As winter approaches, many of us reflect upon what went wrong with our roses during the growing season. We perform a suite of activities to nurture our plants, including careful watering, fertilizing, and pruning. Yet despite our best efforts, come fall our rose bushes have black spots everywhere. What did we do wrong? The answer may be nothing!
Blackspot, a fungal disease, is a fact of life in rose gardening. Understanding this will help gardeners deal with managing the disease on their bushes. Many inquiries about blackspot come in as frantic calls for help. To the impacted gardener, it seems their rose plants have defoliated overnight. Can this happen? The answer is yes. If this occurs, you have lost the battle, for this year, anyway. A rose plant will releaf, but it will also apply a certain amount of unwanted stress on the plants system.
In the book, Diseases and Pests of Ornamental Plants, botanist P. Pirone defines blackspot (Diplocarpon rosea) as, "dark to blackspots appearing on both sides of the leaf." This is caused by the formation of Ethylene gas, which results in early defoliation. Without any protection a rose may be totally infected within six hours. There is no way to save infected leaves, they will drop and must be discarded. Blister-like bodies will produce spores and release them in the spring. A prevention spray program must be started as early as April or May, starting after the spots appear in July and August is too late. The simple act of splashing water on infected leaves, either on the plant or on the ground, will perpetuate the disease.
Implementing a proper spray program can help prevent blackspot. You can ward off the disease by beginning your spraying as soon as young red leaves mature to green, continuing into early fall. The use of combination sprays, or Neem oil, works well. Baking soda/soap is the least effective and needs to be done weekly. If blackspot appears, stop your preventative spray program and apply Mancozeb + Thiophanate-Methyl (i.e. Zyban) once a week for three weeks. After three weeks, cease the Mancozeb + Thiophanate-Methyl (i.e. Zyban) application and proceed with your regular spray program. In the fall the total removal of leaves and debris is also needed.
There is one more step we can take to avoid blackspot. No rose plant is totally immune, but some are more susceptible than others. The type of rose you choose to plant is as important as your spray program. Old Garden Roses (OGR) are the most immune, whereas Modern Roses (MR) are more prone to infection. Hybrid Teas are rarely on the blackspot resistant list. There are a few, such as the Elina (zone 4), shown in the photo. Next to Elina is the Explorer shrub rose J.P. Connell (zone 3). Though very hardy, it is the least resistant to blackspot. You will need to do a bit of research and page flipping to obtain a list of resistant cultivars, but it can be done. You can start with this insert from the Shankman Rose Garden brochure, (also available at the IES Visitor's Center).
We are continuing our evaluation of organically-based methods of rose care. In 2004, we anticipate using and evaluating the effectiveness of biofungicides for deterring the proliferation of fungal pathogens in the rose garden using a strain of Bacillus bacterium.