Black Spot, Rose black spot
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Black Spot of rose is the single most common and impactful disease of roses globally. It is a pretty nasty fungal disease that appears as the name suggests, as black spot on the upper surface of rose leaves. Besides looking unsightly in can seriously weaken a rose plant and affect overall vigour.
Circular black spots that can reach up to 50mm with a perforated or feathery edge and often with a yellowish halo around the spot. Severe infections will cause the spots to run together and cause a large, black mass.
Infections occur during damp and humid periods and wet conditions are required for the disease to spread.
Leaves turn yellow and drop early
Plant vigour is greatly reduced
With times a plant can loose all it's leaves
Black spots on upper side of leaf occurring from the bottom up of a plant
Always remove or destroy infected leaves immediately and try using Neem oil as a biological treatment.
Anti-fungal applications like Chronos, which contains a imidazole prochloraz zinc complex can be used to treat black spot. Always read the instructions supplied on the packaging.
The spores germinate and penetrate rose tissue in 9 to 18 hours or longer; new leaf spots appear in 3 to 16 days and spores in 10 to 18 days. The cycle may be repeated throughout the growing season.
First and foremost, give your rose what it needs. A healthy plant is less susceptible to problems and diseases. Ensure good air circulation for your roses by not planting them close together and pruning often when they become dense. Avoid overhead-watering, especially later in the day and mulch well beneath your plants to reduce water splashing up on leaves. Plant resistant cultivars and last but not least, treat the bush with preventive sulphur spray in spring, before new growth arises. Spores won’t grow on sulphur and you will need to repeat the application after each rain as it will wash off.