Rose Bush Diseases: Black Spots on Leaves
Rose Black Spot (Diplocarpon Rosae) is the most widespread and serious fungal disease affecting members of the rose family. Black Spot attacks the leaves and a bad case can defoliate a rose bush completely. This loss of leaf reduces the vigour of the rose and can ultimately kill it. Black Spot spores which have lain dormant in the soil over winter rise up and attack the plant from spring onwards. Watch out for the typical leaf markings caused by a black spot attack from April through until September or October.
Black Spot is a fungus where new strains develop very quickly. Unfortunately, this means that new Black Spot resistant varieties can lose their immunity quite quickly as the disease mutates. It has become more widespread over the last 20 years or so as the air in the UK has become cleaner. Carbon fuels tend to contain sulphur which is a natural fungicide.
Most prone to attack by Black Spot are the more popular patio, floribunda, hybrid tea and climbing roses. Roses such as the shrub roses and species climbers tend to be pretty immune.
Symptoms of Rose Black Spot
Typical Black Spot symptoms are a black patch (or patches) developing on the upper surface of the leaf. The leaf around the patch usually turns yellow and the affected leaf falls off. Other strains do not cause a yellowing, while some forms of Black Spot can manifest themselves through small black spots that do not get bigger and do not cause the leaf to fall.
How to Control Black Spots on Leaves
Collect and burn fallen leaves in the autumn. Some sources recommend covering them with a mulch - we do not agree as any disturbance allows the spores to rise the following spring.
Cut out any affected stems before the young foliage appears in spring.
Improve the soil. There are few better things a gardener can do than increase the organic content of the soil. The more moisture and nutrient the ground provides the stronger and less stressed the plants that grow in it. Strong plants tend to stay healthy.
Don't plant rose bushes where there is restricted air movement and prune them so they have a nice open shape.
There are a range of fungicides that will attack or prevent Black Spot. The rules about fungicides change fast and so the chemicals available change as well. At the time of writing, we would suggest Bayer Systhane Fungus Fighter and Scotts RoseClear Ultra (which also defends against insect pests). We recommend these two products because they use different chemicals and it is always good practice to rotate products to prevent a build up of resistance.