Geranium Health Research Blooms with DNA Test

Plant disease specialist Gary Moorman and his colleagues developed a new test to detect bacterial blight in geraniums.

Picture yourself at the local garden center, choosing a few bedding plants. Geraniums suit the image you've chosen for the garden, so you buy a dozen, plant them, and three weeks later the plants are as dead as the eight-track tape industry.

This scenario used to occur with alarming frequency in the early days of the greenhouse industry in Pennsylvania. The main culprit, according to plant disease specialist Gary Moorman, was (and still is) bacterial blight, caused by a geranium-attacking pathogen that lives in the plant's water-conducting vessels. Now Penn State has developed a process making the pathogen easier to detect.

"Geraniums are grown for production in two ways," Moorman explains. "Some varieties are grown from seed, and bacterial blight is not carried on the seed. The second method is vegetative propagation, which means producers grow new plants from cuttings from other plants." Before scientists developed varieties that could be grown from seed, all greenhouse geraniums were grown from cuttings. If an infected plant was used as a source for new plants, each of the offspring carried the pathogen. In addition, the knife used to cut the infected plants could pass the bacteria to healthy plants. Under cool, controlled greenhouse conditions, blight symptoms did not appear. But once they were planted in natural conditions, the plants wilted and died. "Bacterial blight came very close to eliminating geraniums as a commercial crop in the 1950s," Moorman says.

Shortly after the industry's near elimination 40 years ago, commercial growers began testing geraniums for bacteria before taking cuttings, using a process called "indexing." Plants are indexed by immersing part of the stem taken from each cutting in a broth medium. If the broth becomes cloudy, bacteria are present and the cutting is eliminated. If the broth does not cloud up, the plant is tested twice more. If bacteria still are not present, the cuttings are used to propagate new plants.

"Indexing effectively eliminates the disease, but it's time-consuming and identifies only the presence of bacteria in general, not the specific pathogen that causes bacterial blight," Moorman explains. Oglevee, Ltd., a Connellsville, Pennsylvania, commercial grower, asked Moorman to develop a fast, efficient test that could definitely identify Xanthomonas campestris pv. pelargoni, the specific bacteria causing blight in geraniums.

Bacteria, like other living things, contain DNA. The DNA of any organism contains unique characteristics that can be identified using a scientific process called the polymerase chain reaction (PCR). The PCR technique recognizes a unique sequence of nucleotides, the building blocks of DNA, in an organism. This unique sequence of nucleotides is the "fingerprint" in DNA fingerprinting. When scientists find such a unique nucleotide sequence, they can use PCR to identify whether that sequence is present in samples of blood or other tissues. When applied to plant disease, PCR tests can give scientists a fast way to confirm that a specific organism or pathogen is present in plant tissue.

Thanks to DNA fingerprinting, geraniums infected with the pathogen that causes bacterial blight, like those below, may soon be a thing of the past.

Penn State plant pathologist Peter Romaine and Michael Sulzinski, a plant pathologist at the University of Scranton, developed a PCR test for geranium bacterial blight using bacteria samples from Moorman's laboratory. "It was an incredibly fast process," Moorman says. "I literally gave them some bacteria on a Tuesday and by Thursday they knew they had an identifiable test."

Moorman says such speed is rare in the world of science, especially considering that there are more than 100 subspecies of this bacteria, only one of which affects geraniums. The scientists have received a patent for the test.

"For indexing to work, there must be at least 1,000 bacteria cells present," Moorman says. "The new test will identify the bacteria if as few as 10 cells are present. Along with saving a few cents in the production of each cutting, it means that producers can create and test new varieties much faster, while at the same time streamlining the process by which geraniums come to the marketplace."

-- John Wall