Journal of a rose gardener...Blackspot 25/07/16
Rose Blackspot, or to give it its official name, ‘Diplocarpon Rosae’, is a fungal disease. It appears as soon as leaves appear on roses in early spring, and will continue until the leaves fall in autumn, but it is at its worst in midsummer.
It infects the leaves, causing, as its name suggests, black spots, often edged with yellow, which spread and cause leaves to turn yellow and fall. Small, black lesions may also appear on young stems.
Some roses are more susceptible than others, and you can buy ‘disease resistant’ varieties, but blackspot is a genetically diverse disease and new strains arise.
To control blackspot, pick off any diseased leaves and remove together with fallen leaves from around the base. A potash deficiency can worsen black spot and it may be worth feeding with a high potash fertiliser. Mulch around the base of roses regularly to supress any spores that may be in the soil.
If you are unable to keep on top of black spot using the above methods, you can spray with a fungicide such as Rose Clear 3 in 1, which also addresses Greenfly, Blackfly, Powdery Mildew and Rust (£7.99 for a 1 litre spray bottle). Spray at the first sign of disease and repeat at regular intervals as directed in manufacturer’s instructions. To minimise the risk to bees spray in the early morning or evening when bees are less active. Also bear in mind that you should not use rose petals for culinary purposes once they have been sprayed.