If you have a rose garden, sooner or later you will have to deal with an outbreak of blackspot. Blackspot is a common fungal disease that affects roses. It is caused by the fungus Diplocarpon rosae. When you notice a case of blackspot coming on, you must act as soon as possible. If you don't your rose will produce fewer blossoms, and even run a risk of not over wintering.
It is very easy to spot blackspot when it is in its later stages, but if you keep a sharp eye out you might be able to find an outbreak in its earlier stages. The first sign that you will see are the tell-tale black spots that form on the leaves of the bush. It is from these spots that the disease gets its name. As blackspot infects a rose bush, yellow rings grow around the original black spots, which start to spread to the underside of the leaves. The symptoms will increase and soon the entire leaf will yellow and wither, before falling off completely.
Blackspot is a fungal growth, and so it will spread from bush to bush. If you leave it untreated it can infect your entire garden. It will over winter in the fallen leaves, and on canes that it previously infected. Because it is a fungus, blackspot thrives under warm and moist conditions.
The only way to keep blackspot from taking over is to take good care of your roses. Dispose of the leaves that provide shelter for blackspot. Cut off infected limbs and canes from the rose bushes. To treat an outbreak, buy sulphur and fungicidal soap in spring before your roses produce leaves. Spray both onto all affected plants. The fungicidal soap will kill living blackspot, and the sulphur keeps its spores from sticking to the plant. It is important that you keep reapplying sulphur with every rain, because water washes it off.
Blackspot cannot germinate if the rose bush is not damp. So do your best to keep your bushes' leaves from getting wet. Don't spray the leaves when you water, or better yet, use a sprinkler hose. Water only early in the day so there will be plenty of sun to dry out the water throughout the day. Keep all your bushes well pruned so that air can circulate around the leaves.
Finally, if you come across a rose that displays the signs of a blackspot outbreak, don't hesitate! Prune it immediately. And don't throw the leaves onto your compost pile, but into the trash. Composting will not kill the fungus. Then apply your sulphur.
With a little be of care, your roses will look stunning, and blackspot will be easily overcome.
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