Look for
Large black or dark purple spots on the surface of the leaves or stems are the main signs of black spot. As the problem gets worse the foliage gradually turns yellow and drops prematurely, weakening the plant.
Plants affected
- Rose black spot is an extremely common and widespread fungal disease of both cultivated and wild rose varieties.
About Rose black spot
- Rose black spot is specific to roses and all types are susceptible.
- The disease causes dark spots or irregular brown or black blotches on both leaf surfaces. Leaves then turn yellow and drop prematurely, resulting in weakened plants.
- The disease is worse in warm, wet weather.
- The black spot fungus produces spores which are released under wet conditions and usually spread by rain-splash.
- The disease can also be passed from plant to plant on hands, clothing or tools.
- Spores overwinter mostly on the shoots, but can survive on fallen leaves and within the soil.
- Recently, rose black spot has become more common in town and city gardens due to less sulphur dioxide within atmospheric pollution. Sulphur is known to reduce fungal problems.
Treatment
Chemical
Products containing the following chemical ingredients are all effective on Rose black spot
- Penconazole
- Myclobutanil – alternating any of these with the protectant Mancozeb to prevent the fungus developing resistance to the fungicides.
Organic
- Remove infected, fallen leaves promptly and burn.
- Hard prune shoots in spring and burn the prunings.
- Treat infections with powdered sulphur, which is deemed organically acceptable.
- Some cultivars are partially resistant to black spot but in a bad year all varieties may succumb. Generally, many of the older cultivars and more yellow than red or pink cultivars are susceptible to the disease. So checking catalogues and choosing resistant varieties, such as ‘Bonita’, ‘Royal William’ or ‘New Dawn’, and growing a mixed planting should help. Rose varieties less than five years old should be relatively disease resistant.
Prevention
- Prevention of rose black spot is very difficult and many strains of the fungi are resistant. However, regular raking around the plant with a hoe and pruning and burning of infected material will certainly help along with the advised treatments.
- Choosing resistant varieties also helps, but don’t rely too heavily on resistance because there are many species of fungus and even new varieties can quickly succumb.
- Adopt a regular, fortnightly spraying regime to protect your plants before the problem begins. It’s a good idea to alternate the chemical that you use to prevent resistance occurring.
- Dig in plenty of organic matter at planting time and keep plants well fed throughout the growing season. Strong healthy plants will generally resist attack better those that are weak.
- Surround plants with a fresh layer of mulch each spring. This will smother any spores that have fallen around your plants and stop them from being splashed on to roses by rain.
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