Black spot
A common fungal disease of roses that causes black spots with feathery edges on leaves, followed by yellowing and leaf drop.
Also known as: Rose black spot
Symptoms and identification
Round black spots up to 1 cm across with fringed edges appear on upper leaf surfaces, starting on lower leaves. The leaf around them turns yellow and drops. Badly affected bushes lose most of their leaves and flower poorly.
Causes
The fungus Diplocarpon rosae spreads by water splash and needs leaves to stay wet for several hours. Warm, humid and rainy weather, overhead watering and crowded growth all encourage it.
Organic treatment
Remove and bin infected leaves, and spray every 7–10 days with neem oil or a baking soda solution (1 teaspoon per litre with a few drops of liquid soap). Prune out infected stems in the dormant season.
Prevention
Water at the base in the morning, space plants for good airflow, and choose resistant varieties. Pick up fallen leaves and remove infected leaves promptly.
Chemical treatment (use with care)
Use a fungicide labelled for black spot on roses, starting at the first sign and repeating at the interval on the label.