Rose Black Spot
Rose black spot is a common and widespread fungal disease of both cultivated and wild rose varieties.
Some modern cultivars show some resistance to this disease and some older cultivars are not affected at all, so when buying plants this is worth considering.
The disease is recognized as large black or dark purple spots on the upper leaf surface sometimes surrounded by a yellow zone.
Once the spots become evident they will produce and spread new spores within ten days.
These spores will be carried on the wind to any other nearby rose bushes.
The disease is worse in warm wet weather, and can be passed from plant to plant on hands, clothing or tools.
During development, the black lesions spread, and more spores are released to be spread by rain to form new infections.
This produces premature leaf fall which in turn weakens the plant, and reduces flower numbers.
The disease is caused by a fungus / mycelium that over winters in fallen leaves, or on green stems and in buds of adjacent plants.
In Spring the over-wintered spores are spread by wind and rain onto emerging young leaves.
To control the disease, hard prune after flowering, and remove fallen leaves.
Annual pruning done in spring will hopefully deal with any remaining spores.
Destroy the prunings and fallen leaves, do not compost them.
Planting the rose bushes in full sun will create strong plants better prepared to resist the onset of the disease.
In full sun, evaporation happens more quickly, which not only helps prevent black spot, but other fungal diseases as well.
Black spot loves moisture, and, in shade, water evaporates much more slowly.
Similarly, plants that are in an area with good air circulation will resist the disease better, as the moisture evaporates more quickly.
Try to avoid overhead irrigation, which wets the foliage as this creates a sticky surface for the spores to stick to.
Spores landing on dry leaves are more likely to be blown away.
Mulch around the plants in spring and this will bury any spores that are lying on the surface of the soil.
Fungicides are available to control black spot, so check with your local garden center as to what is available in this ever changing market.
Although not approved, four level teaspoonsful of Baking Soda, a few drops of washing-up liquid and a desert spoonful of soluble fertilizer mixed into 5litres (1gall)of water can often work when sprayed on the affected plants every two weeks.
As the disease has become more common in city gardens,there are those that say, that this is due to the Smoke Pollution Act of 1989 and 1993 in so far as their is now less sulphur dioxide in the atmosphere, this is thought to be the cause of the increase in the disease.
They base this opinion on the fact that Sulphur is known to reduce fungal problems.