Black spot is a common fungal problem on roses but can be found on other plants too. The spots can be of various colours – grey, brown or black. The spots are in fact dead leaf tissue caused by the fungus which spreads the disease. Spots can sometimes join together to form larger areas of dead tissue. Roses can also develop smaller black spots on their stems.
Black spot will look like somewhat circular black spots on leaves. It usually occurs on the upper sides of leaves, but can also develop on the undersides. The outer margins of the black circles are ragged or feathery and they are usually surrounded by a ring of yellow.
Spots begin on the lower leaves and move upward. They can appear as early as when the leaves first unfurl. These spots can enlarge and eventually merge. Affected leaves often fall off the plants, and if left unchecked, the entire plant can defoliate.
The fungus can also infect young canes, causing dark purple or black blisters on the canes, and even the flowers may show some red spotting. Infected plants will set fewer flower buds and without leaves, the plants become stressed and susceptible to more problems.
How to prevent:
Black spot is easier to prevent than to cure. Existing spores can remain in the soil and overwinter on leaves and stems, waiting for favorable conditions. The spores make contact with the rose by splashing onto it in drops of water.
Give the Plant Ideal Growing Conditions
True for avoiding any plant disease, a healthy, vigorous plant is less susceptible to problems. Roses prefer a sunny location with well-draining soil and regular, weekly watering.
Good Air Flow
Provide good air circulation around and through your rose plants. Do not plant your roses too close to other plants. Prune to open the spaces between canes, if the plant gets too dense and air cannot get through.
Avoid getting the leaves wet while watering. There is not much you can do about rain, but at least limit the time the leaves remain wet.
Remove any infected leaves and always do a thorough cleanup each fall. Remove and dispose of any remaining leaves when you do your dormant pruning in late winter/early spring. Spores can remain on leaves and stems and can reinfect whenever conditions are favorable. Within 10 days of the first symptoms, the disease has already started spreading. Spores can spread by water and wind. Prune out any canes showing signs of infection. Prune 6 to 8 inches below the infection and only prune in dry weather. Disinfect your pruners with a 10 percent bleach solution or alcohol between cuts.
Apply a thick layer of mulch around the plants. Mulch will prevent water from splashing up on the plant and spreading spores.
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