If you have visited the Dean Bond Rose Garden this growing season, you may have noticed a large section of the climbing and hybrid tea beds now contains marigolds, Tagetes patula cultivars, instead of roses. This April we removed about 17 rose shrubs because they were infected with Rose Rosette Disease (RRD).
RRD was first observed by a local gardener and Scott Associate, Judy Penney, on her climbing rose in Swarthmore. The disease has since devastated her once prolific rose and symptoms have begun to appear in the Dean Bond Rose Garden.
Symptoms of RRD are highly variable depending on the species and cultivar of rose. These symptoms include: rapid elongation of new shoot; witches’ brooms with small, distorted leaves; excessive growth of unusually soft and pliable red or green thorns; canes that appear thicker than parent canes; and new growth’s traditional red pigmentation never matures to a green tone. To see a complete list of symptoms read the Plant Disease Fact Sheet from Virginia Cooperative Extension. In the Rose Garden, we have observed witches’ brooms with small, distorted leaves and conspicuous red pigmentation as well as rapid elongation of shoots with excessive thorniness.
RRD is believed to be caused by a virus originally introduced to control the invasive Rosa multiflora. While this virus is effective at overwhelming R. multiflora, cultivated roses are also highly susceptible to it. The disease is transmitted by a microscopic wingless mite, Phyllocoptes fructiphylus, or by grafting. These mites are known to travel on wind currents from infected to healthy plants.
At this time there is no effective control available for infected plants. It is recommended to remove all infected plants to prevent transmission to healthy specimens. The pathogen can survive on old root pieces in the soil, thus be sure to thoroughly remove all parts of the infected plants. Where permitted, burning can be used as removal technique. Once uprooted be sure to remove infected plants from the vicinity of remaining healthy roses. At this time no resistant species or cultivars have been identified.
Because there is no direct treatment for the virus, we are attempting to control the carrier of the disease, the microscopic wingless mite. We are applying a miticide, Avid, along with other treatments on the garden.
Historically, marigolds have been used in vegetable gardens to help mitigate the effects of many problematic pests. In England, marigolds have also been planted in rose beds where roses have been removed to help reduce the effects of Rose Replant Disease, which is another problem in the Dean Bond Rose Garden. It is our hope that the marigolds will ward off the mites. Thus we have planted Tagetes ‘First Lady’, ‘Inca Orange’, and ‘Sweet Cream’ in an attempt control RRD.
Other efforts include sterilizing pruners between pruning and deadheading as well as monitoring and removing any newly infected plants.
This fall the marigolds will be removed and the bed will be replanted with roses. Check back with GardenSeeds to see which species and cultivars we will reintroduce to this location of the Rose Garden.