Also known as Diplocarpon rosae, the fungal black spot disease causes black spots on rose leaves. What often begins as a small black dot here and there can turn into a full-blown illness that causes the leaves to turn yellow and die. Wet, humid conditions combined with an average temperature of 24C create the perfect environment for black spot, which will begin to show signs of its presence within 10 days of fungal spore germination. Left untreated, black spot will not only infect your entire rose garden, but it will linger in the soil as well, affecting your rose bushes in the years that follow.
Prune your rose bushes as soon as you notice the black spots. Use a sharp pair of pruning shears and cut off diseased branches and leaves, as well as attached blooms.
Place all of the cut material in a plastic bag immediately. Do not leave it lying on the ground, or the fungal spores will multiply and spread through your rose garden like wildfire.
Apply a sulphur powder product or fungicide to the rose bushes. Follow the label instructions for application recommendations. Sulphur and fungicides are equally effective and subject to availability. Which one you choose will depend on what is available at your garden centre.
Soak your pruning shears in a household disinfectant after use. A household disinfectant, such as bleach or benzalkonium chloride, will kill blackspot spores. Killing the spores will ensure that you do not transmit the disease to other rose bushes during regular pruning.
Apply sulphur powder to the rose bushes and surrounding soil while the rose bushes are dormant next spring. Early spring treatments will prevent the black spots from returning. Keep in mind that sulphur powder washes off in the rain and will need reapplying after each rainfall.
If you cannot get to a garden centre, baking soda may be able to solve your problems. Dissolve 1 tsp baking soda in 1 litre of water. Put the baking soda solution in a spray bottle and add a squirt of washing-up liquid. Spray the infected rose bushes and the soil around the roses with the baking soda solution. A generous spraying should kill the fungal spores.
Do not water your rose bushes during the late afternoon or at night. Late-day watering does not give the leaves enough time to dry out before dampness sets in. Black spot loves this scenario and will seize the opportunity. Early morning watering will give the sun plenty of time to dry off the leaves before dusk.