Black Spot on Roses
Black spot is one of the most common diseases for roses. It is caused by the fungus Diplocarpon rosae.
Symptoms of Black Spot
Black spot appears as circular brown or dark gray spots surrounded by yellow tissue on rose foliage. In very severe cases, the spots can merge, discoloring the entire leaf. Close inspection of the spots will show the tiny spore-producing bodies. If left untreated early leaf fall and defoliation may occur.
Black spot occurs most frequently during rainy seasons when temperatures are moderate. Leaves on the bottom of the rose tend to become infected earlier because they are shaded by other parts of the plant and thus stay wet longer.
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| Black spot on roses |
How to control and prevent black spot
- Fungicides. Spray roses with a fungicide such as Daconil or Immunox. For the most effective control, spray roses shortly after they start to bud in spring and repeat applications every 14 days up until the first frost. In order to control black spot break outs it is necessary to continue the application of fungicide throughout the growing season. Unfortunately, once your roses are infected in mid-season fungicides are not very effective.
Regular applications of Neem Oil, a botanical insecticide made from the seeds of the neem tree, can help control and prevent black spot. For prevention, apply on a 14 day schedule, if black spot is already present spary every 7 days.
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Environmental controls. Prune and remove all the affected parts on the rose bush. Also rake up and leaves that may have already fallen to the ground. Never compost the affected parts as spores may survive in the soil and spread to other plants. In order to prevent overwintering of the spores thouroughly remove all diseases leaves in the fall and cut canes a couple of inches into the healthy wood.
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Watering. Diplocarpon rosae thrives in wet conditions. When watering your roses, always water from the base of the plant and never from the top. It is also best to water your roses in the morning to allow the leaves the most time to dry.
- Natural Controls. In order to prevent germination of the black spot spores, apply either sulfur dust or wettable sulfur to the plant. Sulfur can be applied every 5-10 days but thourough coverage of the plant is critical. Applying sulfur does not kill the fungi already on the plant.
Another option is to spray the roses with a mixture of baking soda and water (two tablespoons per gallon), however, roses will have to be sprayed weekly and after every heavy rain.
- Resistant Varieties. In general, grandiflora and floribunda varieties are more resistant to black spot than hybrid teas. Most black spot resistant roses will still occasionally get infected with the fungus but infections tend to be on a much smaller scale. Resistant hybrid teas include the Chrysler Imperial and the Tropicana.