Whilst there are some Rose varieties that are very susceptible to Blackspot, Black Spot. This disease (fungus) it can attack most varieties given the right conditions.
Ironically, clean air allows the disease to take hold, whereas the old industrial atmospheres which were generally loaded with sulphur, controlled Rose Blackspot to a large degree! South West UK and S Wales are said to be the worst areas for this disease.
The sure signs of Rose Blackspot, are the rounded black spots on the leaves which themselves are normally surrounded by yellow areas. These yellow areas spread outwards which result in premature leaf drop, and can be so damaging as to affect the stems with dieback if the black spot is not controlled.
Leaf infection is the first sign of blackspot, but in severe infestations, this can spread to rose leaf buds and then stems.
The actual infestation start quite early in the year, but does not start to show through to any extent until late June or July - depending upon season and also the atmospheric conditions. Warm damp conditions are ideal for the spread of Rose Blackspot.
There is no cure for the affected leaves, though a spray of a rose fungicide or combined rose spray such as Multirose will halt the spread. Infected leaves should be removed and burned. Rose leaves affected with blackspot should NOT be put on the compost heap - or even into your recycling bin.
As with many things, Rose Blackspot - Black spot - is best prevented with a preventative spray early in the season - before the foliage starts to show through even. Also spray the ground around the rose bushes at this time.
Most garden roses are prone to this disease, and much depends upon cleanliness for its successful control.
With roses that are susceptible to blackspot, then spraying every two weeks might be necessary.
Hard pruning in the spring - and burning of all pruning material is useful with those varieties of roses that regularly get blackspot - black spot. A feed with a high potash content will also help to allay the disease. This should be carried out early in spring in order that the rose plant may take the potash in as a preventative.