Janna Beckerman, Extension Plant Pathologist
This year, the Perennial Plant Association has declared that Phlox cv. 'David' to be the 2002 perennial plant of the year. 'David' is a beautiful, white-flowered variety of phlox with dark green foliage. In a field of dozens of other phlox varieties, what made 'David' so special? Unlike other varieties of phlox, 'David' stood out in a mass planting at the Brandywine Conservatory in Pennsylvania not only for its great form but because its foliage was powdery mildew resistant, unlike the other phlox in the plantings. Keen-eyed horticulturists recognized the value of a variety of phlox that was powdery mildew resistant and began propagating 'David.'
By naming 'David' as the Perennial Plant of the Year, the Perennial Plant Association is supporting the best method of plant disease management available. By using plant species or varieties or cultivated varieties (termed cultivars (cv.)) that are genetically resistant to diseases, buyers are immediately implementing the most effective means of controlling plant disease in the landscape. Although most plants do not have resistance to the many common diseases present, it is simple common sense to use those plants that are resistant to known disease problems whenever possible to minimize the impact of disease on the home landscape. Although Phlox 'David' is relatively resistant to powdery mildew, proper culture will aid in mildew prevention. Plants perform best when placed in full sun. To increase air circulation (which allows foliage to dry quickly), thin plants to four to six stems. Finally, all plants should be watered at the base of the plant and not the foliage. This reduces both moisture and humidity that is necessary for both spore germination and spread.
Resistance to a given disease is recognized or developed in several different ways. In the case of 'David,' Richard Simon of Bluemount Nurseries (Monkton, Maryland) and F.M. Mooberry (Horticultural Coordinator, The Brandywine Conservancy) recognized that one plant was not affected by powdery mildew when all the surrounding plants were. A plant like 'David' may be planted at multiple sites, to determine how complete the resistance is, and if it functions under different site conditions. This also allows researchers to make certain the their potential resistant plant wasn't an "escapee" that somehow managed to avoid as oppose to resist disease. Often times, resistant cultivars are inoculated with the offending disease causing agent to allow researchers to be certain the plant didn't escape disease, but actively resists it.
Resistance isn't just discovered by accident, but a trait that researchers actively breed for. In the case of chestnut blight, breeding programs have been going on for almost a century, trying to develop the right combination of disease resistance present in the shrubby Chinese chestnut, with the desired form of the 100+ foot- American chestnut tree. In breeding for chestnut blight resistance, the susceptible American chestnut tree is crossed with the resistant Chinese chestnut. In the case of breeding for blight resistant American chestnut, resistance is closely linked to the short stature of the Chinese chestnut. This requires that breeders perform additional crosses with the hybrid offspring back to the parent to introduce additional desirable characteristics, in this case height and disease resistance. A crossing plan for incorporating the resistance of Chinese chestnut into the American chestnut can be seen at: http://www.apsnet.org/online/feature/chestnut/images/figure9.htm The most recent method of developing disease resistance plants is through the integration of foreign DNA (DeoxyriboNucleic Acid-the genetic material that is the blueprint that controls how an organism develops) that consists of a gene to confer resistance to a given disease into the plant genome. The resulting plant is referred to as 'transgenic.' Early attempts to understand this process involved the insertion of a gene that encodes a protein called luciferase from the firefly into the genome of tobacco. Luciferase is the active enzyme that allows fireflies to glow. Insertion of the luciferase gene into tobacco created the first tobacco plant that was able to "light up" by itself. This tobacco plant that "glowed in the dark" demonstrated the power that was available to biologists, and shed light on a new way of studying plant biology. Although still in its infancy, this method of genetic engineering has already changed the way researchers examine and study how disease resistance mechanisms work. Transgenic corn, potato and soybean that possess foreign genes that confer resistance to herbicides or produce their own Bt toxin have been in production for several years; however, these plants are not yet available to the home consumer.
Regardless of how resistance develops, it has an invaluable place in the home landscape. Below, I've included the names of other disease resistant annuals and perennials for you to consider prior to purchasing another variety that may not perform as well under the same disease pressures.
