and no false positives.

"The technique was able to distinguish P. ramorum from every other Phytophthora species-and there are many," says Kubisiak. "But there is a caveat. Although it seems to work very well on DNA extracted from cultured Phytophthorans, we don't know if it will perform as well on tissue taken from plant hosts showing symptoms of sudden oak death. That's a big difference-a sample full of host DNA and possibly other disease organisms, even other Phytophthora species."

Even so, a reliable diagnostic is essential to keep P. ramorum from spreading further, and nursery stock from being quarantined. "We don't want nurseries to ship infected material, nor do we want to shut down nurseries on the basis of false test results."-SA

Back to: Sudden Oak Death