Roses are susceptible to three major diseases, and black spot is one of them. Fortunately for rose growers, it is also one of the easiest rose diseases to treat. Black spot is a fungal disease that most often appears during humid, rainy conditions and causes black patches on stems and leaves of rose plants.
Recognizing Black Spot
During humid, rainy conditions, gardeners should check roses one a week for this disease. The first sign of black spot on roses is small pinhead-sized spots on plant leaves. The spots gradually grow larger and becomes surrounded by a halo of yellow dying leaf tissue. As the disease progresses, rose leaves turn yellow and drop. The good news is, treatments for black spot generally work even in advanced stages of the disease.
Black spot spreads by fungal spores that travel through the air and on surfaces that have come into with infect leaves, such as gardening tools. Fungal spores can also spread by water spray and dead leaves that have fallen and landed on the soil under the plant. Black spot fungal spores are able to survive in the soil during winter.
Treating Black Spot
Treatment for black spot is an ongoing process that includes early prevention, prompt treatment and regular treatments to prevent recurring infections. Early prevention includes monitoring roses as soon as new growth appears and spraying roses with a solution of one part lime to nine parts wettable sulfur during winter.
Ongoing treatment includes removing any infected rose leaves and disposing of them as soon as possible. Once black spot sets in and begins to progress, gardeners need to begin a regular weekly application of a suitable fungicide, such as Daconil or Benomyl .
Good watering practices can help reduce the risk of black spot. Soaker hoses are ideal for roses that are susceptible to the fungal disease. These hoses are placed around the plant’s base to prevent water from splashing up onto plants.
When spraying roses with a fungicide, gardeners must follow label directions, as these will advise them on how to safely apply the product to the plants and which plants may be sensitive to the active ingredients in certain products.
If black spot becomes a recurrent problem on one or more of the rose bushes in the garden, cut the entire plant low to ground in the fall, removing any stems that have been affected or could be carriers for black spot fungal spores.
Healthy plants are better able to withstand black spot infections. To ensure healthy roses, buy healthy plants from a reputable nursery, position plants in the garden to ensure they receive adequate air circulation and water carefully near the plant roots and early in the day. Prune roses as needed to remove an dead or diseased limbs and immediately pick up and discard any leaves that have fallen off the plant onto the soil below. When pruning roses, always use sharp clean pruners, and if they come into contact with black spot fungus, wash them in a bleach solution.