Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
5/12/03
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 May 16, 2003
On the Trail of a Lilac Tree Mystery
By Mark Gleason
Extension Plant Pathologist
Iowa State University
Lilacs are the superstar shrubs of early May. For a week or two every year, their spectacular sprays of blue, purple, magenta, red or white blooms and beguiling scent eclipse everything else in the Iowa landscape. Their deserved reputation as low-maintenance shrubs merely adds to their other perfections.
The Japanese tree lilac (Syringa reticulata) is a relative newcomer to our landscape, but is rapidly gaining in popularity. Unlike the shrub-type lilacs, tree lilacs bloom in late May to early June, throwing out large, fuzzy sprays of white, pleasant-smelling flowers in a spectacular show.
Japanese tree lilacs offer winter interest, too, thanks to their shiny, cherry-like bark, conspicuous seed heads, and elegantly gnarled branching patterns. Topping out at 20 to 30 feet after many years, they fit beautifully into the small spaces typical of most home landscapes. I'm so fond of them that I planted one in my own front yard.
That's where our story takes a darker turn. One summer day a few years back, I noticed spotty, yellowed leaves falling from my beloved tree lilac. I was immediately torn by conflicting emotions: dismay that the tree was sick, and delight that I had found a "new" disease.
To a plant pathologist, finding a new disease is way more exciting than finding a large sum of money on the sidewalk. This sort of reaction may be abnormal, but it's harmless as far as I can tell.
I shared my discovery with some of my abnormal colleagues. We snapped numerous photos of the sick tree, chattering excitedly like first-time visitors to Niagara Falls.
When we checked other Japanese tree lilacs in Ames, the leaf spot problem turned out to be widespread. Then it came out that the leaf spot was a common problem at the Minnesota Landscape Arboretum in southern Minnesota. Interestingly, nobody knew what was causing the problem.
So we started getting scientific. As a first step, we tried to "isolate" the microorganism that we assumed was causing the leaf spots; that is, persuade it to grow on an agar plate, all by itself, in the laboratory.
Tara Barrett, an undergraduate intern in my lab, made the first isolations in her spare time. Some different fungi appeared on her plates, but it wasn't clear which of these, if any, was the bad guy. Since they refused to produce spores (a fungus' primitive version of seeds), they were tough to identify, too.
It became clear that this wasn't an easy nut to crack. A new graduate student, Anne Dombroski, joined the effort. She consulted with Dr. Lois Tiffany, a veteran mycologist and guru of fungal identification at ISU. Dr. Tiffany offered some new ideas for teasing out the identity of the disease-causing fungus, and Anne is testing them now.
Some tests must be passed if you want to prove that a particular microorganism causes a particular disease. Called Koch's Postulates, these tests require that you first isolate the microorganism from a sick plant, then get it into a pure culture and inoculate it into a healthy plant. The healthy plant must then develop the same symptoms as the original sick plant, and finally you have to re-isolate the same microorganism from the inoculated plant.
Mr. Koch didn't invent his Postulates to make graduate students suffer, although suffering can be a side effect. Mainly, fulfilling Koch's Postulates gives you confidence that you're dealing with the right bad guy.
Anne is growing Japanese tree lilacs in the greenhouse right now in order to try out Koch's Postulates on her fungi. If she ends up with leaf spots that look like the ones we've seen before, and isolates a familiar-looking fungus from the spots, she wins mycological bingo.
Even after passing Koch's Postulates, the really interesting questions remain. First, who is this bad guy? Anne, Tara, and Dr. Tiffany think the leaf spot fungus might be in the genus Cylindrosporium. But more culturing tricks, and maybe some molecular genetic tests, need to be done before they'll know for sure.
Back in the real world, how can the leaf spot disease be controlled? Nobody wants Japanese tree lilacs to drop all their leaves in July. Anne hopes to spend time uncovering the ecological secrets of the leaf spot disease, and finding out whether some cultivars of Japanese tree lilac are more disease-resistant than others.
Anne's work could point out better ways to suppress leaf spot. But first she has to deal with those pain-inducing Postulates.
As always with new diseases, whether SARS or Japanese tree lilac leaf spot, there are more questions than answers at the start. But with some time, effort and luck, we hope to solve the leaf spot puzzle, and feel the excitement of solving a puzzle.
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Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 228K and the bottom picture's fullsize photo is 428K. Caption: Lilac 1 |
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| Caption: Lilac 2 |  |
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