Black spot disease is the scourge of rose growers. Not only does it make the plant look unsightly, but it also causes the leaves to drop prematurely, resulting in weakening the plant if it occurs repeatedly.
Black spot is a common fungal problem on roses, but can be found on other plants too. The spots can be of various colours – grey, brown or black. The spots are in fact dead leaf tissue caused by the fungus which spreads the disease. Spots can sometimes join together to form larger areas of dead tissue. Roses can also develop smaller black spots on their stems.
There are few other symptoms than the spots themselves. However, diseased plants frequently have other problems causing weaknesses, which enable the fungal disease to develop. It is worthwhile carefully checking any plant with leaf spot.
TREATMENT AND CONTROL
Immediately remove and destroy all infected leaves and plant parts. Pick up any fallen foliage and destroy it, whenever it happens but especially in autumn. To reduce the chances of re-infection for the following season, prune back plants hard. Where roses were infected with black spot disease in the previous season, then start spraying with a suitable fungicide as new leaves open and repeat at fortnightly intervals.
Spray with a suitable fungicide from April onwards. Continue spraying at recommended intervals throughout the summer.