Extension Communications
Extension 4-H Youth Building
Ames, Iowa 50011-3630
(515) 294-9915
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Extension Communications |
2/2/04
Contacts:
Mark Gleason, Plant Pathology, (515) 294-0579, mgleason@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning Feb. 6
By Mark Gleason
Extension Plant Pathologist
Iowa State University Extension
Bioterrorism is probably the last thing you want to read about in a gardening column. But this is an interesting story, so please keep reading. In an unlikely way, a plant-infecting bacterium has crept onto the domestic bioterrorism radar screen.
The name of the bacterium is Ralstonia solanacearum, and it causes a wilting disease of geranium, also known as florist's geranium or zonal geranium. Zonal geraniums are the ones you see in pots everywhere, beginning in early spring.
In the old days, geraniums were mostly red-flowered, but now they come in a rainbow of colors from deep red to pink to salmon to white. Geraniums are America's most popular flowering annual plant, beloved for their resilience and adaptability almost as much as for their showy flowers. They perform consistently, even for gardeners who inadvertently kill most other garden plants.
However, even Eden had its serpent. The evil being clouding the geranium's blessed existence is Ralstonia, a lowly bacterium. Ralstonia lives quietly in the soil most of the time, minding its own business, until a vulnerable plant comes into range.
That's what happened to the geranium industry in 2003 and again in 2004. R. solanacearum caused wilting of geraniums in several greenhouses last winter. The source of the problem was traced back to a company that had initially grown the plants in a greenhouse in Kenya. Kenya?
The plant industry has gone global in recent decades. The driving forces are economic. Young plants that require manual labor to breed, plant and tend are now nurtured in Central America, Africa or Asia where labor costs are a mere fraction of those in the U.S. Globalization keeps plant prices low for U.S. consumers, but sometimes the world's plant diseases arrive along with the plants.
Ralstonia is more common in the tropics than in the temperate zone. It spreads easily from soil to plants. Once the bacterium invades the water-conducting passages of a geranium, things start to get interesting.
One of the sneakier tricks in Ralstonia's repertoire is that it can inhabit a geranium invisibly. In other words, it can hang out without causing the plant to look sick. No problem, right? Who cares about a bacterium that just hangs around without doing anything bad?
Here's the problem. Geranium growers, in Kenya and elsewhere, propagate the plant by making stem cuttings from healthy-looking "mother plants." But what if the mother plants contain Ralstonia? Once the bacterium contaminates the knives used to make cuttings, every cutting, and then every plant, can be contaminated. And nobody suspects a thing.
The cuttings are packed off to the U.S., where unsuspecting growers transplant them to pots. As the plants grow, the weather begins to warm up. Ralstonia takes its cue from the warm temperatures, multiplying with astonishing speed until the water-conducting passages are clogged with bacteria and the plant starts to wilt.
Enter USDA's Animal and Plant Health Inspection Service (APHIS). After Congress passed agricultural bioterrorism legislation in the wake of 9/11, APHIS compiled a list of plant disease agents that don't normally occur in the U.S. but could potentially be used to wreak havoc on our agriculture. Ralstonia is on the list, because it can devastate potatoes as well as geraniums. A Ralstonia outbreak in geraniums could conceivably escape to potato farms, perhaps endangering our food supply.
The 2003 Ralstonia outbreak on geraniums could not have come at a worse time for hundreds of U.S. greenhouse growers. If they had received geranium cuttings from the company with the contamination problem, their greenhouses were quarantined by federal officials until inspections and tests could clear them. Some greenhouses could sell no plants for many weeks, even though tests eventually showed that they had received no Ralstonia-contaminated cuttings. Even unluckier were the greenhouses where infections were confirmed; many were forced to destroy most or all of their geraniums.
Financial losses quickly mounted into the tens of millions, perhaps even higher. The situation was analyzed in detail, and stringent corrective measures were taken. Then, in early 2004, it happened all over again, and the same company with the Kenya greenhouses was implicated again.
Ironically, this company had earned a reputation as a world leader for its careful, thorough quality-control practices. What happened?
It turns out that lowly Ralstonia is a formidable enemy. Since it contaminates cuttings without showing itself, microbiological tests must be done to reveal its presence. At frequent intervals during the production cycle, samples of cuttings are routinely pulverized and cultured to test for Ralstonia and other disease-causing bacteria. Any bacteria-positive results trigger the destruction of the entire lot of cuttings from which the samples were taken. Foolproof, right?
Not entirely foolproof, as it turns out. For economic reasons, only a small fraction of the cuttings can be sampled and assayed. Even though cuttings are sampled in a scientifically valid way, the vast majority of cuttings are never sampled. So it's always possible for Ralstonia to slip through the system, all the way to the U.S.
The greenhouse industry is re-examining and strengthening its screening procedures in light of the Ralstonia debacle of 2003-2004. The good news for U.S. consumers is that Ralstonia testing, in the era of agricultural bioterrorism fears, has never been as thorough and stringent as it is now. In fact, today's geraniums are probably even more likely to be disease-free than they were a few years ago. So don't hesitate to go out and buy some!
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ml: isugarden