Blackspot is one of the diseases to look out for as we head into Summer and roses can be particularly susceptible during times of stress when the health of the plant declines. It’s a soil borne fungus present at all times, that attacks the leaves of your rose causing dark spots and yellowing. The leaves with black spots drop prematurely and the whole rose plant can even completely defoliate.
Like any disease, prevention is better than cure and it’s important to build up the health of the plant so that they are in a stronger state to fight the disease. Regular watering and feeding with an organic based fertilizer are the basic rules to grow healthy and strong plants. A regular application of sulphate of potash - about 100 to 150 grams per bush four times a year – will go a long way in thickening up the cell walls of the leaf for a healthier plant. And given the wet conditions the roses have lived through over the past few months, nitrogen in the soil may be severely depleted, so it would be a good idea to add a nitrogen-based fertiliser to top it up.
In high humidity conditions, do not wet the rose foliage unless it can dry within 4 hours.
While there are many sprays on the market to treat Blackspot - generally copper or sulphur-based sprays are best - an alternative natural spray for Blackspot can also be made at home. Try using two teaspoons of bicarb soda in five litres of water, and then add a couple of drops of organic detergent or seaweed extract and apply on the leaves using a handheld sprayer. It makes an excellent and inexpensive fungicide. | 
