Authored by Mark Nolan, President of the Indianapolis Rose Society
Diplocarpon rosae is not a prehistoric dinosaur, but, nevertheless, it does strike fear into the hearts of all conscience rose gardeners. In Indiana, diplocarpon rosae is the most common disease of roses, and if a rose grower is going to have any success, a good working knowledge of this disease is essential. In case you don’t recognize the name of the disease it is “#!%^$ black spot.”
The effects of this disease are easily recognized. The lesions start as small black spots on the upper side of the leaf. As the spots enlarge, you will begin to notice that the edges of the spots are feathery; this is a diagnostic symptom of black spot. Eventually, the leaf will turn yellow and drop off. Usually, the disease starts on the lower part of the plant and progresses upward. If left untreated, the disease can defoliate the entire plant within a matter of weeks. Usually, black spot will not cause the death of the plant, but without leaves the plant cannot produce the energy that is necessary for good growth and bloom production. A stunted bloom at the end of a thorny leafless stem is not my vision of a beautiful rose.
Black spot loves water. If you could grow roses in a dry arid region; you wouldn’t have to worry about black spot. Black spot is host specific, it only infects members of the rose family. In a way, it is good that the black spot fungus is host specific, because this means the fungus will not grow on the grass around the rose bed nor will it over winter in the soil or mulch in your rose bed.
So, how do you get black spot in the rose garden? The microscopic spores (conidia) of the black spot fungus are primarily disbursed in water droplets or mist that is driven by the wind. The conidia are smooth and sticky and occur in a white slimy mass. Because the spores are stuck together they are heavier, and they are not generally dispersed by the wind alone. The spores are most often spread by water splashing from an infected plant to another. They can also be spread by insects moving from an infected plant to another. You can inadvertently introduce the fungus into your garden by bringing home an infected rose from the nursery. Perhaps the greatest source is the conidia and mycelia on the dead leaves and other plant debris left in the garden over winter.
Once the spores are present on the rose plant, germination can occur in about five minutes assuming the presence of water or a relative humidity of 100%. However, before the infection can be established, the spore must be immersed in water and continuously wet for seven hours. Successful germination is highest when air temperatures are between 73 and 79 degrees F. Leaves that are still expanding are most susceptible, and lesions can form on the stems and other parts on the plant as well as the leaves. Raised, purple-red, irregular blotches develop on the immature wood of first year canes. Later the blotches will become blackened and may blister.
Armed with this basic knowledge, we can formulate a plan to control black spot. Obviously, water is the essential ingredient in the spread and germination of black spot. We can take steps to minimize the spread of black spot in our gardens.
Your first line of defense is to plant disease resistant roses in an area of your garden that gets the morning sun and has good air circulation. This micro climate will allow the leaves to dry out earlier in the day. When the first bloom cycle is over, prune out all inward growing stems and remove all spindly growth at the base of the plant. Removal of this material will not adversely affect the health of the plant and it will promote good air circulation which will tend to keep the leaves dry. For the same reason, don’t crowd your roses by planting them to close together.
Roses love water, especially when they are putting on a lot of new growth. Since black spot also loves water, we need to be judicious in how we apply water to our roses. If you can, water your roses in the morning on sunny breezy days. Roses benefit from a nice cool shower on a hot summer day; just plan the shower bath so the leaves will be dry going into the evening. If you have to water in the evening, direct the water to the ground at the base of the plant rather than on the plant. Always remember the leaves must be wet continuously for seven hours before the disease can be established.
Keep your rose garden neat and clean. Assume that all plant debris that falls onto the bed is infected with black spot. Pick it up and discard it. In this case all means leaves, stems, peduncles, leaf stalks – it means everything. Don’t forget to pick up debris that has fallen into the lawn too. Make sure you start off every growing season by thoroughly cleaning up the garden. After your spring pruning, spray the canes with a lime-sulfur spray to kill the spores in lesions remaining on the stems. Once the disease is first noted strip off the infected leaves. The infected leaves cannot be cured; they only provide conidia for more infection.
Assuming you have done everything to prevent black spot, you still may get it. Some gardeners may tolerate a little black spot and elect to forego using a chemical fungicide to control the fungus. Some gardeners only use “organic formulations” or old fashioned remedies. I would encourage each gardener to take whatever approach they want, however, I believe the only way to completely control black spot is to use chemical fungicides.
A chemical fungicide spray program should begin early in the spring when the leaves begin to appear and your spray program should last until sometime in late fall. The old adage “an ounce of prevention is worth a pound of cure” is very appropriate when it comes to controlling black spot. There are many fungicides on the market today that are effective in preventing black spot. Choose the one most appropriate for your situation and follow the directions on the container explicitly. It is recommended that two or more different types be used to prevent a resistance build up by the fungus. Use Manzate or any fungicide that has the active ingredient mancozeb when black spot is present. This chemical formulation will kill the spores and prevent further spread of the fungus.